earthquakes. standards ò describe the geological manifestations of plate tectonics, such as...
TRANSCRIPT
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EARTHQUAKESEARTHQUAKES
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StandardsStandards
Describe the geological manifestations of plate tectonics, such as earthquakes
Describe the impact of plate motions on societies and the environment
Describe how waves are used for practical purposes (e.g., seismic data)
Examine investigations of current interest in science
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Major Earthquakes in HistoryMajor Earthquakes in History
The following are just a few of many notable earthquakes through history
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1811: New Madrid Missouri 1811: New Madrid Missouri
Magnitude 7.5 Large areas sank into
the earth New lakes were formed The Mississippi River
changed its course and even flowed backward
Sand blows (geysers) occurred, can still see remnants today
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1906: San Francisco1906: San Francisco
Felt from southern Oregon to south of Los Angeles and inland to central Nevada
Estimated magnitude of 7.8 >3000 killed Massive fires
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San Francisco BurningPhoto: http://www.stvincent.ac.uk/Resources/EarthSci/Tectonics/cons1906.html
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Aerial View of San Francisco from balloonPhoto: http://er1.org/docs/photos/Disaster/san%20francisco%20earthquake%201906%20view%20from%20balloon.jpg
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Photo: http://www.eas.slu.edu/Earthquake_Center/1906EQ/sanfran/m031.html
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San Francisco Financial DistrictPhoto: http://www.sfmuseum.org/hist/pix49.html
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1985: Mexico City1985: Mexico City
Magnitude 8.1 Epicenter 350 km away off Pacific coast Shaking lasted 3 – 4 minutes Collapse of poorly constructed buildings Liquefaction of soils under city ~10,000 killed
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Mexico CityPhoto: http://latimesblogs.latimes.com/laplaza/2010/09/earthquake-mexico-city-1985-memorial.html
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Mexico CityPhoto: http://www.objectlessons.org.uk/default.asp?image=GEO000XP5018&document=500.0021.0020
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1960: Valdivia, Chile1960: Valdivia, Chile Largest earthquake
ever recorded Magnitude 9.5 Caused tsunamis in
many parts of the Pacific, including Hilo, Hawaii
1,655 people killed
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2004: Sumatra EQ and Indian Ocean Tsunami2004: Sumatra EQ and Indian Ocean Tsunami
Magnitude 9.2 Rupture continued for 9 minutes & moved 1300
km along a thrust fault – the longest single fault break ever recorded
Resulted in tsunamis that killed 300,000 on Sumatra, Sri Lanka, Thailand, the Maldive Islands and Somalia
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Banda Aceh Pre-TsunamiJune 23, 2004
Banda Aceh Post-TsunamiDecember 28, 2004
Photos: http://www.baird.com/baird/en_html/indian_ocean/indianocean.html
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Indian Ocean Tsunami, Thailand Photo: http://en.wikipedia.org/wiki/Image:2004-tsunami.jpg
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Original Photo from John Thompson taken on December 26, 2004 in Khao Lak
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2008: Eastern Sichuan, China2008: Eastern Sichuan, China
Magnitude 7.9 Schools and hospitals collapsed ~70,000 killed ~18,000 missing Strong aftershocks and landslides May have been triggered by dam holding 315
million tons of water
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Photo: http://welovecomments.wordpress.com/2009/08/12/reaction-from-someone-who-was-in-china-during-the-2008-magnitude-8-0-earthquake-in-sichuan/
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Photo: http://www.telegraph.co.uk/news/worldnews/asia/china/4434400/Chinese-earthquake-may-have-been-man-made-say-scientists.html
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2010: Haiti Earthquake2010: Haiti Earthquake
Magnitude 7.0 ~212,000 killed ~ 1 million homeless Major damage to city of Port-au-Prince
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Photo: http://image3.examiner.com/images/blog/EXID12837/images/haiti.jpg
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Photo: http://i.telegraph.co.uk/telegraph/multimedia/archive/01558/presidential-palac_1558531i.jpg
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2011: Japan Earthquake and Tsunami2011: Japan Earthquake and Tsunami
Magnitude 9.0 earthquake Rupture along thrust fault at the subduction zone
between the Pacific & N. American plates Fault moved upwards by 30-40 m (this is 98 to 131
feet!) and slip occurred over an area 300 km long Foreshocks occurred over 2 days preceding the quake
(a M 7.2 and 3 greater than M 6 on same day) Several aftershocks have occurred, many over a M 6
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2011: Japan Earthquake and Tsunami2011: Japan Earthquake and Tsunami
Resulting 30 ft tsunami swept through many coastal regions of Japan, reaching as far as 6 mi inland
13,116 people killed, 14,377 missing Caused failure of nuclear power plant
All following photos from MSN.com unless otherwise noted
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Collapsed House_SukagawaCity,Fukushima
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JapanEQ_SplitRoad_SacrementoBee
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TsunamiSwirl_Oarai,Ibaraki_3-11-11
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JapanTsunami_Iwanuma, Miyagi_3-11-11
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JapanTsunami_Natori_3-11-11
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Japan Tsunami_SendaiAirport_3-11-11
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JapanTsunami_3-11-11_Cleveland.com
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JapanTsunami_HousesSweptToSea_NatoriCity_3-11-11
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JapanTsunami_SendaiAirport_3-11-11
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OnagawaTown,MiyagiPrefecture_3-26-11
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What is an Earthquake?What is an Earthquake?
