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ISNS 3359 Earthquakes and Volcanoes Locating EQ’s, EQ Magnitude and Intensity Sound Waves and Seismic Waves • Seismologists record and analyze waves to determine where an earthquake occurred and how large it was • Waves are fundamental to music and seismology • Similarities: – More high frequency waves if short path is traveled • Trombone is retracted, short fault-rupture length (small earthquake) – More low frequency waves if long path is traveled • Trombone is extended, long fault-rupture length (large earthquake) Locating the Source of an Earthquake • P waves travel about 1.7 times faster than S waves • Farther from hypocenter, greater time lag of S wave behind P wave (S-P) • (S-P) time indicates how far away earthquake was from station – but in what direction? Locating the Source of an Earthquake • Need distance of earthquake from three stations to pinpoint location of earthquake: – Computer calculation – Visualize circles drawn around each station for appropriate distance from station, and intersection of circles at earthquake’s location – Method is most reliable when earthquake is near surface Fig. 4.23 Magnitude of Earthquakes •• Richter scale – Devised in 1935 to describe magnitude of shallow, moderately-sized earthquakes located near Caltech seismometers in southern California – Bigger earthquake greater shaking greater amplitude of lines on seismogram – Deﬁned magnitude as ‘logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake’, with corrections made for distance – For every 10 fold increase in recorded amplitude, Richter magnitude increases one number Magnitude of Earthquakes • Richter scale – With every one increase in Richter magnitude, the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time – Bigger earthquake means more people will experience shaking and for longer time (increases damage to buildings) – Many more small earthquakes each year than large ones, but more than 90% of energy release is from few large earthquakes – Richter scale magnitude is easy and quick to calculate, so popular with media • Magnitude of Earthquakes Magnitude of Earthquakes Magnitude of Earthquakes Total Since 2000 Is there a trend of more earthquakes each year? Trend Increasing? • Earthquakes of magnitude 7.0 or greater have remained fairly constant • A partial explanation may lie in the fact that in the last twenty years, we have deﬁnitely had an increase in the number of earthquakes we have been able to locate each year – Increase in the number of seismograph stations in the world and improvements in global communications – In 1931, there were about 350 stations operating in the world – Today, there are more than 8,000 stations and the data now comes in rapidly from these stations by electronic mail, internet and satellite – This increase in the number of stations and the more timely receipt of data has allowed us and other seismological centers to locate earthquakes more rapidly and to locate many small earthquakes which were undetected in earlier years – The NEIC now locates about 20,000 earthquakes each year or approximately 50 per day • According to long-term records (since about 1900), we expect about 17 major earthquakes (7.0 - 7.9) and one great earthquake (8.0 or above) in any given year. Other Measures of Earthquake Size • Richter scale is useful for magnitude of shallow, small-moderate nearby earthquakes • Does not work well for distant or large earthquakes – Short-period waves do not increase amplitude for bigger earthquakes – Richter scale: • 1906 San Francisco earthquake was magnitude 8.3 • 1964 Alaska earthquake was magnitude 8.3 – Other magnitude scales: • 1906 San Francisco earthquake was magnitude 7.8 • 1964 Alaska earthquake was magnitude 9.2 (100 times more energy) Other Measures of Earthquake Size Two other magnitude scales: • Body wave scale (m ): b – Uses amplitudes of P waves with 1 to 10-second periods • Surface wave scale (m ): s – Uses Rayleigh waves with 18 to 22-second periods • All magnitude scales are not equivalent – Larger earthquakes radiate more energy at longer periods not measured by Richter scale or body wave scale – Richter scale and body wave scale signiﬁcantly underestimate magnitudes of earthquakes far away or large Moment Magnitude Scale • Seismic moment (M ) o – Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes – Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault • Moment magnitude scale uses seismic moment: – M w 2/3 log 10 )o– 6 – Scale developed by Hiroo Kanamori Energy Comparison Erg = unit of energy and mechanical work in the centimeter-gram-second system of units Foreshocks, Main Shock and Aftershocks • Large earthquakes are not just single events but part of series of earthquakes over years – Largest event in serimainshocknshock – Smaller events preceding mainshock foreshocksocks – Smaller events following mainshock aftershocksocks • Large event may be considered mainshock, then followed by even larger earthquake, so then re- classiﬁed as foreshock Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies • Fault-rupture length greatly inﬂuences magnitude: – 100 m long fault rupture magnitude 4 earthquake – 1 km long fault rupture magnitude 5 earthquake – 10 km long fault rupture magnitude 6 earthquake – 100 km long fault rupture magnitude 7 earthquake Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies • Fault-rupture length and duration inﬂuence seismic wave frequency: – Short rupture, duration high frequency seismic waves – Long rupture, duration low frequency seismic waves • Seismic wave frequency inﬂuences damage: – but die out quickly with distance from epicenterter – Low frequency waves travel great distance from epicenter so do most damage farther away Ground Motion During Earthquakes • Buildings are designed to handle vertical forces (weight of building and contents) so that vertical shaking in earthquakes is typically safe • Horizontal shaking during earthquakes can do massive damage to buildings • Acceleration – Measure in terms of acceleration due to gravity (g = 9.8 m/s ) – Weak buildings suffer damage from horizontal accelerations of more than 0.1 g – In some locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake) Shake Northridge, CA 1994 Loma Prieta, CA 1989 Kobe, Japan 1995 Kobe, Japan 1995 Landers, CA 1992 Periods of Buildings and Responses of Foundations Just as waves have natural frequencies and periods, so do buildings • Periods of swaying are about 0.1 second per story – 1-story house shakes at about 0.1 second per cycle – 