Earthquakesand Faults

Crustal Strength Profile

Great San Francisco Earthquake

April 18, 1906, 5:12 AMQuake lasted about 60 secondsSan Francisco was devastated by:– Ground shaking– Fires

Most damaging earthquake in US historyFirst quake studied in detail by geologistLead to scientific understanding of quakes

1906 San Francisco

San Francisco Burning Damage

Dead- more than 3000Homeless- 225,000Buildings destroyed- 28,000

Louis Agassiz after the 1906 San Francisco earthquake

Surface Rupture, Olema, CA

Fault Offset2.5 m (8 ft)

1906Rupture-

430 km long

Key Observations

Earthquake was linked to a major faultA huge area was displaced horizontallyDisplacement was sudden

Elastic Rebound Theory

Crust is like a springDeformation (strain) is stored as the fault-blocks moveEventually the fault fails

Strain is released -> QuakeStrain begins to accumulate againetc...

Elastic Rebound (.mov)Earthquake Cycle

Earthquakes are recurrent– Small quakes are frequent– Large quakes are rare

Cycle may be 100s of years for large quakesStrain builds up until an earthquake releases itSize of quake depends on amount of strain stored

Number of Quakes per year in Southern California

Turcotte, 1992

Terminology

Focus- origin of ruptureEpicenter- point at the surface above the focusFault scarp- vertical offset at the surface

Scarp, Alaska 1964

Seismic Waves– Body waves

• P-wave (Primary Waves)• S wave (Secondary Waves)

– Surface waves- most damaging• Love waves• Rayleigh waves

P-Waves

Primary wavesP-waves

Pressure wavesLike soundparticles move in same direction as wave propagationFastest

Secondary WavesS-waves

Shear WavesParticles move perpendicular to wave directionSlower than P-wavesCannot pass through liquids

S-WavesS-Waves

Surface Waves-like sea waves Seismic Record

Focal mechanism Effects of earthquakes

Ground motion FireLandslidesLiquifactionPermanent displacement of land surfaceAfter shocksTsunamis-

Anatolian Earthquake- Turkey 1999

Turkey Quake Triggered Landslide

El Salvador

Kaltag Fault

Denali Fault

Denali Fault, Alaska

Slides Triggered by Denali fault Quake 2003

GPS velocity fieldHector Mine

Quake1999

M=7.0

Fault Map

Mohave Desert0.3 sec Hector Mine

1 sec Hector Mine 3 sec Hector Mine

Vertical Displacement FieldHector Mine Quake interferogram from Synthetic Aperture Radar data. Each fringe is 28 mm of ~vertical displacement.

Hector Mine fault

Landers fault

D.T. Sandwell, EOS 11/11/03

Hector Mine Focal

Mechanism

Shaking during Loma Prieta Quake, 1989

Sequential Ruptures of the Anatolian Fault

Earthquakes prediction

Scientific techniques being explored– Patterns of earthquakes in space & time– Microseisms– Surface tilts & changes of elevation (Strain

accumulation)

Quake probability in the next 50 years

San Francisco Bay

San Francisco Bay

San Andreas Fault

San Andreas Fault Risk1988-2018

New Madrid Seismic

ZoneBrown –74-2000

Green- before 74