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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