Faults and Earthquakes

• Some faults become “locked”– Pressure pushes together the irregular walls of the

fault; surfaces resist sliding

Slip can’t occur until enough stress is built up to overcome resistance

Earthquake !

Focus - point of initial rupture

Epicenter - point on surface directly above the focus

The energy released by the earthquake increases with (a) the area of the fault plane that slips(b) the slip distance(c) the strength of the rocks

It’s easy to break contact points on a small section of fault, that allows a little slip. Large slip on big sections of a fault, requires an rare alignment of contacts points. therefore larger earthquakes are rarer than small earthquakes.

Energy released by global earthquakes, April 30-May 7, 2009

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

1,000,000,000

1 10 100 1000# of quakes larger than E

E, energy per quake (MJ)

Earthquake Damage - ground

displacement

Earthquake Damage - shaking

San Francisco, 1906

Earthquake Damage - fire

Measurement of Earthquake Magnitude, continued

– M 5 earthquake versus M 7 earthquake• M 7 has 100X stronger shaking, ~900X the energy

Magnitude is measured on a Logarithmic Scale An increase in 1 whole number represents an earthquake that has

10 times stronger ground motion ~30 times as much energy release

Sources of Earthquake Damage

shaking

liquifaction- shaking can cause water-saturated soil to behave like a fluid: buildings sink or tip over

fire

tsunami

landslides

ground displacement

Dangerous factors contributing to earthquake deaths

Dense populations

Building style

Time of day

Relief infrastructure

Earthquake Hazard

California Earthquakes

1989 Loma Prieta -6.9

1999 Hector Mine 7.1

1992 Landers 7.31994 Northridge 6.9

1972 San Fernando 6.6

1933 Long Beach 6.4

1952 Kern County 7.3

1872 Owens Valley 7.6

1857 Fort Tejon 7.9

1983 Coalinga 6.7

1906 San Francisco 7.9

Tsunami

(incorrectly called “tidal wave”)

Open seaWave height: cm to 1 mSpeed: 750 km/hr

Near shoreWave height: up to tens of meters

Seismically-induced sea wave

2004 Indian Ocean Tsunami

2004 Sumatra “Tsunami” Earthquake

Dec. 26, 2004 : M 9.0 earthquake (4th largest recorded) occurs off northern Sumatra; ~500 km section of fault moves, with max. of 20 m of slip

Several meters of seafloor movement triggers tsunami

Several meters of seafloor movement triggers tsunami

Wave height on Sumatra - 30 m Thailand - 10 m Sri Lanka - 15 m India - 5 m