Unit 2 Exam Study Guide Packet
Unit 2 Exam Study Guide Packet 2301.001 Environmental Geology
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CH 6 Geology Earthquakes Note bc 2 because abt 2 about yrs 2 years Mm magnitude ppl 2 people E 2 energy btwn 2 between w with wo 2 without Mtn mountain quakes earthquakes amt 2 amount req rreq rs requirerequires SoCaI 2 Southern California directions 5 2 south n 2 north w 2 west e 2 east sw southwest nw northwest etc num number sig 2 significant econ 2 economic pop 2 population vol 2 volume nat natural fed 2 federal gov t 2 government combo combination avg average Case History Catastrophic earthquakes can destroy cities and lots of ppl Human pop increase increase risk Strong earthquake L Aqila in 2009 lots of buildings collapsed and killed 300 ppl California earthquake no or few deaths compared to above Italian event catastrophe b c buildings not built for earthquakes 2010 earthquake in Haiti and lots of ppl killed Lots of buildings collapsed b c not built for quakes and 240000 ppl died 61 Intro to Earthquakes Approx 1 mil earthquakes a yr but only small can be felt Quakes compared to one another by their magnitude how much E they release or by ground motion intensity of shaking 62 Earthquake magnitude location of the surface of Earth above the point at depth where rocks ruptured to produced the earthquake measure of the E released by the earthquake Based on seismic movement area that ruptured along fault plane amt of movement or fault slip and rigidity Use seismographs records earthquake displacement they produce seismographic records or seismograms for seismic movement Before moment magnitude richter magnitude Charles Richter scientist was used it s based on amplitude size of the largest seismic wave produced during an quake 0 Richter magnitude of 7 produces 10 times more displacement than 6 Quake scientists seismologists Event of magnitude 8 M 8 is considered a great earthquake M 7 maj or quake M 6 strong earthquake differences are great btwn them Earthquake Catastrophes Catastrophic earthquakes can destroy large cities and kill thousands of ppl refer to text for castastrophes 63 Earthquake Intensity qualitative way of comparing earthquakes Describes 12 divisions of intensity based on observations concerning the severity of shaking during and earthquake Intensity re ects how ppl perceived the shaking and how structures responded to the shaking Diff levels of intensity may be assigned to same quake at diff locations depending on proximity to epicenter and local geologic conditions the extent of potential damaging shaking following an earthquake Important for directing an effective emergency response to those areas Helps locate areas where gas lines and other utilities are likely to be damaged 64 Plate Boundary Earthquakes Cali frequent quakes b c of 2 lithospheric plates that move past each other Quake in SoCal could in ict 100 bil in damage and kill several thousand ppl The more we know about probable location magnitude effects the better we can estimate damage and make necessary plans Interplate earthquakes btwn 2 plates initiated near plate boundaries and producing nearly continuous linear or curvilinear zones in which most seismic activity takes place Most large US quakes are interpolate in W But large damaging intraplate earthquakes located w in a single plate can occur far from plate boundaries 65 Intraplate Earthquakes Intraplate quakes w M75 that occurred in winter of 18111812 in the central Mississippi Valley nearly destroyed the town of New Madrid Missouri and they killed an unknown num of ppl Forests were attened wide fractures opened in ground Quakes occurred along a seismically active structure known as the New Madrid seismic zone underlies geologic structure known as Mississippi R Embayment a downwarped area of Earth s crust where lithosphere is relatively weak The recurrence interval time btwn events for major quakes is about 500 yrs estimated Believed to be young zone of deformation like 10000 yrs old Chareston S Carolina Aug 31 1886 intraplate quake killed about 60 ppl and damaged destroyed lots of buildings Intraplate quakes in e US generally more damaging and felt over a much larger area than similarM quakes in Cali b c rocks are generally more stronger less fracturd and can more efficiently transmit quake waves than western rocks 66 Earthquake Processes Quakes natural consequence of processes that form the oceab basins continents and mtn ranges of the world Faulting The process of fault rupture or faulting can be compared to sliding two rough boards past one another Friction along boundary btwn boards analogous to a fault plane may temporarily slow their motion but rough edges break off and motion occurs at various places along the plane When stress on rocks exceeds their strength the rocks rupture forming a fault and producing an Fault fracture or fracture system along which rocks have been displaced that is one side of fracture fracture system has moved relative to the other side 0 Longterm rate of movement is known as the slip rate 0 Major great quake displacement of several meters along fault Sudden rupture of rocks produces seismic waves that shake ground Faults are seismic sources and are first step in evaluating risk of quake or seismic risk in a given area Fault Types Strikeslip fault fault in which sides of fault are displaced horizontal Rightlateral if righthand side moves towards you Leftlateral if left hand side moves towards you Dipslip fault fault w vertical displacement Reverse fault normal fault depends on geometry of displacement o Thrust fault reverse fault has angle less than 45 degrees Buried faults associated w folded rocks they don t propagate to suface Anticlines archshaped folds Synclines bowl shaped folds Fault Zones and Fault Segments Faults almost never occur as a single rupture They form fault zones group of related faults roughly parallel to each other in map view Fault zones vary in width from a meter to several km Most long faults or fault zones such as San Andreas are segmented w each segment having an individual history and style of movement those parts of fault zone that have ruptured as a unit during historic and prehistoric quakes Often used to estimate M of future quake If quake history of fault zone is unknown zone may be divided into segments Paleoseismology study of prehistoric quakes form geologic environment The geologic processes that govern and generation of earthquakes on individual segments are subject of active research Active Faults Active fault if it s moved during the past 10000 yrs the Holocene epoch Quaternary period approx past 26 mil yrs most recent period of geologic time Faults that have not moved during the past 26 mil yrs are generally classified as inactive Active fault fault that has moved in the 10000 yrs cali moved at least onece in past 50000 yrs or more than once in last 500000 yrs US Nuclear Regulatory Commission ratio of slip displacement to the time interval over which slip occurred The average recurrence interval of quakes on a particular fault determined by 3 methods 1 paleoseismic data averaging the time intervals btwn earthquakes recorded in the geologic record 2 Slip rate method involves assuming a given displacement per event and dividing that num by slip rate 3 Seismicity method involves using historical quakes and averaging the time interavls btwn events Methods of Estimating Fault Activity Paleoseismicity by investigating landforms produced or displaced by faulting Study of soils can be useful in estimating activity of a fault Tectonic Creep Some active faults exhibit gradual displacement that is not accompanied by felt earthquakes Can slowly damge roads sidewalks building foundations and other structures Slow Earthquakes similar to other quakes in that they are produced by fault rupture M range btwn 67 b c large area of rupture is often involved Newly recognized fundamental earth process 67 Earthquake Shaking 3 factors determine the shaking you will experience during an earthquake 1 earthquake M 2 distance from epicenter 3 local soil and rock conditions Types of Seismic Waves Some seismic waves are generated by fault rupture travel w in Earth and others travel along surface compressional waves faster than S waves and can travel thru solid liquid and gaseous materials velocity thru liquids is much slower Detectable by ear S waves shear waves travels only thru solid materials Speed thru rocks like granite is approx 12 of P waves Up and down or side to side motion at right angles to the direction of wave propagation b c liquids can t spring back when exposed to this motion sideways shear S waves cannot move thru liquids R waves move along Earth s surface slower than P S waves causes much of the quake damage to buildings and other structures Horizontal and vertical ground movement or rolling motion that may crack walls and foundations of buildings bridges and roads Seismograph Seismogram written digital record of an earthquake Written form continuous line that shows vergicalhorizontal Earth motions Difference in arrival times shown by seismograms SP can be used to locate the epicenter of an earthquake Need at least 3 seismograms When the records from seisomgraphs from lots of seismic stations are analyzed the epicenter of a quake can be located Frequency of Seismic Waves Frequency cycles per second or hertz Hz Wave period the avg time period btwn wave peaks Frequency of the waves in cycles per second is 1 cycle divided by 10 seconds wave period or 01 Hz Most PS waves are 05 to 20 Hz or 05 to 20 cycles per second Highfrequency shaking causes low buildings to vibrate an lowfrequency causes tall buildings to vibrate 0 Near epicenter both shorter and taller buildings may be damaged by highlow frequency seismic waves 0 w increasing distance from epicenter highfrequency waves are weakened or removed by a process called attenuation Rapid shaking dies off quickly w distance Material Amplification Diff earth materials like bedrock alluvium sand and gravel silt and mud respond differently to seismic shaking Materials likely to amplify are vulnerable to quakes even if event is several hundred km away 0 Mexico city quake M 81 o 1989 Loma Prieta M 71 which originated south of San Fran Directivity Rupture of rocks on a fault plane starts at a pt and radiates or propagates from the point The larger the area of rupture the larger the quake intensity of seismic shaking increases in the direction of the fault rupture Ground Acceleration During Earthquakes Strong ground motion from earthquakes may be described in terms of the speed or velocity at which primary secondary and surface waves travel thru rocks or across the surface of Earth Damage to