We inhabit a fragile built environment of houses, buildings & transportation systems that is anchored in Earth’s crust
This environment is vulnerable to seismic vibration, ground rupture, landslides and tsunamis
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What is an Earthquake?What is an Earthquake? Plate movements generate forces at the
boundaries that can be described in terms of stress, strain and strength
Stress – local forces per unit area that cause rocks to deform
Strain – relative amount of deformation Rocks fail – break – when they are stressed
beyond a critical value called their strength
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What is an Earthquake?What is an Earthquake? Earthquakes are the result of stress that builds
up over time, as tectonic forces deform rocks on either side of a fault
They occur when the stress exceeds the strength of the rocks, which suddenly break along a new or preexisting fault
The two blocks of rock on each side of the fault slip, releasing the stress suddenly, causing an earthquake, which generates seismic waves
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Elastic Rebound TheoryElastic Rebound Theory
Faults remain locked while strain energy accumulates in the rocks on either side, causing them to deform until a sudden slip along the fault releases the energy
Elastic means the rocks spring back to their undeformed shape when the fault unlocks
The distance of displacement is called the fault slip
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Photo: http://www.winona.edu/geology/MRW/mrwimages/elasticrebound.jpg
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Focus and EpicenterFocus and Epicenter Focus – point at which the slip begins –
somewhere below the surface Most earthquakes in continental crust have focal
depths from 2 – 20 km (rocks behave in a ductile manner below 20 km)
Subduction zone earthquakes can have foci as great as 690 km deep
Epicenter – the geographic point on Earth’s surface directly above the focus
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Photo: http://www.yorku.ca/esse/veo/earth/image/1-10-15.JPG
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Fault RuptureFault Rupture
Does not happen all at once Starts at focus and expands outward on fault
plane at ~2 – 3 km/s Rupture stops when stress can no longer break
the rocks Size of earthquake is related to total area of fault
rupture
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Fault RuptureFault Rupture
Most earthquakes are very small and the rupture never breaks the surface
However, in large, destructive earthquakes, surface breaks are common
Ex: 1906 San Francisco EQ caused surface displacements averaging 4 m (13 ft.) along a 400 km section of the San Andreas
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Tree displaced 15 ft (from where person is standing) Photo: http://www.stvincent.ac.uk/Resources/EarthSci/Tectonics/images/ranch.jpg
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Fault RuptureFault Rupture
Faulting in largest Earthquakes can extend more than 1000 km and the slip can be as large as 20 m (~60 ft)
Stored strain energy is released in the form of frictional heating and seismic waves
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Foreshocks and AftershocksForeshocks and Aftershocks
Aftershocks occur as a consequence of a previous EQ of larger magnitude
Their foci are distributed in and around the rupture plane of the main shock
They can last from weeks to years They can compound damage from the main
shock
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Foreshocks and AftershocksForeshocks and Aftershocks
Foreshocks are small earthquakes that occur near, but before, a main shock
Many large earthquakes have been preceded by foreshocks
Scientists have tried to use them to predict large earthquakes
Hard to distinguish foreshocks from other small earthquakes
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Seismic WavesSeismic Waves Ground vibrations produced by an earthquake Enable us to locate earthquakes and
determine type of faulting that produced them 4 types:1. Body Waves
a. P wavesb. S waves
2. Surface Wavesa. Rayleigh wavesb. Love waves
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Primary or P Waves Primary or P Waves Travel through Earth and are
first to arrive at seismic station
Compressional waves Can be thought of as
push-pull waves: they push or pull particles of matter in the direction of their travel
