FORESHOCK SEQUENCE OF THE 1992 LANDERS, CALIFORNIA, EARTHQUAKE AND ITS IMPLICATIONS FOR EARTHQUAKE NUCLEATION

The June 28, 1992, Landers, California, earthquake (M(w)=7.3) was prec eded for about 7 hours by a foreshock sequence consisting of at least 28 events. In this study we examine the geometry and temporal developm ent of the foreshocks using high-precision locations based on cross co rrelation of waveforms recorded at nearby stations. By aligning wavefo rms, rather than frying to obtain travel time picks for each event ind ependently, we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals. We pe rform a joint relocation using the improved travel times and reduce th e relative location errors to less than 100 m horizontally and less th an 200 m vertically. With the improved locations the geometry of the f oreshock sequence becomes clear. The Landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane. The nucleation zone as defined by the foreshock sequence is southeast trending to th e south and nearly north trending to the north of the right step. This geometry is confirmed by the focal mechanisms of the foreshock sequen ce, which are right-lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault, and are rotated clockwise for foreshocks that occur within the step. The exte nt of the foreshock sequence is approximately 1 km both vertically and horizontally. Modeling of the Coulomb stress changes due to all previ ous foreshocks indicates that the foreshocks probably did not trigger each other. This result is particularly clear for the M(w)=4.4 immedia te foreshock. Since Stress transfer in the sequence appears not to hav e played a significant role in its development, we infer an underlying aseismic nucleation process, probably aseismic creep. Other studies h ave shown that earthquake nucleation may be controlled by fault zone i rregularities. This appears to be true in the case of the Landers eart hquake, although the size of the irregularity is so small that it is n ot detectable by standard location techniques.