structures from strong ground motion is related to both the amplitude of seismic surface waves and the rate of velocity change of the seismic waves w time Acceleration rate of change of velocity w time Acceleration due to gravity on Earth 98 msquot2 Earthquake waves cause ground to accelerate both vertically and horizontally o Accelerometers measures and records acceleration of the ground during earthquakes o Engineers design buildings to w stand seismic shaking the building design criteria are often expressed in terms of max acceleration of the ground that causes most damage to buildings Supershear Supershear occurs when the propagation of rupture is faster than the velocity of shear waves or surface waves produced by the rupture Can produce shock waves that produce strong ground motion along the fault More likely to occur w strikeslip quakes that rupture a long straight fault segment of several tens to a hundred or more km in length Depth of Focus Focus the pt or area w in Earth where an earthquake rupture starts Depth of focus of a quake varies from just a few km deep to almost 700 km below the surface Deepest quakes occur along subduction zones where slabs of oceanic lithosphere sink to great depths Shallow earthquakes occur often in Cali are more destructive than deeper earthquakes of comparable M b c they are deep enough to generate strong seismic shaking yet sufficiently close to surface to cause strong surface shaking 68 Earthquake Cycle hypothesis that proposes that there is a drop in elastic strain after a quake and there is a reaccumulation of strain before the next event Strain deformation resulting from stress Elastic strain deformation that is not permanent provided that stress is released Elastic rebound occurs after an earthquake At time 1 rock on either side of fault segment have no strain built up and no deformation Time 2 elastic strain begins to build caused by tectonic forces that pull the rocks in opposite directions referred to as shear stress rocks begin to bend 3 elastic strain accumulates and rocks have bend still held together by friction 4 deformed rocks finally rupture Stages of the Earthquake Cycle Typical quake cycle has 34 stages First is a long period of seismic inactivity following a major quake and associated aftershocks quakes that occur anywhere from a few minutes to a year or so after the main event Followed by a second stage increased seismicity b c accumulated elastic strain approaches and locally exceeds the strength of rock initiates faulting that produces small quakes 3rd stage may occur only hours or days before next large quake consists of foreshocks small to moderate quakes that occur before the main event The DilatancyDiffusion Model assumes that first stage in quake development is an increase of elastic strain rocks that causes them to dilate inelastic increase in volume after the stress on rock reaches 12 the rock s breaking strength During dilation open fractures develop in rocks first physical changes that might indicate a future quake take place Controversy surrounds the validity of this model one aspect of model gaining lots of favor much water is present if uid pressure caused by deformation is high it can facilitate occurrence of quakes hypothesizes that uid pressure rises until failure occurs thus triggering a quake and upwared water discharge 69 Earthquakes Caused by Human Activity 3 ways that actions of ppl have caused earthquakes 1 loading Earth s crust dam reservoirs 2 disposing of waste deep into the ground through disposal wells 3 setting off underground nuclear explosions ReservoirInduced Seismicity Hoover Dame on Colorado R in Arizona and Nevada Fracture zones may be activated both by increased load of water on land and by increased water pressure in the rocks below the reservoir resulting in faulting Deep Waste Disposal April 1962Nov 1965 several hundred quakes occurred in Denver Co area Source traced to Rocky Mtn Arsenal manufacturing materials for chemical warfare liquid waste was being pumped down a deep disposal well to a depth of about 3600 m rock receiving the waste was a highly fractured metamorphic rock High correlation btwn rate of waste injection and occurrence of quakes Nuclear Explosions Numerous quakes w M s as large as 50 to 63 have been triggered by underground nuclear explosions at Nevada Test Site 610 Effects of Earthquakes Lots of damage and death but also primary effects directly caused by fault movement include ground shaking and its effects on ppl and structures and surface rupture Secondary effects induced by faulting and shaking include liquefaction of ground landslides fires disease tsunami and regional changes in ground and surface water and also land elevation Shaking and Ground Rupture Immediate effects of catastrophic earthquake can include violent ground shakin widespread surface rupture and displacements Fault scarp surface rupture w vertical component like a steep slope or curb on a street 1994 Northridge earthquake caused so much damage b c there was so much there to be damaged Liquefaction transformation of watersaturated granular material or sediments from a solid to a liquid state May result from compaction of sediments during intense shaking Building may tilt or sink into liquefied sediments Landslides Quakes often trigger landslides or hill slope failure in hilly mountainous areas Can be destructive text for examples Fires Fire is a major hazard associated w quakes broken electrical power or gas lines Disease Landslides from 1994 Northridge earthquake raised large volumes of dust some of which contained fungi spores that cause valley fever winds can carry that dust to urban places Quakes can also rupture sewer and water lines causing water to become polluted w diseasecausing organisms Regional Changes in Groundwater and Land Elevation Quakes can alter groundwater levels and stream ow Can also increase pore spaces to increase groundwater ow Vertical deformation including both uplift and subsidence is another effect of some large earthquakes can cause regional changes in groundwater levels Tsunami Sudden vertical displacement of ocean water 611 Earthquake Risk and Earthquake Prediction Quakes often strike wo warning Can make predictions to warn ppl like this magnitude has high probability of occurring etc Estimation of Seismic Risk Quake risk associated w a particular area is shown on seismic hazard maps which are prepared by scientists Some maps show relative hazard where quakes of specified M have occurred Preferable calculate probability of either a particular event or amt of shaking likely to occur to assess seismic risk 4 sources of data used to estimate probability of earthquakes 1 geology mapped location of faults w documented offsets 2 paleoseismology study of past quake history from trenches excavated across faults to determine dates and amt of displacement from past events 3 geodesy determination GPS often used of how fast Earth s tectonic plates are moving 4 seismology occurrence of past quakes detected by seismographs located at many locations that monitor when earthquakes occur and how strong they are ShortTerm Prediction Shortterm prediction or forecast of quakes is an active area of research Specifies a relatively short time period in which event is likely to occur and assigns it a probability of occurring Quake prediction is very complex Iapanese made 1St attempts at earthquake prediction w some success based on frequency of microearthquakes quakes w M lt 2 Chinese scientists made 1St successful prediction of a major quake M 73 Haicheng damaged 90 of buildings in 1975 Used foreshocks there was a foreshock M 24 24 hrs before main shock Foreshocks don t always precede quakes Preseismic Deformation of the Ground Surface Rate of uplift and subsidence esp when they are rapid or anomalous may be significant in predicting earthquakes For more than 10 yrs before 1964 Niigata Iapan quake M 75 there was a broad uplift of Earth s crust of several cm near the Sea of Japan coast Although uplifts not well understood could be indicators of impending quakes Emission of Radon Gas Levels of radon a radioactive gas have been observed to increase significantly before some earthquakes Seismic Gaps Seismic gaps areas along active fault zones or win regions that are likely to produce large quakes but have not caused one recently Useful in mediumrange quake prediction As earth scientists examine patterns of seismicity 2 ideas are emerging First there are sometimes reductions in small or moderate quakes prior to a larger event Second small quakes may tend to ring an area where a larger event might eventually occur Anomalous Animal Behavior Anomalous animal behavior has often been reported before large earthquakes dogs barking unusually chickens refusing to lay eggs horse or cattle running in circles snakes crawling out in winter and freezing etc Reliability of this is difficult to evaluate 612 Toward Earthquake Prediction We are still a long way from a working practical methodology w which to reliably predict quakes but lots of info is being gathered Currently most useful tool is patterns of earthquakes particularly foreshocks and seismic gaps Progress on shortrange days to months quake prediction is still not that great but medium to long range forecast yrs to decades based on probability of quake occurring on a particular fault has progressed faster than expected Borah Peak quake in central Idaho fault scarps up to several m high and numerous ground fractures along 36 km rupture zone of fault were produced as a result of earthquakes important fact was that scarp and faults produced during the quake were superimposed on previously existing fault scarps validating the usefulness of careful mapping of scarps produced from prehistoric quakes 613 Sequence of Quakes in Turkey Can One Quake Set Up Another Controversy do quakes on a given fault have return periods that are relatively constant w relatively constantM quakes or do they tend to occur in clusters over a period of several hundered to several thousand years Pallett Creek LA near San Andreas fault high precision radiocarbon dating methods used most of the quakes tend to cluster individual quakes separated by few decades but time btwn clusters varies from approx 160 to 360 yrs San Andreas fault evidence for 6 quakes since 1360 ad w avg return period fo abt a century Lots of variability involved concerning constant M of earthquakes and their return periods for a given fault system Understanding processes related to clustering of quakes along a particular fault is very important if we are to plan for future seismic events in a given region 614 Response to Quake Hazards Responses to seismic hazards in quake prone areas Earthquake