30-story building sways at about 3 seconds per cycle • Building materials affect building periods – Flexible materials (wood, steel) longer period of shaking – Stiff materials (brick, concrete) shorter period of shaking Periods of Buildings and Responses of Foundations Velocity of seismic wave depends on material it is moving through • Faster through hard rocks • Slower through soft rocks • When waves pass from harder to softer rocks, they slow down • Must therefore increase their amplitude in order to carry same amount of energy greater shaking • Shaking tends to be stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.) Periods of Buildings and Responses of Foundations • If the period of the wave matches the period of the building, shaking is ampliﬁed and resonance results – Common cause of catastrophic failure of buildings Earthquake Intensity – What We Feel During an Earthquake • Mercalli intensity scale was developed to quantify what people feel during an earthquake • Used for earthquakes before instrumentation or current earthquakes in areas without instrumentation • Assesses effects on people and buildings • Maps of Mercalli intensities can be generated quickly after an earthquake using people’s input to the webpage http://pasadena.wr.usgs.gov/shake Earthquake Intensity – What We Feel During an Earthquake Mercalli Scale Variables Mercalli intensity depends on: • Earthquake magnitude – Bigger earthquake, more likely death and damage • Distance from hypocenter – Usually (but not always), closer earthquake more damage • Type of rock or sediment making up ground surface – Hard rock foundations vibrate from nearby earthquake body waves – Soft sediments ampliﬁed by distant earthquake surface waves – Steep slopes can generate landslides when shaken Mercalli Scale Comparison Mercalli Scale Variables Mercalli intensity depends on: • Building style – Body waves near the epicenter will be ampliﬁed by rigid or short buildings – Low-frequency surface waves are ampliﬁed by tall buildings, especially if on soft foundations • Duration of shaking – Longer shaking lasts, more buildings can be damaged What To Do Before and During an Earthquake • Before an earthquake: – Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc. – Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc. • During an earthquake: – Duck, cover and hold – Don’t Panic – If inside, stay inside – If outside, stay outside Design of Buildings in Earthquake-Prone Areas • Eliminate resonance: – Change height of building – Move weight to lower ﬂoors – Change shape of building – Change building materials – Change attachment of building to foundation – Hard foundation (high-frequency vibrations) build tall, ﬂexible building – Soft foundation (low-frequency vibrations) build short, stiff building Design of Buildings in Earthquake-Prone Areas • Floors, Roofs and Trusses – Give horizontal resistance by transferring force to vertical resistance elements • Shear Walls – Designed to receive horizontal forces from ﬂoors, roofs and trusses and transmit to ground – Lack of shear walls typically cause structures like parking garages to fail in earthquakes Design of Buildings in Earthquake-Prone Areas Fig. 4.30 Fig. 4.33b Design of Buildings in Earthquake-Prone Areas • Bracing – Bracing with ductile materials offers resistance • Moment-resisting frames – Devices on ground or within structure to absorb part of earthquake energy – Use wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc. to isolate building from worst shaking Southern California Faults Shake Hazard in Southern California A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971 • Earthquake magnitude – M 6.6, with 35 aftershocks of magnitude 4.0 or higher • Distance from epicenter – Bull’s-eye damage pattern • Foundation materials – Not a major factor • Building style – ‘Soft’ ﬁrst-story buildings were major problem – Hollow-core bricks at V.A. Hospital caused collapse – Collapse of freeway bridges Fig. 4.32 Landslides Santa Cruz Mtns, California , 1989 A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971 • Duration of shaking – Lasted 12 seconds (at that magnitude, can last from 10 to 30 seconds), so relatively short time – Lower Van Norman Reservoir • 11,000 acre-feet of water behind earthen dam, above homes of 80,000 • When shaking stopped, only four feet (of original 30) of dam was still standing above water level • Another few seconds of shaking might have caused catastrophic ﬂood US EQ Hazard Map Earthscope Tsunami EQ&V The Great Wave of Kanagawa By Japanese artist Hokusai Indian Ocean Tsunami, 26 December 2004 • Tsunami swept through Indian Ocean, hitting Asian and African shorelines • Estimated > 245,000 deaths • Seaﬂoor west of Sumatra ruptured northward for 1,200 km over 7 minutes • Caused by third largest earthquake (magnitude 9.1) of last 100 years, on subduction zone of Indian- Australian plate under Burma micro plate • Movements on fault of up to 20 m Indian Ocean Tsunami, 26 December 2004 3 Largest EQ in 100 yrs Indian Ocean Tsunami, 26 December 2004 Tsunami • Japanese word: tsu=harbor, nami=waves • Tsunami reach greater height when they enter harbor or other narrow space – 8 m wave on open coastline 30 m wave in narrow harbor • Japan, 1896 – Offshore earthquake shifted seaﬂoor, causing tsunami to hit coastline 20 minutes later – Highest waves (29 m) in narrow inlets – 27,000 killed • ‘Tidal wave’ inappropriate as not related to tides Tsunami • Created most often by earthquakes – Vertical shift of ocean ﬂoor that offsets water mass, transmitted throughout ocean in tsunami – Usually vertical fault motions at subduction zones, mostly in Paciﬁc Ocean • 70,000 people killed by 141 tsunami in 20 century • Single tsunami on 26 December 2004 killed more than 245,000 people in 13 countries Mw = Moment magnitide At 00:58:53 UTC on 26 December 2004, Great earthquake off northeast coast of Sumatra, Indonesia. Location map of Indonesia from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) Earthquake caused by subduction of Indian plate beneath the Burma microplate Relative motion of Indian plate to Burma microplate = 0.06 m per year (2.4 inches per year) Other tectonic elements in region: Yangtze Plate Eurasia Plate Sunda Trench Sunda Plate Australian Plate Ninety East Ridge Tectonic Setting of Earthquake of 26 December 2004 from United States Geological Survey (http://earthquake.usgs.gov/eqinthenews/2004/usslav/) Mw = 9.1 Hypocenter = 30 km depth Focal mechanism = thrust Largest quake since 1964 Good Friday event, Alaska Large magnitude quakes at