GEO 104-001 Chapter 3 notes
GEO 104-001 Chapter 3 notes Geo 104-001
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This 8 page Class Notes was uploaded by Jennifer Gintovt on Thursday September 22, 2016. The Class Notes belongs to Geo 104-001 at University of Alabama - Tuscaloosa taught by Rona J. Donahoe in Fall 2016. Since its upload, it has received 8 views. For similar materials see Hazardous Earth in Geology at University of Alabama - Tuscaloosa.
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Date Created: 09/22/16
GEO 104 Chapter 3 Earthquake Catastrophe: • Kill a large number of people/cause extensive damage in seconds th o 850,000 killed in 16 century Chinese earthquake o more than 255,000 killed in 7.8 M 1976 Tangshan Earthquake • Contrasting examples: o 2010 Haitian 7 M § killed 316,000 o 1994 Northridge CA 6.9 § killed 60 o Most deaths are “human induced” due to poor building construction § Lack of earthquake construction codes § Disregard for existing codes due to corruption Earthquake Terms: • Focus o Point along a fault plane where rock rupture starts o May not always see surface rupture • Epicenter o Geographic location of the place directly above the earthquake focus Earthquakes: • Many earthquakes in any given day o The majority are too small to be noticed • Earthquakes are compared based on… o Magnitude § Amount of energy released; measuring the amount of shaking that occurs o Intensity § The effects on people and structures • Magnitude o Measured on a logarithmic scale o Moment Magnitude- Depends on: § Area of rupture along fault § Amount of movement along fault (slippage) § Rigidity of the rocks at the point of failure § Used by seismologists today o Richter Magnitude § Developed for Southern Cali. - Charles Richter § Not really used by seismologists anymore; mostly the media o Ground shaking increases by 10x for each magnitude unit o Energy released increases by 32x with each magnitude unit o Ground motion (seismic wave amplitude) is measured by a seismograph § Related to magnitude depth and geologic setting Earthquake Intensity: • Modified Mercalli Scale- measures earthquake intensity o Qualitative scale (1-12) based on damage to structures and people’s perceptions of shaking o Intensity depends on: § Distance from the epicenter § Depth of focus (point of rock failure) and the slip direction (how much movement occurs along the area of the fault plane) § Local geological conditions • The type of rock in the area (consolidated, unconsolidated, (sediment) fluid) o Modified Mercalli Intensity Maps show where the damage is most severe Shake Maps: • Use seismographic data and human accounts to show areas of most intense shaking Earthquake Processes: • Distributed along faults o Places where rocks are broken and displaced o All plate boundaries are faults • 2 basic faults: o Strike Slip o Dip Slip • Long-term rate of movement is the Slip Rate o Measured in mm/yr. or m/100yr. • Sudden rupture of rock produces seismic waves o Release of stored energy o Elastic rebound theory Stress/strain: • Rock deforms (shows strain) under stress • Increasing stress can cause… o Elastic deformation (reversible) o Plastic deformation (permanent) o Rupture § Occurs when rocks strength is exceeded Types of stress: • Compressional Stress o Squeezing stress o Convergent boundaries (COMpressional…CONvergent) • Tensional Stress o Pulling apart stress o Divergent boundaries • Shear Stress o Horizontal motion o Transform boundaries § EX. San Andreas Fault- Cali. Fault Types • Strike-slip (transform) o Crust moves in horizontal direction § Right lateral strike-slip § Left lateral strike-slip • Dip-slip o Vertical movement o Includes two walls defined by miners as § Footwall: where miners put their feet § Hanging wall: where they hang their lanterns o Named by which direction (up or down) the hanging-wall moves relative to the footwall • Normal: o Hanging wall moves down relative to footwall (think normal movement of gravity) o Pull apart (divergent) • Reverse fault o Hanging wall moves up relative to footwall o If angle is 45 degrees or less, it is a thrust fault • Blind faults do not extend to surface Fault Activity: • Active fault o Moved during the past 10,000 years of the Holocene Epoch • Potentially Active Fault o Moved during the Pleistocene (10,000-2 million years) but not the Holocene Epoch • Inactive faults