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Secondary or S WavesSecondary or S Waves
Follow the P waves through Earth, and arrive second at the seismic station
Shear waves Displace material at right
angles to their path of travel
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Surface WavesSurface Waves Arrive last after traveling around Earth’s surface Speed slightly less than S waves Rayleigh waves – travel in rolling motion over
surface Love waves – shake the ground in sideways
motion
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Locating the EpicenterLocating the Epicenter Time interval between
P and S wave arrival depends on distance waves have traveled from focus
If three or more seismic stations know the distance, then the epicenter can be located using triangulation
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Measuring the Size of an Earthquake Measuring the Size of an Earthquake
Magnitude of an earthquake is the main factor that determines the intensity and potential destructiveness of an earthquake
Two scales:1. Richter magnitude2. Moment magnitude
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Richter MagnitudeRichter Magnitude Developed by Charles Richter in 1935 Each earthquake is assigned a number on a
logarithmic scale Two earthquakes differ by one magnitude if the
size of their ground motions differs by a factor of 10 This means the ground motion of a magnitude 6
earthquake is 10 times greater than a magnitude 5 and 100 times greater than a magnitude 4
The energy released as seismic waves increases by a factor of 33 for each Richter unit
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Moment MagnitudeMoment Magnitude Seismologists now prefer a measure of EQ size more
directly related to the physical properties of faulting that causes the EQ
Moment magnitude is the product of the area and the average slip across the fault break
It increases by about 1 unit for every 10-fold increase in the area of faulting
It produces roughly the same numerical values as Richter’s method, but can be measured from seismograms and determined by field measurements of the fault
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Earthquake Size and FrequencyEarthquake Size and Frequency Large earthquakes occur much less often than small
ones Worldwide figures of earthquake size per year: 1,000,000 with magnitudes greater than 2.0 100,000 greater than 3.0 1000 greater than 5.0 10 greater than 7.0 Earthquakes with magnitude above 8.0 occur about once
every 3 years Very large ones like the 2004 Sumatra quake (magnitude
9.2), 1964 Alaska (9.2) and 1960 Chile (9.5) are rare
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Shaking IntensityShaking Intensity
Amplitude of shaking depends on distance from fault rupture
Damage from shaking depends on distance from populated areas
Estimated shaking determined with modified Mercalli intensity scale – values from I (not felt) to XII (damage total) (for full scale see page 307 of textbook)
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Shaking Intensity maps of 1906 & 1989 San Francisco EarthquakesPhoto: http://pubs.usgs.gov/of/2005/1135/1906_Boatwright/download/1906_Boatwright_BA_intensity.jpg
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Earthquakes and FaultingEarthquakes and Faulting
Most earthquakes occur at plate boundariesboundaries Largest earthquakes occur at convergentconvergent
boundaries on megathrustmegathrust faults that form where one plate subductssubducts beneath another
Exs: Sumatra (2004), Alaska (1964) & Chile (1960): largest EQ ever recorded, magnitude 9.5
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Intraplate EarthquakesIntraplate Earthquakes A small percentage of earthquakes occur in
plate interiors Foci are shallow and occur mostly on continents Many occur on oldold faults that use to be part of
plate boundaries and are now areas of crustal weaknessweakness
Examples include some of most famous in American history: New Madrid, Missouri (1811-1812), Charleston, South Carolina (1886), and Cape Ann, near Boston Massachusetts (1755)
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Regional Fault SystemsRegional Fault Systems ZonesZones of deformation between plate boundaries
usually have a network of interactinginteracting faults – a fault systemsystem – rather than a single fault
Ex: in California, the “master fault” is the San Andreas, however, there are many subsidiary faults on either side that generate large earthquakes.