subduction zones are often tsunamigenic Local tsunamis propagate toward nearest shoreline Tsunamis spread out across ocean basins Map showing ep(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)uake Information Center Seismicity 1900-2004 Star is 26 Dec 2004 event Well-defined Benioff Zone associated with subduction of Indian plate beneath Burmas microplate Main event occurred in seismically active area Many earthquakes annually Many have Mw > 6.5 (damaging magnitudes) Map showing historical seismicity (1900 - 2004) from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) Est. rupture length = 1200 km Est. rupture width = 100 km Est. thrust fault offset = 15m Est. seafloor uplift = 20m? Uplift of seafloor is responsible for initiation of tsunamis Map showing histor(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)quake Information Center Areas potentially affected by tsunami inundation and run-up in red Map showing areas potentially inundated by tsunami on 26 December 2004 from UNOSAT. (http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_07January_landcover.pdf) Potential population affected by tsunami in red zone Potential population affected by tsunami on 26 December 2004 from UNOSAT (http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_04January.pdf) Predicted Arrival Time of First Tsunami Wave (in hours after earthquake) Countries Reporting Direct Casualties from Earthquake and Tsunami Indonesia Thailand Andaman Islands (India) Nicobar Islands (India) Myanmar Bangladesh India Sri Lanka The Maldives The Seychelles Kenya Somalia Tanzania In addition, many western nations are reporting fatalities and missing among tourists who were visiting the affected countries Fundamental Features of Water Waves Wave Crest: is the highest portion of the wave. Wave Trough: is the lowest portion of the wave. Wavelength: is the linear distance separating wave crests (or separating wave troughs). Wave Amplitude: is the displacement of a crest or trough about the mean position or water level. Wave Height: is the total vertical distance from crest to trough (equal to twice the amplitude). Wave Period: is the time required for successive wave crests (or troughs) to pass a fixed point. from An Introduction to the World's Oceans, 8th edition by K.A. Sverdrup, A.C. Duxbury, and A.B. Duxbury (2004) Deep-water waves: water depth is greater than one-half the waves' length. Shallow-water waves: depth becomes shallower than 1/20th of the wavelength Tsunamis have wavelengths ranging from 100 – 200 km Tsunamis are, therefore, shallow-water waves (even in the open ocean) Though tsunamis have very long wavelengths, their amplitude in the open ocean is often relatively small - commonly only a meter or two - and this amplitude is distributed over the very long wavelength so that tsunamis are quite imperceptible on the surface of the ocean. In the deep ocean, tsunamis travel at speeds up to 750 km per hour Wave Refraction occurs when waves enter shallow water over an irregular bottom Wave traveling in shallowest water will also move with the slowest celerity Waves in deeper water continue moving with relatively greater speed Wave crests will curve or refract as the wave moves forward Causes of Tsunami Tsunamis are waves generated by displacement of the ocean by impulsive events Events known to generate tsunamis: Submarine earthquakes Explosive volcanic eruptions Submarine landslides Terrestrial landslides that enter water bodies Impacts of large extraterrestrial objects (e.g. asteroids or comets) in the ocean Other examples of Tsunami Triggered by Submarine Earthquake (other than 26 December 2004) The Great Chilean Earthquake and Tsunami, 1960 (largest earthquake ever recorded, Mw = 9.5) Tsunamis devastated Pacific Rim (Chile, Hawaii, Japan) Good Friday Earthquake, Anchorage, Alaska, 1964 (also an Mw = 9.2 earthquake) Tsunamis destroyed coastal Alaskan communities Tsunami also devastated Crescent City, California Earthquake-Caused Tsunami Chile, 22 May 1960 • Magnitude 9.5 subduction event was most powerful earthquake ever recorded, created large tsunami • Three waves, each successively larger, hit Chilean coast, killing 1,000 Chileans • Adequate warning was given in Hawaii but 61 people killed • Tsunami continued to Japan, killing 185 people • Could continue to be measured in Paciﬁc Ocean for a week Travel Time • Aerial view of coastal area on Isla Chiloe, Chile, showing tsunami damage and wave extent. Two hundred deaths were reported here. • The inhabitants, fearing the earthquake, took to small boats to escape the shaking. • The trough of the tsunami arrived just 10 to 15 minutes after the earthquake, along more than 500 m of the coast. Upon the return of the sea in a huge breaker, all boats were lost. • Aftermath of the Chilean tsunami in the Waiakea area of Hilo, Hawaii, 10,000 km from the generation area. • Parking meters were bent by the force of the debris-ﬁlled waves. In the area of maximum destruction, only buildings of reinforced concrete or structural steel, and a few others sheltered by these buildings, remained standing. Photograph Credit: U.S. Navy. • Downtown Hilo, Hawaii, devastated by the tsunami Earthquake-Caused Tsunami Alaska, 1 April 1946 • Two large subduction earthquakes in Aleutian islands, shook Scotch Gap lighthouse (steel- reinforced concrete, 14 m above low-water level) • Twenty minutes after second earthquake, 30 m tsunami swept lighthouse away (ﬁrst wave was biggest) • Tsunami traveled across Paciﬁc at 485 mph, slowing to 30 mph near Hilo • Rushed ashore and killed 159 people in Hilo, despite warnings (April Fool’s Day) Earthquake-Caused Tsunami Alaska, 27 March 1964 • Magnitude 9.2 subduction earthquake killed 122 people in sparsely populated Alaskan coast • Tsunami hit Vancouver Island, then California • Series of waves, with ﬁfth one largest • Which wave in series will be largest is not predictable Earthquake-Caused Tsunami Alaska, 27 March 1964 Earthquake-Caused Tsunami • Fault movements of sea ﬂoor – must be vertical movement, result in uplifting or down dropping seabed, earthquake of at least magnitude 7.5 • Tsunami Warnings – Feel the earthquake – See sea level draw down signiﬁcantly – Hear wave coming – Seek high ground immediately – Go upstairs in well-built building – Warning system • First sensors activated in 2003 • Tsunami warning center in Honolulu for Paciﬁc Ocean Earthquake-Caused Tsunami British Columbia, Washington, and Oregon – upcoming • Most killer tsunami generated at subduction zones • All oceans have at least some short subduction