SEISMOTECTONICS OF STRIKE-SLIP PLATE BOUNDARIESNon-Ridge Transform/Transcurrent Faults

San Andreas strike-slip boundary betweenPacific and North American plates.

Right-lateral strike-slipfault. Formally a transform fault connecting spreadingridge in Gulf ofCalifornia to ridgetransform fault ofMendocino fault.

The San Andreasdoes not have uniform frictionalsliding properties.Stick-slip in northand south, but stablesliding (creeping)in the middle.

The creeping sectionhas many smallevents, but no recordof large events.

Last large event innorth was 1906,in south was 1857.

Strike-slip faults tend to have irregularitiesthat produce steps, jogs, and associatedalong-strike features.

Salton Sea is depressionin a right-stepping jumpin the fault between theImperial fault and SanAndreas. This is likelyhard for earthquake torupture through, so issouthern limit of bigruptures on San Andreas.

Transpression inthe big bendleads to slippartitioning withthrust on shallowdipping faults onsouthern side ofthe San GabrielMountains andvertical strike-slipon the San Andreas.

TRIPLE JUNCTIONS EVOLVE & MIGRATE WITH TIME:Pacific-North America - Farallon (Juan de Fuca) evolution caused

formation of San Andreas and opening of Gulf of California

USGS

TRIPLEJUNCTIONSEVOLVE &

MIGRATE WITHTIME

Kula-Farallon-Pacific TJmigrates

Kula platevanishes

Farallon breaksup into Juan de

Fuca, Rivera, andCocos

Cocos-Nazcaspreading center

forms

Atwater website

Atwaterwebsite

Atwaterwebsite

FORMATION OFTRANSVERSERANGES

LA

SAF

There are a lot of seismic stations operatingin California, most areoperated by the USGS,Caltech, Berkeley andCDMG.

With so many stations, wehave extensive catalogsof small earthquakes.

Events prior to 1906lack seismic recordings,but are well documentedfrom shaking reportsand surface offsets.

1872 is Owen’s valleyNormal faulting.

1952 is Kern CountyEarthquake on GarlockFault

1868 is on Hayward fault

1872

1857

18681906

1838

1812

1952

1992

Seismicity near the San Andreas fault is shallow, almost all events are at depthsless than 15 km.

The northern end ofthe San Andreas isthe Cape Mendocinotriple junction withMendocino transformfault, Gorda plate subduction zone, andSan Andreas Fault.

The seismicity is lowon the SAF. High inlandin the crust above theslabless window.

The Bay Area is distinctivein splaying of the San Andreas into the Calaverasand Hayward faults andthe northward extension alongthe Rodgers Creek Fault.

The creeping sectionhas lots of small eventsand ends just northof Parkfield, which hashad repeated magnitude6.0 size events. 6 since1857. The SAF is veryquiet to the south, alongthe rupture zone of the1857 event.

With extensive seismic stations, can determine focal mechanisms for all eventsdown to very small magnitude of ~2.0.