VISCOELASTIC COUPLING MODEL OF THE SAN-ANDREAS FAULT ALONG THE BIG BEND, SOUTHERN CALIFORNIA

The big bend segment of the San Andreas fault is the 300-km-long segme nt in southern California that strikes about N65 degrees W, roughly 25 degrees counterclockwise from the local tangent to the small circle a bout the Pacific-North America pole of rotation. The broad distributio n of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional m odel of strain accumulation (continuous slip on the fault below the lo cking depth at the rate of relative plate motion), whereas the observe d seismicity and laboratory data on fault strength suggest that the lo cking depth should be no greater than 10 to 15 km. The discrepancy is explained by the viscoelastic coupling model which accounts for the vi scoelastic response of the lower crust. Thus the broad distribution of deformation observed across the big bend segment can be largely assoc iated with the San Andreas fault itself, not subsidiary faults distrib uted throughout the region. The Working Group on California Earthquake Probabilities [1995] in using geodetic data to estimate the seismic r isk in southern California has assumed that strain accumulated off the San Andreas fault is released by earthquakes located off the San Andr eas fault. Thus they count the San Andreas contribution to total seism ic moment accumulation more than once, leading to an overestimate of t he seismicity for magnitude 6 and greater earthquakes in their Type C zones.