SEISMICITY REMOTELY TRIGGERED BY THE MAGNITUDE 7.3 LANDERS, CALIFORNIA, EARTHQUAKE

The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remar kably sudden and widespread increase in earthquake activity across muc h of the western United States. The triggered earthquakes, which occur red at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity an d strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress chang es calculated for elastic models of the earthquake appear to be too sm all to have caused the triggering. The most promising explanations inv olve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps includin g crustal magma).