The Modern San Andreas Fault

Abstract

Although the San Andreas fault was once viewed by most geologists as a grossly aberrant tectonic feature, it is now recognized as representing normal plate-tectonic processes and is, in fact, one of the simpler of the transform faults that cut the continents. The fault zone is continuous for at least 1100 km, but individual surface traces are en echelon segments of much more limited extent, and they represent fracture processes that currently are not fully understood. Studies of both the fault's physiographic expression and its recent earthquake history emphasize the remarkable temporal repeatability of seismic events along it; during Holocene time, earthquakes and their associated effects have repeated one another at given localities with almost uncanny reproducibility. Two great earthquakes occurred along the fault in 1857 and 1906, and although two remaining segments have not broken in large events during the historic record, the most likely next great earthquake is a repeat of the 1857 event. Seismographic stations in California now number perhaps 500, and the recent dramatic increases have shown that epicenters tend to markedly "pull in" toward the active fault trace in some segments but not others. In general, the parts of the fault showing concentrated alignments of small shocks are the same parts that display continuous or episodic surface creep. Currently locked segments of the fault, presumably those of highest seismic hazard, are characterized by very low, scattered seismicity. Short-term epicenter maps are thus not necessarily a good representation of long-term seismic hazard, which is much better portrayed by geologic studies of the fault's Holocene history, using radiometric dating of offset and disturbed strata. The 1857 and 1906 earthquakes represent the types of events we must be prepared for in the future, although their engineering effects were not as great as often imagined. The hazards of both surface fault displacements and heavy ground shaking are not yet fully understood, but appear to be problems capable of solution in the foreseeable future.

Additional details