THE STATE OF STRESS ON SOME FAULTS OF THE SAN-ANDREAS SYSTEM AS INFERRED FROM NEAR-FIELD STRONG-MOTION DATA

We present a simple method to calculate the stress produced on an eart hquake fault during rupture. This method allows the complete evaluatio n of the stress spatio-temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault sys tem over the last 20 years: the Imperial Valley earthquake of 1979, th e Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomog raphic models of the fault rupture obtained from the inversion of the near-field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary great ly over the fault. The peak values obtained for the four earthquakes s tudied range from about 20 to 100 MPa. These high values imply that th e initial shear stress level on the fault at the onset of the earthqua ke was high on at least a significant portion of the fault. The region s of the fault which break with a high stress drop are also the region s where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the ''strong'' areas of the fault . Low slip regions tend to break with low stress drops. After the eart hquake, the shear stress is increased over a significant portion of th e fault, which corresponds to low slip regions. Aftershock activity te nds to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heter ogeneous. The rupture velocity seems to be inversely correlated with t his apparent fault strength, the rupture accelerating over the ''weak' ' areas of the fault and slowing down over the high strength areas.