Preearthquake and Postearthquake Creep on the Imperial Fault and the Brawley Fault Zone

Abstract

Taken together, 12 years of alinement-array data, 4 years of creepmeter records from four instruments, and 2 years of surveys from two nail files suggests that creep events on the Imperial fault 2 to 5 months before the October 15 earthquake are consistent with longterm trends and not indicative of any imminent event. No discernible creep occurred on the fault in the hours and days before the earthquake. Records of coseismic displacement imply that response of the soil to the fault slip at depth was brittle rather than plastic; they uniquely demonstrate that the minimum rate of surface fault displacement was 1.8 cm/s. Continuing measurements of afterslip show that all motion is due to discrete 0.2- to 1.5-cm creep events occurring less frequently over time. The accumulating displacement for the first 35 days after the earthquake is well approximated by linear logarithmic functions of time. Use of this accumulating displacement to predict future slip rates implies that for 6 years the afterslip rate from the 1979 earthquake should be greater than the 0.5-cm/yr average preearthquake creep rate. The maximum amount of slip on the surface trace of the Imperial fault associated with the 1979 earthquake, including afterslip, amounts to more than 60 cm.