Most of the damaging earthquakes in California during the last century have occurred on these subsidiary faults
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San Andreas Fault SystemPhoto: http://pubs.usgs.gov/gip/earthq3/map1a.gif
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Earthquake DestructivenessEarthquake Destructiveness
Over the last century, earthquakes worldwide have caused an average of 13,000 deaths per year and hundreds of billions of dollars of damage
Two California earthquakes – 1989 Loma PrietaLoma Prieta (mag 7.1 & $10 billion in damage) and 1994 NorthridgeNorthridge (mag 6.8 & $40 billion in damage) – were among the costliestcostliest disasters in U.S. history because of nearby urban areas
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Loma Prieta Earthquake DamageLoma Prieta Earthquake Damage
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Nimitz Freeway after the Loma Prieta Earthquake, 1989Photo: http://www.dot.ca.gov/hq/esc/geotech/photos/south2/cypress03.jpg
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Column collapse along Cypress Viaduct, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom
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Marina District after Loma Prieta Earthquake, 1989Photo: GSA, Explore Earthquakes CD-Rom
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Damage to garages in Marina District, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom
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House that slid off foundation during Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom
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Collapsed walls of house, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom
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Collapse of 5 story tower, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom
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Northridge Earthquake DamageNorthridge Earthquake Damage
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Collapse of Interstate 5, Northridge EQ, 1994 Photo: http://pubs.usgs.gov/fs/1999/fs110-99/
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Highway damaged during Northride EQ, 1994
Photo: http://boxer.senate.gov/students/resources/features/1906/committee.cfm
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Highway Damage, Northridge Earthquake, 1994Photo: http://mceer.buffalo.edu/research/resilience/default.asp
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Damaged Building, Northridge EQ, 1994 Photo: http://www.calstatela.edu/dept/geology/Geocareer.htm
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Earthquake DestructivenessEarthquake Destructiveness Destructive earthquakes are even more common in
JapanJapan than in California Japan is the best prepared nation to deal with
earthquakes, with strong public education campaigns, building codes and warning systems
Despite this, more than 5600 people were killed in a mag 6.9 EQ in Kobe in 1995
Casualties and structure failure (50,000 buildings destroyed) occurred because of less stringent building codes that were in effect when most of the city was built and the proximity of the rupture to the city
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How Earthquakes Cause DamageHow Earthquakes Cause Damage
PrimaryPrimary effects: Faulting (breaks in ground surface) Ground shaking (from seismic waves) SecondarySecondary effects: Landslides Tsunamis Fires
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Faulting and ShakingFaulting and Shaking
Ground surfacesurface can subside or uplift during faulting Ground accelerationsaccelerations near the epicenter can exceedexceed
the acceleration of gravitygravity, so objects lying on the surface can be thrownthrown into the air
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Faulting and ShakingFaulting and Shaking Seismic waveswaves can shakeshake structures so hard
that they collapsecollapse, which is the leading cause of casualties and economic damage
Examples: Tangshan, China 1976: >240,000 killed Spitak, Armenia 1988: 25,000 killed Izmit, Turkey 1999: 15,600 killed Etc…
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Landslides and Other Ground FailuresLandslides and Other Ground Failures LandslidesLandslides can bury towns Ex: debris flow in China’s Kansu Province, 1920,
covered >100 km2, 200,000 killed WaterWater saturated soils can behave like a liquid –
called liquefactionliquefaction – and flow away, taking buildings, bridges, etc along with it
Ex: cause of massive building collapse in Mexico City EQ: Mexico City built on unstable soils of ancient lakebed
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Liquefaction in Niigata, 1964Photo: http://www.ce.washington.edu/~liquefaction/selectpiclique/nigata64/tiltedbuilding.jpg
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Tsunamis Tsunamis
Destructive seasea wave triggered by earthquake beneath the ocean
NOT called tidal wave – this term is incorrect, has nothing to do with tides
Deadliest and most destructive hazard associated with largest earthquakes – megathrustmegathrust quakes that occur in subductionsubduction zones
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TsunamisTsunamis Megathrust ruptures can push the seafloor
upward by as much as 10 m, displacing the overlying ocean water
Resulting wave travels at speeds of up to 800800 km/hr, as fast as a jetliner
They are hardly noticeable in deep ocean, but waves slowslow down and pilepile up when they reach shallowshallow coastal waters
Resulting wave can be tenstens of meters tall
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TsunamisTsunamis Most common in PacificPacific Ocean, why? RingRing of FireFire – subduction zones ring the Pacific Examples: 1964 Alaska EQ caused tsunamis that hit
thousands of kilometers from epicenter. At one location, near Valdez, AK, the tsunami ran up a mountainside to a height of 67 m (that’s 220220 feet)!