o Faults that haven’t moved during the past 2 million years o Determined by studying the paleoseismicity (prehistoric period) of the fault Tectonic Creep/Slow Earthquakes • Tectonic Creep o Occurs when movement is so gradual that earthquakes are not felt § Can produce slow earthquakes § Also called fault creep • Ex. Hayward fault (along San Andreas fault) o Can slowly damage roads, sidewalks, and building foundations o Can last from days-months o Slow earthquakes can have magnitudes between 6-7 § Large areas of rupture, but small displacements Seismic Waves: • Body waves o Caused by release of energy from rupture of a fault o Travel through the body (interior) of the earth o P waves § Primary/compressional waves • Move fast w/ push/pull motion • Move through solid, liquid, and gas • Is possible to hear them o S waves § Secondary/shear waves • Move slower with up/down motion • Only travel through solids • Surface Waves o Move along Earth’s surface o Pass through everything o travel slower than body waves o move vertically and horizontally with rolling motion o responsible for most of the damage near the epicenter o Love Wave § Horizontal ground shaking o Rayleigh Wave § Wave-like rolling ground motion § Knock people off their feet Earthquake shaking: • Shaking experience depends on o Magnitude § Strong shaking occurs even for moderate earthquakes § Shaking increases w/ magnitude o Location in relation to epicenter, depth of focus and direction of rupture § Determined from earthquake seismogram records § Shaking usually decreases with distance from the epicenter and increasing depth of focus o Local soil and rock conditions § Unconsolidated (loose sediment) and wet materials amplify seismic waves (liquefaction) § Certain geologic structures can focus seismic waves • Ex. Graben, Syncline Local geologic conditions: • Nature of earth materials and geologic structures affect ground motion o Different materials respond differently to an earthquake • Shaking depends on their degree of consolidation o Seismic waves move faster through consolidated material (bedrock) o Move slower through unconsolidated sediment and soil o Move slowest through unconsolidated materials with high water content • Material Amplification o Energy is transferred to the vertical motion of the surface waves The Earthquake cycle: • Elastic strain drops after an earthquake + reaccumulates before the next event o Recurrence interval for a fault can sometimes be estimated from paleoseismicity studies o Elastic rebound occurs when rock strain energy is released through rupture • The earthquake cycle consists of four (five) stages o 1: period of inactivity along a segment of fault o 2: period of small earthquakes produced by accumulated elastic strain o 3: Foreshocks may occur hours to days prior to a major release of stress o 4: Mainshock o 5: Aftershock Foreshocks and Aftershocks: • small earthquakes before the major event are called foreshocks (days/months leading up to main event • adjustments after a major earthquake can generate small quakes called aftershocks Plate boundary earthquakes: • Subduction zones o Site of world’s great earthquakes (megathrust earthquakes) § Ex. 9.0 japan earthquake in 2011 § Ex. Cascade mountains (continental and oceanic plate convergence) § Ex. Aleutian Islands (convergence between two oceanic plates) • Transform Fault Boundaries o San Andreas Fault in California (Between N. American and Pacific Plates) § Ex. 1989 Loma Prieta, 1994 Northridge earthquake Intraplate Earthquakes • Earthquakes that occur within plates = rare, but can be large and extremely damaging o Lack of preparedness and earthquake building codes o Stronger basement rocks transmit seismic waves • New Madrid Seismic Zone o South of St. Louis, MO o 1811-1812 2 earthquakes with M 7.5+ felt from New Orleans to Quebec City, rand church bells in Boston • Charleston, S.C. o 1889 Earthquake was felt from Canada to Cuba (M ~7.3) and as far west as Arkansas Primary effects of earthquakes: • ground rupture o displacement along the fault causes surface cracks § horizontal displacement (especially common along a transform fault) § vertical displacement (produces fault scarp) • cliff created from vertical displacement • Shaking o causes damage to buildings, bridges, damn, tunnels, pipelines, etc. § measured as ground acceleration § buildings may be damaged due to resonance • matching of vibrational frequencies between ground and building Secondary effects of earthquakes: • liquefaction o near-surface layer of water-saturated sand chances rapidly from solid to liquid o similar to “quick sand” o can produce “sand blows” o common in ≥5.5M earthquakes in sediments <10,000 years’ old o after shaking stops, ground re-compacts and becomes solid o things that are buried in the ground can also rise to the surface • Elevation changes o Regional uplift and subsidence o Can cause substantial damage on coasts and along streams • Landslides o Earthquakes= most common trigger, especially in mountainous areas o Loss of human life can be substantial • Tsunamis o Earthquake induced tidal wave • Fires o Displacements cause power/gas lines to break+ ignite o Hard to put out- water lines are often broken • Disease o Caused by loss of sanitation/housing, contaminated water supplies, disruption of public health services, and the disturbance of natural environment § Ex. sewage contaminating drinking water supplies, sewage being discharged into a stream, trash pileup, etc. § Northridge – 1994 earthquake- caused “Valley Fever” • Traced back to the inhalation of fungal spores Natural Service Functions: • Water, oil, or nat. gas may be rerouted or trapped due to faults • New mineral resources may accumulate along or be exposed by faults • Scenic landscapes may form over long time intervals (mountains, etc.) • Large feature earthquakes may be reduced due to release of energy by frequent, small events Human interaction with earthquakes: • Reservoir-induced seismicity o Weight from water reservoirs may create new faults or lubricate old ones • Deep Well disposal o Liquid waste disposal deep in the earth can increase fluid pressure on faults • Nuclear explosions o Can cause the release of stress along existing faults § Proposed as an earthquake control method… Can earthquakes be predicted? • Parkfield, CA experiment o Moderate earthquakes have occurred at regular intervals in area since 1857 § Approx. every 22 years o 1985- predicted that next event would happen by 1993 o finally occurred on Sept. 28, 2004 Minimizing Earthquake hazard • focus of hazard minimization is on forecasting and warning • National Earthquake Hazard Reduction Program goals (USGS) o Know where the earthquake may originate (know information about faults, etc.) o determine earthquake potential o predict effects of earthquakes o apply research results with public education on hazards/planning Estimating seismic risk • hazard maps show earthquake risk • show either o locations and magnitudes of historic events o probability of a particular event o amount of shaking Short term prediction • Prediction o Gives timeframe and location of a certain magnitude event o Relies on precursors § Events/changes that occur before the earthquake mainshock o Pattern and frequency of earthquakes § Foreshocks, microearthquake swarms o Deformation of ground surface § Changes in land elevation and tilt o Seismic gaps along faults § Areas that have not experienced recent earthquakes • The fault is “locked” because no events have happened for a long time o Geophysical and geochemical changes § Changes in Earth’s magnetic field, gravity, conductivity, groundwater levels, temperature & chemistry o Animal behavior can be a precursor event § Ex. birds flying away seconds before an earthquake § Used by Chinese scientists to successfully predict 3 earthquakes in the past –not foolproof Earthquake warnings: • Plan for issuing an earthquake prediction or warning developed by USGS 30 years ago • Warning systems are technically feasible o Radio wave system (Japan) o Seismometer/transmitter system (Southern California) o Liability concerns? § What if nothing happens? § A lot of bureaucracy involved in the warning process Community preparations for earthquake hazards • Location of critical facilities o Hospitals, schools, police, fire, etc. must be located in earthquake safe zones o Need for details maps of ground response (microorganizaton) • Structural protection o Buildings, bridges, pipelines, etc. need to be designed to withstand the vibrations created by an earthquake o Retrofitting of older buildings may be necessary o Costs must me balanced with risk to human life • Education • Increased insurance and relief measures
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