Date of this Version
The June 28, 1992, Landers, California, earthquake(Mw=7.3) was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events. In this study we examine the geometry and temporal development of the foreshocks using high-precision locations based on cross correlation of waveforms recorded at nearby stations. By aligning waveforms, rather than trying to obtain travel time picks for each event independently, we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals. We perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100m horizontally and less than 200m vertically. With the improved locations the geometry of the foreshock sequence becomes clear. The Landers foreshocks occurred at a fight step of about 500m in the mainshock fault plane. The nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step. This geometry is confirmed by the focal mechanisms of the foreshock sequence, which are rightlateral 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 extent of the foreshock sequence is approximately 1 km both vertically and horizontally. Modeling of the Coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other. This result is particularly clear for the Mw=4.4 immediate foreshock. Since stress transfer in the sequence appears not to have played a significant role in its development, we infer an underlying aseismic nucleation process, probably aseismic creep. Other studies have shown that earthquake nucleation may be controlled by fault zone irregularities. This appears to be true in the case of the Landers earthquake, although the size of the irregularity is so small that it is not detectable by standard location techniques.