zones (Atlantic Ocean’s Puerto Rico trench had magnitude 7.3 earthquake on 11 October 1918, causing submarine landslide and 6 m tsunami hitting Puerto Rico coast) • British Columbia, Washington and Oregon coastlines slipped in magnitude 9 earthquake on 26 January 1700, generated massive tsunami recorded in Japan • Next event will be deadly 1700 Cascadia EQ Induced Tsunami • Fault rupture was about 1000 km with a slip of 20 meters • Caused a tsunami that struck the coast of Japan • Evidence suggests that it took place ~ 9:00 pm January 26, 1700 – Although there were no written records in the region at the time, the earthquake's precise time is calculated from Japanese records of a tsunami that has not been tied to any other Paciﬁc Rim earthquake – The most important clue linking the tsunami in Japan and the earthquake in the Paciﬁc Northwest comes from studies of tree rings which show that red cedar trees killed by lowering of coastal forests into the tidal zone by the earthquake have outermost growth rings that formed in 1699, the last growing season before the tsunami • Oral traditions describing a large quake also exist among the region's inhabitants, although these do not specify the date Geo Evidence • Sand beds cover the remains of two ﬁre pits dug by Native Americans, perhaps not long before the tsunami • The layers are well preserved partly because much of this part of the Oregon coast permanently subsided about 0.5-1.0 m during the earthquake • the sand layers, protecting them from later erosionlowed tidal sediments to quickly bury Tsunami Triggered by Explosive Volcanic Eruption Krakatau, Indonesia, 1883 •On 26-27 August 1883, Krakatau volcano erupted •Among most violent volcanic eruptions in last 200 years •Tsunamis triggered by volcanic explosion and caldera collapse •Destroyed 165 coastal Indonesian villages on Java and Sumatra •36,000 Fatalities •Prior to 26 December 2004, most destructive tsunami on record Volcano-Caused Tsunami Krakatau, Indonesia, 26-27 August 1883 • Volcanic eruptions and explosions increased in frequency and strength, with pyroclastic materials ﬂowing into sea and creating tsunami • Culmination of eruption sequence was collapse of mountain into partially emptied magma chamber, creating tsunami 40 m high • More than 36,000 people killed Krakatau, Indonesia • Ships as far away as South Africa recorded tsunamis hitting them • Bodies of victims were found ﬂoating in the ocean for weeks after the event • The tsunamis which accompanied the eruption are believed to have been caused by gigantic pyroclastic ﬂows entering the sea; each of the four great explosions was accompanied by a massive pyroclastic ﬂow resulting from the gravitational collapse of the eruption column. • This caused several cubic kilometers of material to enter the sea, displacing an equal volume of seawater. Tsunami Triggered by Submarine Landslides Storrega, Norway •Series of submarine slides at ca. 35,000 and ca. 7,000 years ago •Triggered by earthquake or decomposition of gas hydrate in seafloor sediment • Gas hydrate is a crystalline solid consisting of gas molecules, usually methane, each surrounded by a cage of water molecules •Storegga 1 (30,000-35,000 years before present) •Storegga 2(approximately 7,000 years before present) •Storegga 3(approximately 7,000 years before present) •Largest mass movement affecting the northwest •European continental margin in the last 50,000 years •Tsunami deposited sediment widespread on Scottish coast USGS Position • Seaﬂoor slopes of 5 degrees and less should be stable on the Atlantic continental margin, yet many landslide scars are present. • The depth of the top of these scars is near the top of the hydrate zone, and seismic proﬁles indicate less hydrate in the sediment beneath slide scars. • Evidence available suggests a link between hydrate instability and occurrence of landslides on the continental margin. • A likely mechanism for initiation of landsliding involves a breakdown of hydrates at the base of the hydrate layer. • The effect would be a change from a semi-cemented zone to one that is gas-charged and has little strength, thus facilitating sliding. • The cause of the breakdown might be a reduction in pressure on the hydrates due to a sea-level drop, such as occurred during glacial periods when ocean water became isolated on land in great ice sheets. Gas Hydrates Landslide-Caused Tsunami • Volcano Collapses – Hawaii in the Paciﬁc Ocean • Deposits of slumps and ﬂank-collapses cover more than ﬁve times land area of islands • Huge tsunami when chunk of island collapses into ocean • Coastal area southeast of Kilauea (active volcano on Big Island of Hawaii) slides at up to 25 cm/yr into ocean, would create tsunami up to 30 m high, directed to southeast Figure 5.14, 5.15 Landslide-Caused Tsunami Landslide-Caused Tsunami • Volcano Collapses – Canary Islands in the Atlantic Ocean • Three of Canary Islands have had mega-collapses, last one 15,000 years ago • Next mega-collapse could send powerful tsunami to coastlines of Africa, Europe, North and South America • Models simulate 10 to 20 m tsunami across Atlantic Ocean Figure 5.17 • Flank collapses occur globally about every 10,000 years Landslide-Caused Tsunami • Earthquake-Triggered Movements – Newfoundland, Canada, 18 November 1929 • Magnitude 7.2 earthquake offshore, triggering submarine mass movement, which set off tsunami • Waves arrived at coast of Newfoundland 2.5 hours later, in three pulses over 30 minutes – Papua New Guinea, 17 July 1998 • Magnitude 7.1 earthquake 20 km offshore, triggered underwater landslide that caused tsunami • Hit coastline of Papua New Guinea about 5 minutes later, washing four villages on barrier beaches into lagoons • Rethinking tsunami threat – not caused just by large earthquakes, also by landslides from moderate earthquakes Tsunami Triggered by Terrestrial Landslide that Entered Water Body Lituya Bay, Alaska 9 July 1958 Large earthquake created massive rock avalanche into fjord in southeast Alaska Resulting impulsive wave is the highest ever recorded = 525 m (over 1,700 ft) Several eyewitnesses lived to tell about it! Landslide-Caused Tsunami • In Bays and Lakes – Lituya Bay, Alaska, 9 July 1958 • Largest historic wave run-up • Magnitude 8 earthquake on Fairweather fault, causing collapse of more than 900 m of rock and ice into Lituya Bay • Three boats anchored in bay, hit by huge wall of water about 30 m high, faster than 100 mph • Crews of two boats survived being lifted and dropped by wave • Wave sent surge of water 525 m up side of bay Details • The rockslide occurred along the eastern wall of the Gilbert Inlet. • The mass of rock striking the surface of the bay created a giant splash, which sent water surging to a height of 1720 feet (see across the point opposite the inlet. • This initial sheet of water stripped all vegetation from the point, leaving a bare rock face. • The in addition to this initial splash, the rock slide also sent a giant local tsunami sweeping across the bay. • Eyewitness accounts from the few unfortunate boaters who happened to be anchored in the bay for night, state that the wave was at least 100 feet tall at its maximum height near the head of the bay. • Two of these boaters were killed by the wave while making a run for open water, the rest, amazingly, survived. • The tsunami inundated approximately 5 square miles of land along the shores of Lityua Bay, sending water as far as 3,600 feet inland (see ﬁgure above), and clearing millions of trees. • The barren shoreline left by the tsunami shows up nicely on the map above, and provides a good approximation of the inundation area. Landslide-Caused Tsunami • In Bays and Lakes – Lake Tahoe, California and Nevada • High in Sierra Nevada, created by active normal faults dropping land between (10 deepest lake in world) • Predictions suggest 4% probability of magnitude 7 earthquake on lake-bounding faults in next 50 years – Would drop lake bottom about 4 m, generate 10 m waves across lake Tsunami Triggered by Impacts of Large Extraterrestrial Objects Chicxulub, Mexico Asteroid impact site and tsunami trigger terminating the Cretaceous Period Asteroid believed to have had 10 km diameter (6.2 miles) Impact in shallow tropical seas Tsunami deposits widespread across Caribbean basin & Gulf of Mexico Impact event also associated with mass extinction of some terrestrial and marine biota ET Impact Tsunami vs. Wind-Caused Waves • Wind waves – Single wave is entire water mass – Velocity depends on period of wave • Tsunami – Huge mass of water with tremendous momentum – Velocity: V = (g x D) • g – acceleration due to gravity; D – depth of water • For average D = 5,500 m, v = 232 m/sec (518 mph) • Actual observations of tsunami speed peak at 420 to 480 mph • Wave will slow as approaches shore, but still fast Tsunami vs. Wind-Caused Waves • Tsunami – Height: ~1 m in open ocean, 6 to 15 m in shallow water, higher in narrow topography – Wave height is leading edge of sheet of water that ﬂows on land for minutes – Usually a series of waves separated by 10 to 60 minutes • Tsunami at the shoreline – Not a big breaking wave – Very rapidly rising tide, rushing inland Tsunami vs. Wind-Caused Waves Tsunami vs. Wind-Caused Waves Tsunami at Hilo, Hawaii, 1 April 1946 • Large earthquake in Aleutian Islands of Alaska created tsunami across Paciﬁc • Eyewitness accounts of loud hissing sound, with advancing and retreating waves for several minutes Tsunami at Oahu, Hawaii, 9 March 1957 • Advancing sheet of water Wavelength and Period versus Height • Destructive power of tsunami is not due to height, but due to momentum of large mass, with ultra- long wavelength and period • Tsunami rushes inland for 30 minutes before water pulls back to form next wave • Long wavelengths and periods mean waves can bend around islands and hit all shores – no protected shores, as with wind waves Waves spread or disperse upon passing through the gaps in barriers. This process is called “wave diffraction” Barriers with many gaps scatter wave energy, diminish wave height and power Seiches • Oscillating waves in enclosed body of water – sea, bay, lake, swimming pool • Energy from strong winds or earthquakes • Hebgen Lake, Montana, 17 August 1959 – Two faults under lake shifted in 6.3 and 7.5 M earthquakes – Eyewitness accounts of water migrating from one end of lake to other, over 11.5 hours Hebgen Lake EQ 1959 Tsunami and You • If You Feel the Earthquake – Mild shaking for more than 25 seconds: powerful, distant earthquake may have generated tsunami – Sea may withdraw signiﬁcantly, or may rise, before ﬁrst big wave – Water may change character, make unusual sounds Tsunami and You: Lessons • Simuele Island, Indonesia, 26 December 2004 – Closest inhabited land to epicenter of magnitude 9.2 earthquake – After shaking stopped, residents ﬂed uphill immediately – Only 7 out of 75,000 inhabitants were killed – Oral history reminded people: when ground shakes, run to hills before giant wave arrives • Nicaragua, 1 September 1992 – Subduction earthquake shifted ground very slowly, creating little ground shaking, but transmitting energy into water efﬁciently, generating large tsunami Tsunami and You • Tsunami Warnings – Coastal Maps • Tsunami-hazard map of Hawaii Big Island, based on local topography • Coastline mapping of Indonesia, India and Sri Lanka after 2004 tsunami indicated where human activities increased damage and loss of life – Removal of vegetation & reefs increased impact of waves – Buoys and Pressure Sensors • Before 2004, NOAO operated six-buoy warning system in northern Paciﬁc Ocean • Funding since provided for 32 buoys around world, transmitting information to scientists • New tide-gauge stations and seismometers along coastlines Warning • US National Weather Service runs: – Paciﬁc Tsunami Warning Center in Hawaii – West Coast and Alaska Tsunami Warning Center in Alaska • Others established in Japan, Indonesia, Australia, and New Zealand……with more on the way in the Caribbean and Mediterranean seas • Similar to NEIC, but catalogs seismic events in ocean basins – Most importantly----issues warnings to those potentially in harms way ISNS 3359 Earthquakes and Volcanoes Historic Earthquakes I Wave Motion: Another View Earthquake Damage • Ground Shaking and Displacement – Earthquake waves arrive in a distinct sequence – Different waves cause different motion • P waves are the 1st to arrive – They produce a rapid up and down motion Earthquake Damage • S waves arrive next (2nd) – They produce a pronounced back and forth motion – This motion is usually much stronger than from P- waves – S waves cause extensive damage Earthquake Damage • Surface waves lag behind S waves – Love waves are the ﬁrst to follow – Ground writhes like a snake Earthquake Damage • Rayleigh waves are the last to arrive – The land surface behaves like ripples in a pond – These waves may last longer than others – Cause extensive damage EQ Damage Earthquake Damage • Landslides and Avalanches – Shaking causes slopes to fail – Hazardous slopes bear evidence of ancient slope failures – Rockslides or snow avalanches follow earthquakes