HazardReduction Programs Major goals of National Earthquake Hazard Reduction Program Develop an understanding of the earthquake soured like physical properties and mechanical behavior of faults Determine quake potential characterizing seismically active regions rates of crustal deformation identifying active faults paleoseismicity slip rate recurrence level etc Predict effects of earthquakes gathering data to predict ground rupture shaking and predicting response of structures that we build in quakeprone areas Apply research results inform ppl communityies states and nation Adjustments to Earthquake Activity Reliable protective measures we can take Structure protection building better buildings and structues that can withstand quakes reinforced concrete wood frames etc Land use planning siting of important structure like schools hospitals and police stations away from active faults or sensitive earth materials that are likely to increase seismic shaking 0 Zoning the grounds response to seismic shaking on a blockbyblock basis Increased insurance and relief measures to help adjust after quakes Earthquake Warning Systems Technically it is feasible to develop a quake warning system that would provide 10 s or more warning to Cali cities before arrival of damaging quake waves from an event several hundred km away Based on the principle that warning sent by a radio signal via satellite relay travels faster than seismic waves Could use seismometer along San Andreas fault and other faults that can sense movement and relay the massage Earthquake warning system is not a prediction tool only warns when an earthquake has already occurred Perception of Quake Hazards We are vulnerable to catastrophic loss Haiti Northridge Kobe Turkey Pakistan ex in text from large earthquakes Minimizing hazard req rs new thinking about the hazard Personal and Community Adjustments Before During and After an Earthquake Preparation is important Community level est building codes to provide safeguards against loss of life and major structural failures thru better design of buildings to allow them to better w stand quake shaking Education is also important Practicing an earthquake drill in schools During an earthquake strong ground motion will greatly restrict motion and you should duck cover and holdquot Geology Ch 7 Tsunami Note bc 2 because abt 2 about yrs 2 years Mm magnitude ppl 2 people E 2 energy btwn 2 between w with wo 2 without Mtn mountain quakes earthquakes amt 2 amount req rreq rs requirerequires SoCaI 2 Southern California directions 5 2 south n 2 north w 2 west e 2 east sw southwest nw northwest etc num number sig 2 significant econ 2 economic pop 2 population vol 2 volume nat natural fed 2 federal gov t 2 government combo combination avg average Case History Indonesian Tsunami Tsunami countries surrounding Indian ocean No warning Dec 26 2004 Earthquake 91 caused a large tsunami Indian and Australian plates were subducted to the northeast beneath Burma microplate Seismic waves cause several minutes of shaking on nearby islands Sea oor shifted about 20 m horizontally and rose several meters vertically People were caught surprisingly by the tsunami Sea recedes prior to a tsunami Khao Luk Thailand Elephants trumpeted at the time of the M 91 earthquake Possible elephants heard the earthquaes because earthquaes produces sound waves w low tones referred to as infrasonic sound and they could ve also sensed the motion Education could ve saves lots of lives 71 Introduction Japanese Large harbor wavesquot produced by the sudden vertical displacement of ocean water Serious natural hazard that can cause a catastrophe thousands of km from where they originate Can be triggered by several events large quake that causes rapid uplift or subsidence of sea oor underwater landslide submarine volcanic explosion etc asteroid impact a wave that is about 100 times higher than the largest tsunami produced by a quake 0 Frequency of this is very low though 0 Tsunami by quakes most common Damaging tsunami in historic time have been relatively frequent and mostly in the Pacific Basin How Can an Earthquake Cause a Tsunami An earthquake can cause a tsunami by movement of the sea oor and by triggering a landslide Sea oor movement is probably the more common of these two mechanisms Sea oor sits on a block of earth s crust that shifts up or down during a quake 0 This movement displaces the entire mass of water from sea bottom to ocean surface I If quake rupture uplifts sea oor the water surface above the uplift initially forms and elongate dome parallel to geologic fault Dome collapses and generates a tsunami wave I Deep ocean tsunami waves move very rapidly and are spaced long distances apart Sailors rarely notice a passing tsunami in the deep ocean I As tsunami nears land water depth decreases so velocity of tsunami also decreases spacing btwn wave crests decrease too As water slows down and piles up height of waves increases too I First tsunami wave when it reaches the shore and moves inland it may be several meters to several tens of meters high and destroy nearly everything in its path When wave arrives it s like a strong and fast rising rise in sea level Movement of tsunami inland is called the of the wave It s the furthest horizontal and vertical distance that the largest wave of a tsunami moves inland I Edge waves travel back and forth parallel to shore I Interaction btwn edge waves is called and they can move thousands of km across ocean to strike remote shorelines w very little loss of energy I heads in opposite direction toward the nearby land can arrive quickly following an earthquake How Can a Landslide Cause a Tsunami These landslides can take place underwater submarine landslides or rock avalanches that fall mtns into sea In most cases landslides triggered by an earthquake Quake off of north shore of New Guinea 1998 M 71 tsunami from a submarine landslide epicenter 50 km offshore so little warning time Lituya Bay Alaska 1958 M 77 lots of rock fell from cliff into the bay and displaced huge volume of seawater Lots of water surged upward to elevation of about 524 m above normal water level 72 Regions at Risk Heightened risk comes from geographic location of a coast in relation to potential tsunami sources such as quakes landslides and volcanoes Coasts in proximity to major subduction zone or directly across ocean basin from a major subduction zone M 9 or greater are at greatest risk Greatest tsunami hazard w return periods of several hundred years is adjacent to those major subduction zones w a convergence of a few cm per year Tsunamis range from height of few cm to 30 m or more Runup height of at least 5 m significant tsunamis and commonly produced by highM quakes and associated submarine slides Not enough known about global seismicity to accurately predict return periods of quakes w M 9 or above Major subduction zones found on margins of the Pacific 73 Effects of Tsunamis and Linkages to Other Natural Hazards Primary effects related to inundation of water and resulting ooding and erosion Wave energy of tsunami is sufficient to tear up beaches and most coastal vegetation and also homes and buildings Effects diminish w distance Secondary effects occur in minutes hours days and weeks following the event Fires in urban areas from ruptured natural gas lines Polluted water supplies Rotting animal carcasses and plants 2004 Indonesian tsunami pneumonia like disease tsunami lungquot Tsunamis closely linked to submarined and coastal quakes and landslides as well as island volcanic explosions and oceanic impacts of asteroids and comets 74 Minimizing the Tsunami Hazard The magnitude and frequency of tsunami are not in any way tied to human activity People who move to coasts that have an elevated risk for tsunami are increasing the likelihood that they will be affected by this natural process Rapid development of coastal areas in recent decades and increasing human pop in coastal cities Lessons to be learned from past tsunamis o Buffer zones of trees along coast to absorb impact 0 Strength of buildings 0 Warning and evacuation Strategies to minimize tsunami hazard 0 Detection and warning Structural control Construction of tsunami runup maps Landuse planning Probability analysis Education 0 Tsunamiready status Detection and Warning Nearly all large tsunamis are associated w giant earthquakes First warning comes from an earthquake in an offshore area that is large enough to produce a tsunami For distant tsunami we are capable of detecting them in open ocean and accurately estimate their arrival time to w in a few minutes 0 Info used to create a successful tsunami warning system in the Pacific Ocean 3 components to tsunami warning system Network of seismographs to accurately locate and determine the depth and M of submarine and coastal earthquakes Automated tidal gauges measures rises and falls of sea level Network sensor connected to oating buoys 0 Surface buoys w bottom sensor tsunameter detect small changes in the pressure exerted by the increased volume of water as a tsunami passes overhead 0 Info relayed by satellite to warning center combined w tidal gauge to predict tsunami arrival time Similar systems being created in Indian and Atlantic oceans after Indonesian tsunami Local tsunami strike land close to source of quake little warning time OOOOO People close to source can feel the quake and can move inland or to higher ground Structural Control Tsunamis that are even 12 m high have such power that many houses and small buildings are unable to w stand their impact Building designs for larger structures highrise hotels critical facilities etc can be engineered in a way to greatly reduce or minimize destructive effects of a tsunami Tsunami Runup Maps Following a tsunami straightforward procedure to produce a shows level to which the water traveled inland Created for island of Oahu Hawaii following 1946 tsunami Before tsunami strikes can use hazard map that shows area that is likely to be inundated by a given height Huntington Beach Cali has such a map although probability of large tsunami in So Cal is low but consequences are high Land Use Planning In the aftermath of the 2004 Indonesian tsunami scientists discovered that tropical ecology played a role in determining tsunami damage Large waves massive destruction Some coastal villages destroyed while others were less damaged They were protected by coastal mangrove forest or several rows of plantation that reduce velocity of incoming water Land use planning identifying where serious tsunami damage has occurred in the past and avoiding the most hazardous places Rapid coastal development coastal mangroves removed and replaced by homes hotels and vulnerable to tsunamis But