STRESS-FIELD ASSOCIATED WITH THE RUPTURE OF THE 1992 LANDERS, CALIFORNIA, EARTHQUAKE AND ITS IMPLICATIONS CONCERNING THE FAULT STRENGTH AT THE ONSET OF THE EARTHQUAKE

We investigate the space and time history of the shear stress produced on the fault during the 1992 Landers earthquake. The stress is direct ly calculated from the tomographic image of slip on the fault derived from near-source strong motion data. The results obtained shed some li ght on why the earthquake rupture cascaded along a series of previousl y distinct fault segments to produce the largest earthquake in Califor nia in over 40 years. Rupture on the 30 km long northernmost segment o f the fault was triggered by a large dynamic increase of the stress fi eld, of the order of 20 to 30 MPa, produced by the rupturing of the ad jacent fault segments. Such a large increase was necessary to overcome the static friction on this strand of the fault, unfavorably oriented in today's tectonic stress field. This misorientation eventually led to the arrest of rupture. The same mechanism explains why rupture brok e only a small portion of the Johnson Valley fault on which the earthq uake originally started, before jumping: to an adjacent fault more fav orably oriented. We conclude from these results that the dynamic stres s field could not sustain and drive the rupture along the strongly mis oriented NW-SE strands of the preexisting fault system. Instead, the d ynamic stress field produced new fractures favorably oriented in a N-S direction and connecting parts of the old fault system.