Earthquake Research on the San Andreas Fault Zone, California

Abstract

The United States Geological Survey (USGS) carries out an extensive program of seismic, geodetic, and geologic studies in California that may be relevant to designing new experiments on the North Anatolian Fault Zone. A telemetered microearthquake network of ~350 stations monitors seismicity down to a threshold level of M=2 or better, accurately locates these events, and obtains fault-plane solutions for the larger earthquakes. Several types of repeated geodetic measurements determine the spatial and temporal pattern of crustal movements over a ~100 km wide zone across the major faults. Historic triangulation measurements determine the horizontal deformation in California since ~1900. Annually- repeated laser ranging (geodolite) meausrements of ~600 line lengths have precisely monitored the strain field since 1971. Lines vary in length from 1 to 35 km and are measured to a precision of 3 to 8 mm. Repeated alinement array measurements (~100 m aperture) and continuously recording wire strainmeters determine the slip rates on aseismically slipping (creeping) portions of the San Andreas fault system. Repeated leveling surveys monitor the vertical movement related to the compressional tectonics of the “big bend” region of the San Andreas in southern California. Both repeated and continuously recording magnetometer observations monitor magnetic field variations along major faults. Detailed geologic strip maps at 1:24,000 scale locate the most-recently active traces of major faults and are essential aids in planning geophysical measurement programs.