large tsunami waves can erode large trees from the soil and their trunks and branches can then move inland w water producing a serous hazard to people and structures Probability Analysis The risk of a particular event may be defined as the product of the probability of that event occurring and the consequences should it occur Important to determine the likelihood or probability of a tsunami to analyze potential hazard Past tsunamis used to determine hazard The approach taken in developing a probabilistic analysis of the tsunami hazard is to 0 Identify and specify the potential earthquake sources and their associated uncertainties o Specify relationships that will either attenuate or reduce tsunami waves as they travel from the source area 0 Apply probabilistic analysis to the tsunami hazard similar to what is currently being done for quake hazard analysis 0 Difficulty is that tsunami at a particular location are generally rare events 0 Monte Carlo simulation Education Education concerning the tsunami hazard is critical to minimize risk Important to educate coastal residents and visitors as to the difference btwn a which is a notification that an earthquake that can cause a tsunami has occurred and an actual that a tsunami has been detected and is spreading across the ocean toward their area Distant tsunami several hours before waves arrived after warning issued Local little warning time Ppl also should be taught that tsunami comes in a series of waves Also that the water returning to ocean once a wave has runup is just as dangerous as incoming water Tsunami Ready Status For community to be Establish an emergency center w 24 hr capability Have ways to receive tsunami warnings Have ways to alert the public Emergency drills Community awareness program to educate ppl concerning a tsunami hazard Education is very important 75 Perception and Personal Adjustment to Tsunami Hazard Ppl can take following actions when warning issued Strong earthquake felt leave beach and low lying coastal area immediately Trough of tsunami wave arrives first than ocean will recede Tsunami may be small at one location but larger nearby Coastal communities as they gain tsunami readiness status will have warning sirens Tsunami warning don t go to the beach Geology Ch 8 Volcanic Activity Note bc 2 because abt 2 about yrs 2 years Mm magnitude ppl 2 people E 2 energy btwn 2 between w with wo 2 without Mtn mountain quakes earthquakes amt 2 amount req rreq rs requirerequires SoCaI 2 Southern California directions 5 2 south n 2 north w 2 west e 2 east sw southwest nw northwest etc num number sig 2 significant econ 2 economic pop 2 population vol 2 volume nat natural fed 2 federal gov t 2 government combo combination avg average Case History Mt Unzen Nearly 200 yrs ago Mt Unzen in sw Iapan killed lots of ppl Authorities ordered evacuation of thousands of ppl Lots of lava and hot ash Mt Unzen one of the ash ow centers of the world 81 Introduction to Volcanic Hazards 5060 volcanoes erupt each yr Most eruptions in US occur in Alaska US experiences 23 eruptions a yr Lots of ppl live close to volcanoes More pop more ppl close to volcanoes Densely pop countries w many active volcanoes such as Japan Mexico esp near Mex City the Philippines and Indonesia W US including Alaska Hawaii and Pacific NW also has many active or potentially active volcanoes 82 Volcanism and Volcanoes Volcanism related to tectonic activity As spreading sinking lithospheric plates interact w other Earth materials or molten rock including a small component of dissolved gases mostly water vapor and carbon dioxide is produced magma that has emerged from a volcano onto Earth s surface 23 of active volcanoes on Earth are located in ring of firequot that circumscribes the Pacific Ocean an area corresponding to subduction zones on the border of the Pacific plate 83 Volcano Types Each volcano characteristic style of activity that is partly a result of viscosity of the magma Viscosity a liquid s resistance to ow high viscosity high resistance to ow Magma viscosity determined by both silica SiOZ content which can vary from 50 70 and its temp Higher silica content higher viscosity Colder temp higher viscosity Highly viscous magma erupt explosively less viscous ows Shield Volcanoes the largest volcanoes common on Hawaiian islands and also found in Iceland and some Islands in Indian ocean Shaped like a gentle arch or shield Among the tallest mtns on Earth when measured from base often located on ocean oon Characterized by non explosive eruptions low silica content abt 50 Common rock type formed by magma of shield volcanoes is basalt composed mostly of feldspar and ferromagnesian minerals Built up entirely from numerous lava ows but they can also produce a lot of tephra o Tephra pyroclastic debris includes all types of volcanic debris that are explosively ejected from a volcano o Ranges from ash less than 2m in diameter to cinders 432 mm Slope of shield is gentle near top but increases on anks change due to viscosity of owing lava When magma comes out of vents or openings at top of volcano it is very hot and ows easily Magma can ow down sides of volcano and can move away from vent in a number of ways Magma can also move for many km underground thru lava tubes close to surface and insulate magma keeping it hot and uid When lava tubes cool they form natural conduits for movement of groundwater and may cause engineering probs Shield volc can also have a summit caldera a steep walled basin often 10 km or more in diameter 0 Formed from collapse of summit of volcano in which a lava lake may form and from which lava may ow during an eruption Commonly occur along linear fractures known as rift zones on ank of volc I Ex Rift eruptions at Hawaiian shield Volc Kilauea forms new land to island Composite volcanoes Composite Volcanoes beautiful cone shape ex Mt St Helens and Mt Rainier intermediate silica content abt 60 more viscous than shield Common rock type formed andesite Characterized by mixture of explosive activity and lava ows These volcanoes also called stratovolcanoes composed of alternating layers of pyroclastic deposits and lava ows Steep anks due to angle of repose or max slope angle for loose material b c of their explosive activity and relatively common occurrence composite volcanoes are responsible for most volc hazards and have caused lots of death 0 1980 Mt St Helens Volcanic Domes characterized by viscous magma w high silica content abt 70 common rock type produced by magma rhyolite mostly explosive dangerous ex Mt Lassen in ne Cali last series of eruptions 19141917 Cinder Cones relatively small volcanoes formed from tephra mostly volcanic ash and larger particles including volcanic bombs Bombs blobs of ejected lava that spin in the air and take on a rounded shape w tapered ends Cinder cones grow from accumulation of tephra near a volcanic vent Often found on anks of larger volcanoes or along normal faults and long cracks or fissures o Paricutin cinder cone Itzicuaro Valley of central Mex Feb 20 1943 0 Ground swelled sulfurous smoke and ash came from bold at night it was ejecting glowing red rock fragments high into air 0 No one killed and win a decade it became a dormant volc I Ash caused failed crops and sick livestock 84 Volcano Origins Plate tectonics in the role for making a volcano 1 Volcanism occurring at midoceanic ridges produces basaltic rock decompression melting a Low silica content in basaltic rock mixes little w other materials 2 Shield volcs are formed above hot spots located below the lithospheric plates a Hawaiian volcs located w in Pacific plates rather than near a plate boundary b Currently believed there is hot spot below Pacific where magma is generated magma moves upward thru plate and produces a volcano on the bottom of the sea may eventually become an island 3 Composite volcanoes are associated w andesitic volcanic rocks and subduction zones melting due to addition of volatiles a Most common in Pacific Rim b Andesitic rocks produced at subduction zones where rising magma mixes w both oceanic and continental crust 4 Calderaforming eruptions may be extremely explosive and violent a Associated w rhyolitic rocks produced when magma moves upward and mixes w continental crust little silica b c of high silica content of continental crust 85 Volcanic Features Craters Calderas and Vents Depressions commonly found at the top of volcanoes are craters Form by explosion or collapse of upper portion of volcanic cone and ma be at oored or funnel shaped Usually few km in diameter Calderas are gigantic often circular depressions resulting from explosive ejection of magma and subsequent collapse of the upper portion of the volcanic cone may be 20 or more km in diameter and contain volcanic vents and other volc features Volcanic vents openings through which lava and pyroclastic debris are erupted at the surface of Earth Vents may be roughly circular conduits and eruptions construct domes and cones Others may be elongated fissures or rock fractures often normal faults which produce lava ows Flood basalts results from extensive fissure eruptions that produce huge accumulations Hot Springs and Geysers Hot spring thermal spring groundwater that comes into contact w hot becomes heated and in some cases discharges at the surface Geyser rare cases the subsurface groundwater system involves circulation and heating patterns that produce periodic release of steam and hot water at the surface Caldera Eruptions Rare but violent eruptions At least 10 have occurred in past million yrs 3 in N America May explosively extrude up 1000 km of pyroclastic debris consisting mostly of ash The most recent calderaforming eruptions in N America occurred abt 600000 yrs ago at Yellowstone National Park in Wyoming and 700000 yrs ago in Long Valley Cali Main events in a calderaproducing eruption can occur quickly few days to few weeks but intermittent lesserM volcanic activity can linger for a million yrs 0 Yellowstone even has left us w hot springs and geysers and Long Valley even has left us a potential volc hazard 0 Both sites still capable of volc activity 86 Volcanic Hazards Volcanic hazards include the primary effects of volcanic activity that are direct results of the eruption and secondary effects which may be caused by primary effects Primary effects lava ows pyroclastic activity ash fall ash ows release of gases etc At planetary level large eruptions can cause global cooling of the atmosphere for a yr or so Lava Flows Lava ows when magma reaches the surface and over ows the crater or a volcanic vent along the anks of the volcano 3 major groups of lava use rock names basaltic most abundant andesitic rhyolitic Lava ow can be viscous and slow or uid and fast Basaltic