in uplands – An earthquake started the landslide that “uncorked” Mount St. Helens on May 18th, 1980 Earthquake Damage • Liquefaction – Waves liquefy H O-ﬁlled sediments 2 – High pore pressures force grains apart reducing friction – Liqueﬁed sediments ﬂow as a slurry – Sand becomes “quicksand”: clay becomes “quickclay” • Sand dikes • Sand volcanoes • Contorted layering Liquefaction • Water saturated sediments turn into a mobile ﬂuid • Land will slump and ﬂow • Buildings may founder and topple over intact Prediction Earthquake Prediction • Prediction would help reduce catastrophic losses • Can we predict earthquakes? Yes and no – They CAN be predicted - long-term (10-100s of years) – They CANNOT be predicted - short-term (hours-months) • Seismic hazards are mapped to assess risk • This information is useful for… – Developing building codes – Land-use planning – Disaster planning Earthquake Prediction • Long-Term Predictions – Probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more – Based on the premise that earthquakes are repetitive Earthquake Prediction • Long-Term Predictions require determination of seismic zones, by… – Mapping historical epicenters (after ~ 1950) – Evidence of ancient earthquakes (before seismographs) • Evidence of seismicity – Fault scarps, sand volcanoes, etc… • Historical records Earthquake Prediction • Long-Term Predictions – Recurrence interval - Average time between events. • Historical records • Geologic evidence – Requires radiometric dating of events – Sand volcanoes – Offset strata – Drowned forests Earthquake Prediction • Long-Term Predictions – Seismic gaps, places that haven’t slipped recently, are more likely candidates Earthquake Prediction • Long-Term Predictions – Seismic gaps, places that haven’t slipped recently, are more likely candidates. Historic Earthquakes I Largest Earthquakes in the World 1900-2011 Old List in 2004 • 1.Chile 1960 9.5 mag • 2.Alaska 1964 9.2 mag • 3.W Coast of N Sumatra 2004 9.1 mag • 4.East Coast of Honshu, Japan 2011 9.0 mag • 5.Kamchatka 1952 9.0 mag • 6.Offshore, Chile 2010 8.8 mag • 7.Off the Coast of Ecuador 1906 8.8 mag • 8.Rat Islands, Alaska 1965 8.7 mag • 9.N Sumatra, Indonesia 2005 8.6 mag • 10.Assam – Tibet 1950 8.6 mag Historic Events Lisbon, Portugal 1755 • Greatly inﬂuenced art and literature in Europe • Population of Lisbon at the time ~ 275,000, one of the largest in Europe • Some argue that the Lisbon EQ marked a major turning point in human perception of natural disasters – Dynes at the University of Delaware argues that it was the ﬁrst “modern” disaster in the sense that for the ﬁrst time, many people looked for natural explanations rather than attributing the disaster to the wrath of some god/deity Lisbon 1755 The Great Palace Square of Lisbon http://www.drgeorgepc.com/Tsunami1755Lisbon.html Lisbon 1755 • November 1, 1755: Lisbon experienced two major earthquakes in close succession – First caused widespread ﬁres – Second caused tsunamis, which swept many away • A few hours later, Lisbon was again shaken by an earthquake, epicenter in Fez, Morocco (550 km away) • 70,000 people killed and 90% of structures destroyed or damaged • Changed people’s attitudes about the world – To many, earthquakes were a sign of God’s wrath Lisbon 1755 http://neic.usgs.gov/neis/eq_depot/world/1755_11_01_loc.gif Lisbon 1755 • A little after 9:30 in the morning on a Sunday - most people in the city were either in church or on their way to church • Estimates of the number killed vary, but up to 100,000 in Lisbon alone (most put the number at 60-70k) • In Lisbon, perhaps 30k killed initially - church roof collapse, others related to ﬁres and Tsunami • Signiﬁcant numbers also killed elsewhere, especially N. Africa, including maybe 10-12,000 in Morocco, with severe damage to Fez and Marakesh Lisbon 1755 Lisbon 1755 • The Lisbon earthquake caused damage to other parts of Portugal and Spain (especially Madrid, Seville). • The shock waves were felt throughout Europe (Spain, France, Italy, Switzerland, Germany, Luxembourg and Sweden) & North Africa ~ 1.3 million square miles • Seiches were observed in Finland • In Italy, an ongoing volcanic eruption of Vesuvius stopped abruptly • Precursory phenomena also had been widely observed prior to the great earthquake. – For example in Spain, there had been reports of falling water levels in wells, turbid waters and a decrease in water ﬂow in springs and fountains had been reported in both Portugal and Spain Lisbon 1755 • Lots of candles were burning for All Saints Day (Nov 1), ﬁres raged for 5-6 days after the main earthquake(s) • Lisbon housed noted museums and libraries - including unique documents dealing with the history of Portugal's past, most were consumed in the ﬁres • Archives and other precious documents were completely destroyed. • Works of art, tapestries, books, manuscripts, including the invaluable records of the India Company were destroyed – Over two hundred priceless paintings , including works by Titan, Reubens, and Coreggio, were burned in the palace of the Marques de Lourcal • Also burned the King's palace and its 70,000-volume library Lisbon 1755 Lisbon 1755 • Some 30-40 minutes after the initial shocks, Lisbon was further impacted by tsunami, with three main waves • It is estimated that some 20,000 additional people who had survived the collapse of the churches, houses and the ﬁres were killed by the tsunami • All boats moored in Lisbon's harbor were destroyed • "You may judge of the force of this shock, when I inform you it was so violent that I could scarce On a sudden I heard a general outcry, "The sea is coming in, we shall be all lost." Upon this,mer. turning my eyes towards the river, which in that place is nearly four miles broad, I could perceive it heaving and swelling in the most unaccountable manner, as no wind was stirring. In an instant there appeared, at some small distance, a large body of water, rising as it were like a mountain. It came on foaming and roaring, and rushed towards the shore with such impetuosity, that we all immediately ran for our lives as fast as possible; many were actually swept away, and the rest above their waist in water at a good distance from the banks.” • Rev Charles Davy, who survived the events that day Lisbon 1755 Lisbon 1755 • Rupture length of ~ 200 km • Rupture dip of 30° • Slip dislocation of 20 - 30 m of seaﬂoor • Mw estimate of around 8.7 – Data according to Baptista et al, 2003 Lisbon 1755 From: Science, vol 308, 50-52 Lisbon 1755 Gutscher, M-A., 2004-Science, 305, 1247-1248 Lisbon 1755 • Immanuel