Paleomagnetism-based limits on earthquake magnitudes in northwestern metropolitan Los Angeles, California, USA

We have used paleomagnetism to estimate the maximum moment magnitude (M (sub w) ) of earthquakes in the east Ventura Basin and San Fernando Valley, in metropolitan Los Angeles. Magnetic declinations show differential rotation between crustal blocks with linear dimensions of 10-20 km, similar to the thickness of the seismogenic layer. The maximum magnitude of an earthquake based on blocks of this size is M (sub w) = 6.8, comparable to the 1971 San Fernando and 1994 Northridge earthquakes and consistent with paleoseismic trenching and surface ruptures of the 1971 earthquake. The paleomagnetic results suggest that the blocks have retained their configuration while moving relative to each other for the past approximately 0.8 m.y. Therefore, it is unlikely that in this area multiple blocks combined to trigger much larger shocks during this period, in contrast to adjacent regions where events with M (sub w) >7 have been postulated on the basis of paleoseismic excavations.