approx 50 silica range of velocities 0 Lower viscosity and higher eruptive temps faster 0 Called pahoehoe lavas smooth ropy surface texture when hardened o Cooler more viscous basaltic lava has blocky texture and is slower called aa Kilauea Hawaii Methods to Control Lava Flows Can use methods like hydraulic billing and wall construction Mixed success Can t be expected to modify large ows and effectiveness w small ows reqr s further evaluation Hydraulic chilling cooling the ow w water sometimes successful 0 Icelandic island of Heimaey Mt Helgafell o 3 favorable conditions 1 ows were slow moving so more time 2 transport by sea and local roads allowed for transport of pipes pumps and heavy equipment 3 water was readily available 0 watering near ow helped to slow it and stop it too in some places Pyroclastic activity explosive volcanism in which tephra is physically blown from a volcanic vent into the atmosphere tremendous quantity of rock fragments natural glass fragments and gas is blown high into the air by explosions from the volcano several hazards o vegetation crops trees etc destroyed contamination of water structural damage to buildings health hazards like irritation of respiratory system and eyes engines of jetliners may ame out as melted silicarich ash forms a thin coating of volcanic glass in engines explosions of gas and ash from side of a volc that destroy part of the mtn lots of speed destructive avalanches of very hot pyroclastic materials ash rock volcanic glass fragments and gas that are blown out of a vent and move very rapidly down sides of the volcano can be hot as 800 degrees Celsius and move as fast as 200 km per hr down sides of a volcano very dangerous 1902 W Indian Island of Martinique Mt Peleetown of St Pierre Poisonous Gases Lots of gases like water vapor C02 C0 802 and H28 are emitted Water and C02 make up more than 90 of all emitted gases Lake Nyos Cameroon Africa C02 released and suffocated lots of people C02 started to accumulate there again there is an alarm system installed in lake if levels become high Sulfur dioxide can produce acid rain downwind of an eruption can be absorbed by volc ash and fall onto land Volcs can also produce a type of smog known as vog volc material and fog emissions of sulfur dioxide and other gases interact w oxygen and moisture in atmosphere to produce vog conditions mixed w acid rain hazardous to ppl and other living things Debris Flows and Mud ows Most serious secondary effects of volcanic activity are and known collectively by Indonesian name Produced when large vol of loose volc ash and other ejecta becomes saturated w water and becomes unstable allowing it to suddenly move downslode Debris ows Even small eruptions of hot volc material may quickly melt large vols of snow and ice produce oods that may erode the slope of volc and incorporate sediment such as volc ash and other material forming debris ows Debris ows Fast moving mixtures of sediment including blocks of rock and water w general consistency of wet concrete Can travel many km down valleys from anks of volc where they were produced O O O O Mud ows Mud ow composed mostly of volcanic ash is often called a lahar Hundreds of thousands of ppl now live on the area covered by these old ows and there is no guarantee that similar ows will not occur again US Geological Survey has developed an automated solarpowered lahardetection system for several volcs in US that sense ground vibrations from a moving lahar can warn that a ow is moving down valley Largest active landslides on Earth located in Hawaii presently these landslides are moving slowly but they could become giant and fast avalanches Canary Islands located in Atlantic Ocean off w coast of Africa 87 Two Case Histories Mt Pinatubo June 15 16 1991 Second largest volc eruption of 20th century on Mt Pinatubo on Luzon Island in Philippines Combined effects of ash fall debris ows mud ows and a typhoon resulted in deaths of abt 300 ppl also collapse of buildings as heavy wet volcanic ash accumulated on roopfs Cloud of ash 400 km wide to elevations of 34 km remained in atmosphere for more than a year ash particles and sulfur dioxide scattered incoming sunlight and slightly cooled the global climate during the year following the eruptions Evacuation of 250000 ppl from villages and a us military base win radius of 30 km from summit saved thousands of lives Mt St Helens May 18 1980 erupted in sw corner of Wahington Awoke after 120 yrs of dormancy w seismic activity and small explosions as groundwater came into contact w hot rock May 1 prominent bulge on n ank of mtn could be clearly observed M 51 quake on may 18 quake was registered on the volcano triggered a large landslide debris avalanche that involved the entire bulge area Lateral blast directly from the area that the bulge had occupied moved at speeds up to 1000 km per hr Vertical cloud had risen quickly to an altitude of approx 19 km eruption of vertical column continued for more than 9 hrs and large amts of volc ash fell on a wide area of Wadhington Several mud ows mixture of water volc ash rock and organic debris such as logs minutes after start of eruption 54 ppl killed and more than 100 homes destroyed by ooding after eruption extensive programs was established to monitor volc activity 11 smaller eruption in the following 3 yrs where lava was extruded near top of dome and slowly owed toward base 88 Forecasting Volcanic Activity forecast of volcanic eruption is a probabilistic statement concerning the time place and character of an eruption before it occurs analogous to forecasting weather unikely to forecast accurately in near future but lots of info being gathered about phenomena before eruptions Most forecasts req r experience w actual eruptions before mechanism is understood Seismic Activity Quakes often provide the earliest warning of an impending volc eruption St Helens quakes started midmarch before eruption in May Hawaii quakes used to monitor the movement of magma as it approaches the surface 1991 Mt Pinatubo quakes increased in number and M before the catastrophic eruption Geophysicists have proposed a generalized model for seismic activity that may help in predicting eruptions In reawakening volcano magma must fracture and break previously solidified igneous rock above magma chamber in order to work its way to surface several weeks after increasing pressure creates numerous fractures in plugged volc conduit above the chamber Thermal Magnetic and Hydrologic Monitoring Monitoring of volcs is based on fact that before an eruption a large volume of magma moves up into some sort of holding reservoir beneath the volcano Hot material changes local local magnetic and geochemical conditons as surrounding rocks heat the rise in temperature of surficial rock may be detected by remote sensing or infrared aerial photography When older volcanic rocks are heated by new magma magnetic properties originally imprinted when the rocks cooled and crystallized may change Topographic Monitoring Monitoring topographic changes and seismic behavior of volcanoes has been useful in forecasting some volcanic eruptions Hawaiian vocs esp Kilauea have supplied most of the data Bc of characteristic swelling and quake activity before eruptions scientists expect Hawaiian vocs to continued to be more predictable than others Monitoring Volcanic Gas Emissions Primary objective of monitoring volc gas emissions is to recognized changes in the chemical composition of the gases Changes in both gas composition rel amts of gases such as steam C02 and 02 and emission rates are thought to be correlated w changes in subsurface volcanic processes these factors may indicate movement of magma toward the surface Geologic History Geologic history of volc or volc system is useful in predicting types of eruptions likely to occur in future Geologic mapping of volc rocks and deposits lava ows volcanic mud ow deposits pyroclastc deposits and ash deposits are dated to determine when eruptions occurred in the past Volcanic Alert or Warning Important question at what pt should public be alerted or warned that a volcanic eruption may occur Evacuation is definitely necessary 89 Adjustment to Perception of the Volcanic Hazard A person s age and length of residence near a volcanic hazard are significant factors in a person s knowledge of volcanic activity and possible adustments Science of volcanoes is becoming well known a situation in which the science suggests that a volcanic eruption is likely in the near future CH 9 Geology Rivers and Flooding Note bc 2 because abt 2 about yrs 2 years Mm magnitude ppl 2 people E 2 energy btwn 2 between w with wo 2 without Mtn mountain quakes earthquakes amt 2 amount req rreq rs requirerequires SoCaI 2 Southern California directions 5 2 south n 2 north w 2 west e 2 east sw southwest nw northwest etc num number sig 2 significant econ 2 economic pop 2 population vol 2 volume nat natural fed 2 federal gov t 2 government combo combination avg average Case History Pakistan Floods of 2010 Water covers 70 of Earth s surface and critical to support life Can also cause sig hazard Asia greatest num of ppl killedaffected along w greatest econ loss due to ooding High pop along rivers land use changes climate and intense precipitation Monsoon seasonal shift in wind pattern and precipitation patterns ie dry winter to wet summer Asian monsoon several months of summer rain to India Pakistan and China July Aug 2010 greatest monsoon rains for decades catastrophic ooding lots of ppl killed entire villages washed away thousands of homes ooded Pop of Pakistan has grown greatly lots of ppl put at risk As pop grows we need to reduce ood hazard in ways that don t req r massive response but are proactive future ood hazard reduction 91 Rivers Historical Use For more than 200 yrs Americans have lived and worked on oodplains enticed to do so by rich alluvial ie steam deposited soil abundant water supply ease of waste disposal and proximity to the commerce that has been developed along the rivers the at surface adjacent to the river channel that is periodically inundated by oodwater is in fact produced by the process of ooding Of oodplainriver relationship are not recognized ood control and drainage of wetlands including oodplains become prime concerns Pioneers moving west caused damage 0 Clear land by cutting and burning trees and then modify natural drainage 92 Streams and Rivers Streams and rivers are part of water hydrologic cycle and hydrology is study of this cycle Transport of water by evaporation from Earth s surface mostly from oceans to atmosphere and Via surface and subsurface runoff from land back again to the oceans water that falls on land as rain or snow that doesn t infiltrate soilds