Kant (Prussian philosopher) noted that EQ events are natural phenomena and suggested that we note where EQ’s occur and then NOT build cities there!! http://geology.about.com/library/bl/bllisbon1755eq.htm Lisbon 1755 • The earthquake accentuated political tensions in Portugal and disrupted the country's eighteenth- century colonial ambitions • The event was widely discussed and dwelt upon by European Enlightenment Philosophers, and inspired major developments in theodicy (attempts to justify the actions of God) • Considered to be the ﬁrst earthquake studied scientiﬁcally for its effects over a large area, leading to the birth of modern seismology and earthquake engineering San Francisco 1906 San Francisco 1906 Transform Fault Earthquakes in California San Francisco, 1906 • Population about 400,000 people • Intense ground shaking for about one minute in early morning hours • Damage much worse in areas constructed on artiﬁcial ﬁll rather than rock or consolidated sediment • Many ﬁres broke out and water lines were ruptured, making ﬁre-ﬁghting impossible – ten times more damage than shaking Transform Fault Earthquakes in California San Andreas Fault Earthquakes • In 1906 earthquake, 430 km of San Andreas fault between Cape Mendocino and San Juan Bautista shifted up to 6 m • This section of the fault has experienced no major earthquakes since 1906: ‘locked’ section of fault – Stress is stored until large rupture releases energy Transform Fault Earthquakes in California San Andreas Fault Earthquakes • Different sections of the fault behave differently: – South of 1906 section, between San Juan Bautista and Cholame ( best know for James Dean fatal acc):ent site • Frequent small to moderate earthquakes and creep (millimeters per year of ongoing offset) – South of creeping section, between Cholame and San Bernardino: • Locked section with recent deﬁcit of earthquake activity ~ seismic gap • Most recent large rupture in 1857 Fort Tejon earthquake of magnitude 7.9 – South of locked Fort Tejon section: • Complex zone with locked sections San Francisco 1906 Gradual slip, stress not accumulating San Francisco 1906 Locked, stress accumulating Gradual slip, stress not accumulating San Francisco 1906 San Francisco 1906 San Francisco 1906 http://quake.wr.usgs.gov/info/1906/offset.html San Francisco 1906 • Shaking lasted from 45-60 seconds, based on eye witness accounts (image = Hotel St. Francis) http://quake.wr.usgs.gov/info/1906/shaking.html San Francisco 1906 http://quake.wr.usgs.gov/info/1906/got_seismogram_lp.html San Francisco 1906 Stanford University http://quake.wr.usgs.gov/info/1906/casualties.html Alaska 1964 Alaska 1964 http://neic.usgs.gov/neis/eq_depot/usa/1964_03_28.html Subduction Zone Earthquakes • Subduction of Paciﬁc plate under Alaska creates truly large earthquakes • The Good Friday Earthquake, Alaska, 1964 – Major subduction movement created magnitude 9.2 earthquake, as Paciﬁc slab shoved under Alaska in seven lurches – Four minutes of shaking induced avalanches, landslides, ground settling and tsunami, killing 131 people – Relatively low loss of life because area is sparsely inhabited, few people downtown, low tide and warm weather Alaska 1964 • Epicenter located between Valdez and Anchorage, near Prince William Sound. • Earthquake occurred on a thrust fault. This fault was a subduction zone, where the Paciﬁc plate plunges underneath the North American plate. • The ﬁrst slip occurred at a depth of 25 km, considered a shallow depth. Alaska 1964 Alaska 1964 Station Code: DAL; Southern Methodist University; Dallas, Texas; Vertical- Component Sprengnether Seismometer; Magniﬁcation: 25,000; Start Time of Record: March 27, 1964, 17 hours, 39 minutes UTC http://neic.usgs.gov/neis/eq_depot/usa/1964_03_28.html Alaska 1964 http://neic.usgs.gov/neis/eq_depot/usa/1964_03_28.html Alaska 1964 http://neic.usgs.gov/neis/eq_depot/usa/1964_03_28.html Alaska 1964 http://neic.usgs.gov/neis/eq_depot/usa/1964_03_28.html Alaska 1964 1904-1992 seismicity: http://www.owlnet.rice.edu/~geol108/eq19/Alaska_Hist/EQHistory Alaska 1964 • The sudden uplift of the Alaskan seaﬂoor caused a tsunami, which was responsible for 122 of the 131 deaths • The tsunami propagated at speeds over 400 miles per hour, reached the Hawaiian Islands, also struck Crescent City, California, killing 10 people, 6 people died in Oregon • Seiches occurred in rivers, lakes, bayous, and protected harbors and waterways along the Gulf Coast of Louisiana and Texas, causing minor damage ISNS 3359 Earthquakes and Volcanoes Historic Earthquakes II Historic Earthquakes II Today we will discuss several major earthquakes – Kobe Japan, 1995 – Izmit, Turkey, 1999 – Bam, Iran, 2003 – New Madrid, US, 1811-1812 Kobe, Japan, 1995 • The effects of any earthquake depend on a number of widely varying factors. These factors are: – Intrinsic to the earthquake - its magnitude, type, location, or depth; – Geologic conditions where effects are felt - distance from the event, path of the seismic waves, types of soil, water saturation of soil; – Societal conditions reacting to the earthquake - quality of construction, preparedness of populace, or time of day (e.g.: rush hour). Kobe, Japan, 1995 Kobe, Japan, 1995: most expensive earthquake in history • Kobe/Osaka region had a population of 10 million • Magnitude 6.9 earthquake with 50 km long rupture of Nojima fault and 100 seconds of shaking • Tile roofs and little lateral support caused collapse of buildings causing > 6,300 fatalities • More than 140 ﬁres resulted • State-owned buildings and facilities losses > $ 150 billion Kobe, Japan, 1995 http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html Kobe, Japan, 1995 Aftershock locations http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html Kobe, Japan, 1995 http://neic.usgs.gov/neis/eq_depot/world/1995_01_16.htmll-kobe.htm Kobe, Japan, 1995 http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html Kobe, Japan, 1995 http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html Kobe, Japan, 1995 http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html Comparison • How might the Kobe quake compare to the next big quake in Oakland, California? Kobe, Japan, 1995 vs. Oakland, California, 20?? Oakland, California, 20?? • Hayward fault has 27% probability of causing magnitude 6.7 or greater earthquake before 2032 • San Francisco Bay region, 62% probability of 6.7 magnitude earthquake before 2032 • Comparison of Hayward fault and Nojima fault: – Both about 50 km long – Both run through densely populated areas, with large areas of weak