2004 IndianIndian Ocean EQ caused tsunamis that killed 300,000300,000 people in several countries
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1946 Tsunami Hilo, Hawaii. Caused by earthquake in Aleutian IslandsPhoto: http://static.howstuffworks.com/gif/tsunami-5.jpg
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Damage from 1946 tsunami, Hilo, HawaiiPhoto: http://soundwaves.usgs.gov/2005/01/fieldwork2.html
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Aftermath of 1960 tsunami at Hilo, Hawaii; caused by earthquake in ChilePhoto: http://earthquake.usgs.gov/regional/world/events/images/1960_05_22_hilo.gif
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Damage to hotel from Indian Ocean tsunami, 2004Photo: http://www.calstatela.edu/dept/geology/G351.htm
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FiresFires
Are ignited by ruptured gasgas lines or downed electricalelectrical power lines
Damage to waterwater mains can making fighting them impossible, as happened in the 1906 San Francisco EQ
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Reducing Earthquake RiskReducing Earthquake Risk
Seismic hazardhazard – describes the intensityintensity of seismic shaking and ground disruption that can be expected
Seismic riskrisk – describes the damagedamage that can be expected for a specific region
Risk depends on the seismic hazard, populationpopulation, and number of builtbuilt structures
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Reducing Earthquake RiskReducing Earthquake Risk
CaliforniaCalifornia leads the nation in seismic risk at 7575% of the national total, with Los Angeles county accounting for 2525%
But 46 million people are at risk outside of California, including: Hilo, Honolulu, Anchorage, Seattle, Tacoma, Portland, Salt Lake City, Reno, Las Vegas, AlbuquerqueAlbuquerque, Charleston, Memphis, Atlanta, St. Louis, New York, Boston & Philadelphia
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United States seismic hazard mapPhoto: http://pubs.usgs.gov/fs/2005/3038/images/seismic-hazard-map.jpg
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Land Use PoliciesLand Use Policies Exposure of built structures to earthquake risk
can be reduced by policies that restrictrestrict land use It is unwise to erect buildings on known active
faults, as was done in residential areas of San Francisco.
California law now restricts construction across active faults.
Real estate agents are required to disclose information about houses built on a fault
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Earthquake EngineeringEarthquake Engineering Risk from seismic shaking can be reduced by good
engineeringengineering and constructionconstruction Building codes specify the forcesforces a structure must be
able to withstand from a seismic hazard U.S. building codes have been largely successful in
preventing loss of life during earthquakes Ex: from 1983 to 2004, 131 people died in nine severe
earthquakes in the western U.S., whereas >460,000 people were killed by earthquakes worldwide
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Warning SystemsWarning Systems When an earthquake occurs, automated seismicseismic
systems can send warnings tens of seconds before the arrival of destructive seismic waves
Tsunamis travel 10 times slower than seismic waves, so distant shorelines can be given up to hourshours of warning time
Unfortunately, no system had been installed in the Indian Ocean during the 2004 quake
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Can Earthquakes be Predicted?Can Earthquakes be Predicted?
Prediction means specifying timetime, locationlocation and sizesize
Information from plate tectonicstectonics and geologic mappingmapping of fault systems can allow geologists to forecastforecast which faults are likely to produce earthquakes over the longlong term
To specify precisely when a particular fault will rupture is very difficult
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Long-Term ForecastingLong-Term Forecasting
The longer the time since the last big EQ, the sooner the next one will be
RecurrenceRecurrence interval – the average time between large earthquakes.
Determined by strainstrain rate – how long it takes for a fault to build up enough strain that rock strength is exceeded
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Short-Term PredictionShort-Term Prediction There have been a few successful short-term
predictions Ex: in 1975, an EQ was predicted only hours before
occurring near Haicheng, China Seismologists used precursors of swarms of tiny
earthquakes to make prediction The next year, however, an unpredicted quake struck
the Chinese city of Tangshan, killing more than 240,000 No reliable method of short-term prediction has been
found