and rocks or evaporate and drains runs off Finds its ways to streams and may merge to form a larger stream or a Streams small rivers in geology it s any body of water that ows in a channel 2 basic types of rivers Common alluvial rivers where bed and banks of river are sediment such as gravel and sand Less common bedrock rivers where bedrock is commonly exposed in the bed and banks 0 Steep mtn areas watershed the region drained by a single river or river system River s slope or gradient is the vertical drop of the channel over some horizontal distant Steepest at higher elevations in the drainage basin and levels off as the stream approaches its base level theoretical lowest level to which a river may erode usually at sea level River usually has a steepersided and deeper valley at high elevations near its headwaters than closer to its base level where a wide oodplain may be present Higher elevations steeper slope deeper erosion of valley 93 Sediment in Rivers The total quantity of sediment carried in a river total load includes the bed load the suspended load and dissolved load Bed load moves by the bouncing rolling or skipping or particles along the bottom of the channel usually composed of sand and gravel relatively small component of total load Suspended load mainly silt and clay carried above the streambed by the owing water 90 of total load and makes rivers look muddy Dissolved load carried in chemical solution and is derived from chemical weathering of minerals in rock sediment and soil in the drainage basin may make water taste salty and make stream water hard if it contains high concentrations of Mg and Ca 0 Most common constituent ions bicarbonate ions sulfate Ca Na and Mg ions 94 River Velocity Discharge Erosion and Sediment Deposition Rivers are basic transportation system of the part of the rock cycle that involves erosion and deposition of sediments primary erosion agent Velocity affects erosion vol of water moving by a particular location in a river per unit time Cubic meters per second Q Wwidth of ow x D depth of ow x V velocity of ows This equation known as and is one of the most important relationships in understand the ow of waters in rivers If crosssectional area of ow decreases velocity increases narrow rivers have higher velocity Explains why rapids are common in narrow canyons Faster rivers has ability to erode its banks more than a slowermoving one Streams that ow from mountains onto plains may form fanshaped deposits known as alluvial fans Rivers owing into ocean or some other body of still water may deposit sediments that form a delta a triangular or irregularshaped land mass extending into the sea or a lake Alluvial fans ood hazard diff from river valley and oodplain environment Alluvial fan or delta environments often split into a system of tributary channels No more one channel but has several channels that carry oodwaters to different parts of the fan or delta Reasons erosion or deposition occurs in a specific area or fiver channel or on alluvial fans or deltas are complex but they can be correlated to the physical properties of the river 0 Change in channel width depth or slope 0 Composition of channel bed and banks rock gravel sand silt or clay 0 Type and amt of vegetation 0 Land use like deforestation Deposition on alluvial fans occurs in part b c of changes in shape and slope of distributary channels 0 Wider and shallower w decreasing slope decreases velocity and favors deposition 0 Vice versa 0 Largest particle a river may transport is called its competency 0 Total load by mass or weight of sediment that a river carries in a given period of time is called its capacity 95 Effects of Land Use Changg Streams and rivers are open systems that generally maintain a rough dynamic equilibrium or steady state between the work ie the sediment transported by the stream done and the load imposed ie the sediment delivered to the stream from tributaries and hill slopes Stream tends to have a slope and crosssectional shape that provides the velocity of ow necessary to do the work of moving the sediment load Increase decrease of amt of water or sediment receive by stream changes channel s slope or crosssectional slope and changes velocity Change in velocity may increase decrease amt of sediment carried in the system Land use changes that affect stream s vol of sediment or water vol may set into motion a series of event that results in a new dynamic equilibrium 0 Land use change from forest to agri row crops increased soil erosion and increase in sediment load supplied to stream slope will increase b c of increased deposition increase in velocity 0 Vice versa where agri crop chages to forest sediment load to stream will decrease erosion of channel will lower slope lowers velocity of water I Predominance of erosion over deposition will continue until equilibrium btwn the total load imposed and work done is achieved again Appalachian Mountains and the coastal plain 1800s forestland cleared for farming o Accelerated soil erosion and deposition Dam on stream 0 Upstream at head of reservoir water in stream will slow down deposition 0 Downstream water coming down little sediment since most of it s trapped in reservoir 0 Channel erosion will predominate over deposition downstream of the dam slope will decrease until equilibrium 96 Channel Patterns and Floodplain Formation the configuration of the channel as seen in an aerial view Can be braided characterized by numerous gravel bars and islands that divide and reunite the channel 0 Steep slope and coarse sediment favor transport of bed load material important in development of gravel bars that form the quotislandsquot 0 Braided channels wide and shallow compared w meandering rivers steep rivers owing through areas that are being rapidly uplifted by tectonic processes Meanders bend and migrate back and forth across oodplain 0 Outside of a bend cut bank water moves faster during high ow events causing more bank erosion 0 Inside of curve more slower and sediment deposited forming point bars 0 Meanders migrate laterally by erosion on the cut banks and by deposition on point bars a process that is prominent in constructing and maintaining some oodplains 0 Much of the sediment transported in rivers is periodically stored by deposition in the channel and on adjacent oodplain I These areas collectively called the riverine environment and are the natural domain of the river 0 Meandering channels often contain a series of regularly spaced pools deep areas produced by scour or erosion at high ow and characterized at alow ow by relatively deep slow movement of water and rif es shallow areas produced by depositional processes at high ow and shallows fast moving water at low ow 97 River Flooding natural process of overbank ow Most river ooding is function of total amt and distribution of precipitation in drainage basin rate at which precipitation infiltrates rock or soil and topography Some oods rapid melting of ice snow in spring or failure of dam Land use can affect ooding a lot Channel discharge at the pt where water over ows the channel is called the ood discharge indication of M of ood Stage height of water in river at any given time Flood stage water has reach a highwater condition can cause damage Flash Floods and Downstream Floods occur in the upper parts of drainage and are generally produced by intense rainfall or short duration over a relatively small area Can be severe locally 1976 Front Range of Colorado highM ash ood rapid rise of oodwaters downstream oods cover a wide area and are usually produced by storms of long duration that saturate the soil and produce increased runoff ooding on small tributary basins is limited but the contribution of increased runoff from thousands of tributary basins may cause a large ood downstream This type of ood characterized by downstream movement of the oodwaters w a large rise and fall of discharge at a particular location 98 Urbanization and Flooding Human use of land in urban environments has increased both the magnitude and frequency of oods in small drainage basins of a few square km Rate of increase is a function of the percentage of the land that is covered w roofs pavement and cement referred to as impervious cover and the percentage of area served by storm sewers Storm sewers important let runoff reach stream channels more quickly than naturally As size of drainage basin creases high M oods w frequency of approx 50 yrs are not significantly affected by urbanization One study urban runoff from large storms is nearly 5 times that of preurban runoff More runoff b c of more storm sewer coverage and impervious areas Urbanization increased runoff b c less water infiltrates the ground 99 The Nature and Extent of Flood Hazards Flooding is one of the most frequently experienced natural hazards Num 1 nat type disaster in US durng 20th century Factors That Cause Flood Damage Land use on the oodplain M or depth and velocity of the water and frequency of ooding Rate of rise and duration of ooding Season Sediment load deposited Effectiveness of forecasting warning and emergency systems Effects of Flooding Primary directly by ood or secondary disruption and malfunction of services and systems due to the ood Primary death injury damage by swift currents debris and sediment to farms homes buildings railroads bridges roads and communication systems Secondary shortterm pollution of rivers hunger and disease displacement of persons who ve lost homes fires in electrical circuits or gas mains 910 Adjustments to Flood Hazards 19th cent humans responded to ooding by attempting to prevent the problem Physical barriers dams levees etc Every new ood control project lures more ppl b c false hope Yet to build a dam or channel capable of controlling the heaviest runoff There are advantages to alternative adjustments Controlling land use and ood insuance The Structural Approach Physical Barriers Levees oodwalls reservoirs to store water for later release at safe rates onsite stormwater retention basins but potential benefits of barriers are lost b c increased development on oodplains 1986 Yuba river Cali levee broke b c of big storm some engineering structures designed to prevent ooding have actually increased the ood hazard in the long term Structural controls must go hand in hand w oodplain regulations if the hazard is to be minimized Channelization methods of this include straightening deepening widening clearing or lining existing stream channels 0 Used to control oods drain wetlands control erosion and improve navigation 0 Opponents of channelization say the practice is antiethical to production of fish and wetland wildlife and causes stream to suffer 0 Not all channelization cause serious