ground materials – Both capable of generating magnitude 7 earthquakes Transform Faults and Earthquakes • Horizontal movements cause major earthquakes • Turkey, 1999: – Segment of North Anatolian fault ruptured for 120 km in magnitude 7.4 earthquake near Izmit – Followed weeks later by rupture to the east in magnitude 7.1 earthquake – Residential buildings on soft ground • Adding sand to concrete resulted in buildings collapsing during shaking – This compromised the concrete, making it weaker than it could have been – > 17,000 people dead Izmit, Turkey • Long history of damaging earthquakes • Ancient account from 358 AD reveals striking similarities with the 1999 event • Marcellinus provides detailed descriptions of the shaking, the sound of the earthquake, and the ground failure on the hillsides and ensuing destruction of the houses that were built there • He describes the clearing of the air (presumably the settling of dust raised by the shaking and landslides) a few hours after main shock, and the vast piles of rubble that were then revealed • He speaks of the human tragedy, as well, describing in graphic terms injuries and unfortunate fates of earthquake victims • The similarity between what Marcellinus described and what we have all witnessed from the 1999 event is incredible • Unlike the 1999 Izmit earthquake, but very much like the 1906 earthquake in San Francisco, the earthquake in 358AD was followed by a great ﬁre that consumed much of what remained standing Izmit, Turkey 1999 • Turkey, 1999: – Turkey is pushed westward along the North Anatolian fault, which runs for 1,400 km along the Black Sea – Since 1939, the North Anatolian fault has ruptured in 11 earthquakes, from east end of fault to west • Unique, semi-regular pattern • Next event? Probably to west of Izmit, closer to Istanbul • Probably within next 30 years Izmit, Turkey 1999 http://quake.wr.usgs.gov/research/geology/turkey/images/CA-Tu_comp.jpeg Izmit, Turkey 1999 Mw ≥ 5 http://www.earthquakes.bgs.ac.uk/images/turkey_5plus.jpg Izmit, Turkey 1999 http://quake.wr.usgs.gov/research/geology/turkey/images/CA-Tu_comp.jpeg Izmit, Turkey 1999 http://quake.wr.usgs.gov/research/geology/turkey/images/CA-Tu_comp.jpeg Izmit, Turkey 1999 http://pubs.usgs.gov/circ/2000/c1193/c1193.pdf Izmit, Turkey 1999 http://pubs.usgs.gov/circ/2000/c1193/c1193.pdf Bam, Iran 2003 • Friday, Dec 26, 2003 at 5:26:52 AM local time • Magnitude (Mw) 6.6 earthquake • Depth of hypocenter = 10 km • At least 30,000 dead, 30,000 injured (dead = 26, 271 according to www.farsinet.com/bam) • 85% of buildings were destroyed Bam, Iran 2003 http://neic.usgs.gov/neis/world/iran/ Bam, Iran 2003 http://www.usaid.gov/our_work/humanitarian_assistance/disaster_assistance/resources/pdf/iran_pop_Bam123103.pdf Bam, Iran 2003 Seismicity in Iran since 12/26/03 http://www.iiees.ac.ir/EQSearch/ShowMap.aspx CO 01 Fig. 1.01 U.S. and Canadian Earthquakes • Earthquakes occur throughout North America, not just in California • Occur in clusters, mostly in western North America but also in eastern North America and Hawaii Western North America: Plate Tectonic-Related Earthquakes • North American plate moves southwest at 2.5 cm/yr • Paciﬁc plate moves northwest 8 cm/yr • Much of Farallon plate has subducted under North America – Western United States uplifted, creating Rocky Mountains, Sierra Nevada, Colorado Plateau – Earthquakes throughout western United States Intraplate Earthquakes: “Stable” Central United States • Clusters of earthquakes at a few locations • Away from active plate edges • Fewer earthquakes, but can be just as large, especially where Old Plate Boundaries or Old Structures Exist – Although earthquakes are small, they occur on the Meers fault in Southern Oklahoma for example… Intraplate Earthquakes: “Stable” Central United States New Madrid, Missouri, 1811-1812 • Series of earthquakes, with four very large events – Eight considered violent, ten very severe – Total of 1,874 events – Hypocenters beneath thick sediments of Mississippi and Ohio Rivers at Mississippi River embayment, near town of New Madrid (called ‘Gateway to the West’ before destruction by earthquakes) – Long-lasting effects on topography • Two new lakes • Low cliffs and domes formed • Waterfalls in streams New Madrid, Missouri, 1811-1812 • Felt Area – Felt area was largest for any U.S. earthquake – Must consider difference in wave propagation in eastern vs. western North America • Young, tectonically fractured rocks of west coast impede wave propagation and cause wave energy to die out faster than older, more homogeneous rocks of central U.S. New Madrid, Missouri, 1811-1812 • Magnitudes – Using felt area to estimate magnitudes 8 to 8.3 – Studies of small earthquakes occurring today (aftershocks of 1811-1812 events) in this area can map out the faults • First earthquake on Cottonwood Grove fault, triggered two of following earthquakes on Reelfoot blind thrust • Remaining earthquake difﬁcult to locate New Madrid, Missouri, 1811-1812 • Magnitudes – Using fault-rupture length estimates from aftershock locations gives smaller moment magnitudes 7.3 to 7.7 – Soft, water-saturated sediment of ground ampliﬁes shaking; accounting for ampliﬁcation gives magnitudes 7.0 to 7.5 New Madrid, Missouri, 1811-1812 • The Future – 1811-1812 New Madrid earthquakes did not cause great damage because population of area at time was so low • Future earthquakes will affect population of St. Louis, Memphis • Buildings not designed for earthquake shaking • Soft sediments will amplify ground shaking • Very large area will be affected New Madrid, Missouri, 1811-1812 • The Future – Neotectonic analyses show signiﬁcant earthquakes in the area around 500, 900, 1300 and 1600 • Magnitude 7 or higher earthquakes occur here about every 500 years • U.S. Geological Survey forecasts 90% probability of magnitude 6-7 earthquake in next 50 years • Why do earthquakes occur here in middle of continent? Reelfoot Rift Reelfoot Rift: Missouri, Arkansas, Tennessee, Kentucky, Illinois • Why do earthquakes follow same linear pattern as deposition of sediments by Mississippi River system? • Linear structural depression underlying New Madrid region ancient rift valley, Reelfoot Rift • Formed 550 million years ago and since ﬁlled with sedimentary rocks and covered with younger sediments Ancient Rifts in the Central United States • As Pangaea tore apart around 200 million years ago, many rifts formed • Some separated the landmass and formed Atlantic Ocean • Others were failed rifts, left
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