EARTHQUAKESEARTHQUAKES

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

Major Earthquakes in HistoryMajor Earthquakes in History

The following are just a few of many notable earthquakes through history

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

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

San Francisco BurningPhoto: http://www.stvincent.ac.uk/Resources/EarthSci/Tectonics/cons1906.html

Aerial View of San Francisco from balloonPhoto: http://er1.org/docs/photos/Disaster/san%20francisco%20earthquake%201906%20view%20from%20balloon.jpg

Photo: http://www.eas.slu.edu/Earthquake_Center/1906EQ/sanfran/m031.html

San Francisco Financial DistrictPhoto: http://www.sfmuseum.org/hist/pix49.html

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

Mexico CityPhoto: http://latimesblogs.latimes.com/laplaza/2010/09/earthquake-mexico-city-1985-memorial.html

Mexico CityPhoto: http://www.objectlessons.org.uk/default.asp?image=GEO000XP5018&document=500.0021.0020

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

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

Banda Aceh Pre-TsunamiJune 23, 2004

Banda Aceh Post-TsunamiDecember 28, 2004

Photos: http://www.baird.com/baird/en_html/indian_ocean/indianocean.html

Indian Ocean Tsunami, Thailand Photo: http://en.wikipedia.org/wiki/Image:2004-tsunami.jpg

Original Photo from John Thompson taken on December 26, 2004 in Khao Lak

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

Photo: http://welovecomments.wordpress.com/2009/08/12/reaction-from-someone-who-was-in-china-during-the-2008-magnitude-8-0-earthquake-in-sichuan/

Photo: http://www.telegraph.co.uk/news/worldnews/asia/china/4434400/Chinese-earthquake-may-have-been-man-made-say-scientists.html

2010: Haiti Earthquake2010: Haiti Earthquake

Magnitude 7.0 ~212,000 killed ~ 1 million homeless Major damage to city of Port-au-Prince

Photo: http://image3.examiner.com/images/blog/EXID12837/images/haiti.jpg

Photo: http://i.telegraph.co.uk/telegraph/multimedia/archive/01558/presidential-palac_1558531i.jpg

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

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

Collapsed House_SukagawaCity,Fukushima

JapanEQ_SplitRoad_SacrementoBee

TsunamiSwirl_Oarai,Ibaraki_3-11-11

JapanTsunami_Iwanuma, Miyagi_3-11-11

JapanTsunami_Natori_3-11-11

Japan Tsunami_SendaiAirport_3-11-11

JapanTsunami_3-11-11_Cleveland.com

JapanTsunami_HousesSweptToSea_NatoriCity_3-11-11

JapanTsunami_SendaiAirport_3-11-11

OnagawaTown,MiyagiPrefecture_3-26-11

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

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

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

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

Photo: http://www.winona.edu/geology/MRW/mrwimages/elasticrebound.jpg

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

Photo: http://www.yorku.ca/esse/veo/earth/image/1-10-15.JPG

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

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

Tree displaced 15 ft (from where person is standing) Photo: http://www.stvincent.ac.uk/Resources/EarthSci/Tectonics/images/ranch.jpg

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

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

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

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

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

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

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

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

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

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

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

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

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)

Shaking Intensity maps of 1906 & 1989 San Francisco EarthquakesPhoto: http://pubs.usgs.gov/of/2005/1135/1906_Boatwright/download/1906_Boatwright_BA_intensity.jpg

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

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)

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

San Andreas Fault SystemPhoto: http://pubs.usgs.gov/gip/earthq3/map1a.gif

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

Loma Prieta Earthquake DamageLoma Prieta Earthquake Damage

Nimitz Freeway after the Loma Prieta Earthquake, 1989Photo: http://www.dot.ca.gov/hq/esc/geotech/photos/south2/cypress03.jpg

Column collapse along Cypress Viaduct, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom

Marina District after Loma Prieta Earthquake, 1989Photo: GSA, Explore Earthquakes CD-Rom

Damage to garages in Marina District, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom

House that slid off foundation during Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom

Collapsed walls of house, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom

Collapse of 5 story tower, Loma Prieta EQ, 1989Photo: GSA, Explore Earthquakes CD-Rom

Northridge Earthquake DamageNorthridge Earthquake Damage

Collapse of Interstate 5, Northridge EQ, 1994 Photo: http://pubs.usgs.gov/fs/1999/fs110-99/

Highway damaged during Northride EQ, 1994

Photo: http://boxer.senate.gov/students/resources/features/1906/committee.cfm

Highway Damage, Northridge Earthquake, 1994Photo: http://mceer.buffalo.edu/research/resilience/default.asp

Damaged Building, Northridge EQ, 1994 Photo: http://www.calstatela.edu/dept/geology/Geocareer.htm

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

How Earthquakes Cause DamageHow Earthquakes Cause Damage

PrimaryPrimary effects: Faulting (breaks in ground surface) Ground shaking (from seismic waves) SecondarySecondary effects: Landslides Tsunamis Fires

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

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…

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

Liquefaction in Niigata, 1964Photo: http://www.ce.washington.edu/~liquefaction/selectpiclique/nigata64/tiltedbuilding.jpg

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

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

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

1946 Tsunami Hilo, Hawaii. Caused by earthquake in Aleutian IslandsPhoto: http://static.howstuffworks.com/gif/tsunami-5.jpg

Damage from 1946 tsunami, Hilo, HawaiiPhoto: http://soundwaves.usgs.gov/2005/01/fieldwork2.html

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

Damage to hotel from Indian Ocean tsunami, 2004Photo: http://www.calstatela.edu/dept/geology/G351.htm

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

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

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

United States seismic hazard mapPhoto: http://pubs.usgs.gov/fs/2005/3038/images/seismic-hazard-map.jpg

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

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

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

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

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

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