environmental degradation some drainage projects are beneficial Channel Restoration Alternative to Channelization Constructing roads utilities and buildings w associated sediment production is sufficient to disrupt small streams uses various techniques cleaning urban waste from channel allowing stream to ow freely protecting the existing channel banks by not removing existing trees or planning additional native trees and other vegetation where necessary Objective is to create a more natural channel by allowing stream to meander and provide variable lowwater ow conditions when possible Flood Insurance 1968 private companies became reluctant to continue to offer ood insurance the fed gov t then took over Insurance program intended to provide shortterm financial aid to victims of oods as well as to establish longterm landuse regulations for the nation s oodplains Nearly all communities w a ood risk in the US have basic ood hazard maps and have initiated some form of oodplain regulations FloodProofing Several methods of oodproofing Raising the foundation of building above the ood hazard Constructing oodwalls or earth berms around buildings to seal them from oodwaters Waterproofing construction Installing improved drains w pumps to keep ood waters out Floodplain Regulation Floodplain as a landform produced by a river We often determine that discharge by analyzing past ow records the best approach to minimizing ood damage in urban areas Purpose is to obtain the most beneficial use of oodplains while minimizing ood damage and the cost of ood protection Floodplain regulation is a compromise btwn indiscriminate use of oodplains resulting in loss of life and tremendous property damage and complete abandonment of oodplains FloodHazard Mapping Preliminary step to oodplain regulation is ood hazard mapping Direct observation and measurement of physical parameters Floodplain Zoning Floodhazard info is used to designate a oodhazard area Relocating People from Floodplains Examples of North Carolina and North Dakota September 1999 Hurricane Floyd 50cm of rain to N Carolina and ooded many areas Churchs Ferry N Dakota there has been a wet cycle since 1992 causing nearby Devils Lake to rise approx 8m Had not outlet Look at book Personal Adjustment What to Do and What nOT T0 d0 Table 911 Perception of Flooding At institutional level gov t and ood control agency level perception and understanding of ooding are adequate for planning purposes Progress at institutional level includes mapping of oodprone areas of areas w a ash ood potential downstream from dams and areas where urbanization is likely to cause problems in the near future Fed gov t has encouraged states and local communities to adopt oodplain management plans Ch 10 Slope Processes Landslides and Subsidence Note bc 2 because abt 2 about yrs 2 years Mm magnitude ppl 2 people E 2 energy btwn 2 between w with wo 2 without Mtn mountain quakes earthquakes amt 2 amount req rreq rs requirerequires SoCaI 2 Southern California directions s 2 south n 2 north w 2 west e 2 east sw southwest nw northwest etc num number sig 2 significant econ 2 economic pop 2 population vol 2 volume nat natural fed 2 federal gov t 2 government combo combination avg average Case History La Conchita Landslide of 2005 A small beach side community called La Conchita 80 km nw of LA Cali Fast moving landslide damaged or destroyed 36 homes and killed 10 ppl Reactivation of a 1995 landslide that destroyed several homes but caused no deaths Both 1995 and 2005 events were part of an older prehistoric landslide less than 6000 yrs old on the steep slope directly above La Conchita Winter of 20042005 wet and lots of intense rain Lots of ppl trapped In homes Landslides in that area have been occurring for thousands of years 101 Introduction to Landslides Landslides and related phenomena cause lots of damage and loss of life Natural phenomena that would occur w or wo human activity a comprehensive term for any type of downslope movement of Earth materials Refers to a downslope movement of rock or soil as a more or less coherent mass consists of earth ows mud ows rock falls and snow or debris avalanches Subsidence a type of ground failure characterized by nearly vertical deformation 102 Slope Processes and Types of Landslides Slope Processes Slopes are the most common landforms on Earth and they are dynamic evolving systems 2 different types of slopes First has a high cliff or freeface a straight nearly vertical slope segment Rock fragments that fall from the freeface may accumulate at base of slope to form a talus slope Free face and talus are segments of the slope Hill slopes formed from rocks not as hard as granite gentler 5 slope segment types Free face more common on strong hard rocks in arid environment w little vegetation Convex concave common on softer rocks or w humid wet climate where thick soil and vegetation are present Material on most slopes is constantly moving down slope at rates that vary from and imperceptible creep of soil and rock to thundering avalanches and rockfalls that move at big velocities Types of Landslides Rotational slumps involve sliding along a curved slip plain producing slump blocks Translational sliding is downslope movement of earth materials along a planar slip plane Rock fall free fall of Earth materials from a freeface of a cliff Creep very slow owage of rock or soil Rapid owage may be an earth ow mud ow or debris ow Debris or mud ow is a mix of rock soil organic matter that mixes w air and water to ow rapidly down a slope Debris ow less than 50 fines sand silt and clay Mud ow more than 50 fines by vol Lateral spreads type of landslide that often occurs on nearly at slopes or very gently slope Starts w liequefaction of silts clays or fine sands during earthquake or another disturbance Often start suddenly than become larger in a slow progressive manner Subsidence may occur on slopes or on at ground and involves sinking of a mass of Earth material below the level of surrounding surface Landslides commonly complex combos of sliding and owage Failures such as Upper slump that is transformed to a ow in the lower part of the slide such complex landslides May form when watersaturated Earth materials ow from the lower part of the slope undermining the upper part and causing slumping of blocks of Earth materials important variables in classifying downslope movement are 0 type of movement slide fall ow slump complex movement 0 slope material type amt of water present and rate of movement 0 generally considered rapid if movement can be seen w naked eye 103 Slope Stability Forces on Slopes Slope stability must be examined to determine causes of landslides stability of slope relationship btwn driving forces move materials down the slope and resisting forces oppose the movement most common resisting force is the strength or resistance to failure by sliding or owing of the slope material acting along potential slip planes geologic surfaces exhibiting weakness in the slope material Slope stability is evaluated by computing a ratio of resisting forces to driving forces reisisting force exceeds driving force factor of safety is greater than 1 considered stable FS less than 1 driving force more than resisting considered unstable Driving and resisting forces determined by 0 Type of earth materials slope angle and topography climate vegetation water and time The Role of Earth Material Type Material composing a slope affects both type and frequency of downslope movement or slumps sliding occurs along a curved slip surface tend to produce topographic benches that can be rotated and tilted in the upslope direction slumps most common on soil slopes but some also occur on rock slopes weak rock like shale planar occur along inclined slip planes Common translation slip planes in rock slopes include fractures in all rock types bedding planes weak clay layers and foliation planes in metamorphic rock Soil slip translational slide very shallow slides in soil over rock that occur parallel to slope Material type facto in falls as well as slides Resistant rocks form steep slopes weathering or erosion along fractures may cause a rockfall Weak volcanic pyroclastic materials or shale slopes slow downslope movement of soil and rock creep occurs also earth ows mud ows downslope ow of saturated Earth materials slumps or soil slips 0 These do not happen on resistant rock The Role of Slope and Topography Slope hill slope angle measure of how steep a hill slope is Greatly affects the relative M of driving forces on slopes As slope increases driving force also increases making landslides more frequent on steeper slopes 0 US rocky mtns and Appalachian Mtns have greatest frequency of landslides o Steep slopes are often associated w rock falls and debris avalanches very rapid downslope movement of soil rock and organic debris I Cali shallow soil slips common on steep saturated slopes more downslope they transform to earth debris ows I Earth ows moderate slopes I Creep gentle slopes o Debris ows downslope ow of relatively course materials more than 50 of particles are coarser than sand I Debris ows debris avalanches and mud ows vary in size I Small to moderate sized in valleys but larger on mtns The Role of Climate Climate can be defined as characteristic weather at a particular place or region over seasons years or decades Avg air temp and amt of precipitation Also seasonal patterns of weather Climate in uences the amount and timing of water in form of rain and snow that may infiltrate erode a hill slope and affect the vegetation abundance that grows on a hill slope Arid semiarid climates sparse vegetation and more soilrock exposed o Rockfall debris ow and shallow soil slips more common The Role of Vegetation Vegetation more abundant in subhumidhumid climates and slopes have many more convex and concave slope segments Also has more thick soil cover Vegetation is a significant factor for slope stability b c Vegetation provides a cover that cushions the impact of rain falling on slopes filtration of water into slope Vegetation has root systems that tend to provide an apparent cohesion to slope materials increases resistance Adds weight to slope In some cases vegetation increases probability of a landslide especially for shallow soil slips on steep slopes Southern Cali ice plants take up water and add weight to slopes and increases driving force Plant also causes increases in infiltration of water into slope which decreases the resisting forces The Role of Water Water is almost always directly indirectly involved w landslides so it s important 3 ways water can affect stability 1 landslides such as shallow soil slips can develop during rainstorms when slopes become saturated 2 Landslides like