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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? 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? 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 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 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 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 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 .ac.uk/Resources/EarthSci/Tectonics/images/ranch.jpg

Fault 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 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 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 Waves Ground vibrations produced by an earthquake Enable us to locate earthquakes and determine type of faulting that produced them 4 types: Body Waves a. P waves b. S waves Surface Waves a. Rayleigh waves b. Love 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 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 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 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 Magnitude of an earthquake is the main factor that determines the intensity and potential destructiveness of an earthquake Two scales: Richter magnitude Moment magnitude

Richter 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 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 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 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 Earthquakes Photo: http://pubs.usgs.gov/of/2005/1135/1906_Boatwright/download/1906_ Boatwright_BA_intensity.jpg

Earthquakes and Faulting Most earthquakes occur at plate boundaries Largest earthquakes occur at convergent boundaries on megathrust faults that form where one plate subducts beneath another Exs: Sumatra (2004), Alaska (1964) & Chile (1960): largest EQ ever recorded, magnitude 9.5

Intraplate Earthquakes A small percentage of earthquakes occur in plate interiors Foci are shallow and occur mostly on continents Many occur on old faults that use to be part of plate boundaries and are now areas of crustal weakness 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 Systems Zones of deformation between plate boundaries usually have a network of interacting faults – a fault system – 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 System Photo: http://pubs.usgs.gov/gip/ earthq3/map1a.gif

Earthquake 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 Prieta (mag 7.1 & $10 billion in damage) and 1994 Northridge (mag 6.8 & $40 billion in damage) – were among the costliest disasters in U.S. history because of nearby urban areas

Loma Prieta Earthquake Damage

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

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