slumps and translational slides can develop months or even yrs after infiltration of water deep into slope 3 Water can erode the base or toe of a slope decreases slope stability Water s ability to erode affects stability of slopes This problem is critical especially if it s an old inactive landslide that is likely to move again if stability is reduced Water can also cause liquefaction of clayrich sediment or quick clay When disturbed some clays temporarily lose their shear strength behave as a liquid and ow The Role of Time Forces on slopes often change w time Ex Driving and resisting forces may change seasonally as the moisture content or water table position alters Water is often acidic b c it reacts w carbon dioxide in atmosphere and soil to produce weak carbonic acid chemical weathering 0 Increased frequency of landslides during or after wet years Other slopes continuous reduction in resisting forces overtime due either to weathering reduces cohesion to slope materials or a regular increase in pore water pressure in slope from natural or artificial conditions 104 Human Use and Landslides effect of human use on Magnitude and frequency of landslides varies from nearly insignificant to very significant We need to learn how to prevent landslides and how to control recognize and control them Many landslides caused by interactions of adverse geologic conditions excess moisture and artificial changes in landscape and slope material Timber Harvesting Timber harvesting activities such as clear cutting and road building over approx a 20 yr observation period on geologically stable land did not greatly increase landsliderelated erosion But logging on weak unstable slopes did increase landslide erosion by several times compared w landslide erosion on forested land Construction of roads on areas to be logged interrupts surface drainage and surface movement of water and change distribution of mass on a slope Urbanization Human activities most likely to cause landslides in urban areas where there are high densities of people and supporting structures roads homes and industries Rio de Ianeiro big pop lots of slopestability problems steep slopes and fractured rocks w thin soil Urbanization vegetation cover usually removed lack of room on at ground so more structures built on slopes Feb 1988 intense rainstorm in Rio de Ianeiro caused mudslides and ooding many of the landslides were initiated on steep slopes where housing was precarious and control of storm water runoff was nonexistent Also Jan 2011 ash ooding debris ows debris avalanches and shallow landslides LA and southern Cali landslides associated w hillside development 0 Landslides in s Cali result from complex physicsal conditions in part b c of great local variation in topography rock and soil types climate and vegetation 0 LA leads the nation in developing building codes concerning grading for development 0 S Cali grading process in which benches are cut into slopes is responsible for many landslides o Machines can carve the landscape at a much faster pace than glaciers rivers leads to numerous artificially induced landslides 0 Any project that steepens or saturates a slope increases its height or places extra load on it may cause a landslide o More ex In book pg 346 105 Minimizing the Landslide Hazard Need to identify areas where landslides are likely to occur designing slopes or engineering structures to prevent landslides warn ppl in danger areas and control slides after they ve started moving Identifying Potential Landslides Identify areas w most potential for landslides Once a landslide hazard is identified it must be evaluated Landslide inventory shows areas that have experienced slope failure or at a more detailed level it may be a map that shows definite landslide deposits in terms of their relative activity 0 Only show general guideline for land use planning 0 Determining landslide risk and making landslide risk maps is more complicated b c it involves probability of occurrence and assessment of potential losses 0 Grading codes to minimize landslide hazard have been in effect in LA area since 1963 o The application of geologic and engineering info before hillside development can help minimize the hazard Preventing Landslides Preventing large landslides is difficult We should avoid loading tops of slopes cutting into sensitive slopes placing fills on slopes or changing water conditions on slopes Common engineering techniques for landslide prevention include provisions for surface and subsurface drainage removal of unstable slope materials construction of retaining walls or other supporting structures or some combo of these Drainage control Surface and subsurface drainage control are usually effective in stabilizing a slope 0 Objective divert water to keep it from running across or infiltrating into slope 0 Do this by covering slope w impermeable layer soilcement asphalt or even plastic constructing subsurface drains Grading Grading has increased landslide hazard in many areas but if carefully planned it can increase slope stability 0 Material from upper part of slope is removed and it contributes to driving forces and is placed at toe of slope I Not practical on steep high slopes 0 Alternative steep slope is cut into benches or steps reduces slope of land and is a good collecting site for falling rocks and small slides Slope Supports Retaining walls concrete stonefilled wire baskets piles designed to provide support at base of slope 0 Should be anchored well below base of slope backfilled w permeable gravel or crushed rock Preventing landslides is expensive But it s still more beneficial to prevent landslides than to pay for it afterwards Warning of Impending Landslides Landslide warning systems provide time to evacuate people and their possessions and to stop trains or reroute traffic Surveillance areas can be visually inspected for apparent change Electrical systems tilt meters geophones Shallow wells can be used to monitor when slopes contain a dangerous amt of water Correcting Landslides Best way to stop a slide once it has begun is to attack the process that started the slide Most cases cause of slide is an increase in water pressure Solution is to add an effective drainage program Draining tends to increase resisting force of slope material more stable 106 Snow Avalanches rapid downslope movement of snow and ice sometimes w addition of rock soil and trees Most damaging avalanches occur when a large slab of snow and ice weighing millions of tons fails due to overloading of a slope w fresh snow or to development of zones of weakness w in the snowpack These slabs move rapidly downslope Avoiding hazardous areas the preferred and least expensive adjustment to avalanches 0 Can also clear excess snow w carefully placed explosives construct buildings and structures to divert or retard avalanches or planting trees in avalanche prone areas 107 Subsidence Interactions btwn geologic conditions and human activity have been factors in numerous incidents of the very slow to rapid sinking or settling of Earth materials Subsidence caused by withdrawal of uids from subsurface reservoirs or by collapse of surface and nearsurface soil and rocks over subterranean voids Withdrawal of Fluids Can cause subsidence withdrawal of stuff like oil ground water etc Fluids in Earth materials below Earth s surface have a high uid pressure that tends to support the material above Buoyancy produced by liquid tends to lift rock liquid removed than subsidence occurs Thousands of sq km of central valley of Cali have subsided as a result of overpumping groundwater in the area Also Phoenix AZ Las Vegas HoustonGalvestan and Mexico City Sinkholes Removal of subterranean earth materials by natural process also causes subsidence Voids large open spaces such as caves often form by chemical weathering win soluble rocks resulting lack of support for overlying rock may cause it to collapse circular area of subsidence caused by the collapse of a nearsurface subterranean void or room in a cavern Collapse over the void results in collapse at the surface which is reported as a sinkhole 0 Natural or artificial uctuations in the water table are probably the trigger mechanism 0 High water table conditions enlarge cavern closer to surface of Earth by dissolving material and buoyancy helps support the overburden o Lowering the table lowers buoyancy collapses May 8 1981 Winter Park Florida large sinkhole began developing had to be drained 0 They form nearly every year in central Florida June 23 1986 large subsidence pit developed at site of an unrecognized filled sinkhole in Lehigh Valley near Allentown in e Pennsylvania More examples in text on pg 354 Salt Deposits Serious subsidence events have been associated w salt mining Water injected through wells into salt deposits salt dissolves and water supersaturated w salt is pumped out Leaves a cavity in rock and weakens support for overlying rock leads to subsidence Nov 21 1980 southern Louisiana lake Peigneur 0 Oil drilling operation left a hole water entering mine left an enlarged hole large subsidence pit claimed more than 025 kmquot2 of Jefferson Island and remaining gardens were disrupted by large fractures that dropped the land down to the new edge of the lake 0 Lake began refilling water from a canal connecting it to Gulf of Mexico 0 Hole was sealed by debris in form of soil and lake sediment 0 Structural integrity of salt mines questioned Coal Mining Coal mining subsidence problems Most common where underground mining is close to surface or land or where rocks left as pillars after mining are weak or intensely fractured Overtime pillars weather weaken and collapse producing surface subsidence 108 Perception of the Landslide Hazard Landslide hazard maps will probably not prevent all ppl from moving into hazardous areas Rocky Mountains Appalachian Mountains California and other areas lots of ppl still build in areas subject to future landslides What You Can Do to Minimize Your Landslide Hazard landslides involve complex geology and geologic evaluation by a pro geologist of any property on slope is recommended Avoid homes at mouth of a canyon debris ows and mud ows hazard Consult local agencies that may know more about landslide prone areas Watch out for little landslidesquot they usually get larger w time Check for cracks when purchasing homes Look out for leaks in swimming pools or trees tilted downslope Look for linear or curved cracks while walking the property Active landslides often have hummocks or steplike ground features Fixing a potential landslide prob is often costeffective but can still be expensive don t purchase land w potential landslide hazard
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