ANALYSIS OF BROAD-BAND RECORDS FROM THE 28 JUNE 1992 BIG-BEAR EARTHQUAKE - EVIDENCE OF A MULTIPLE-EVENT SOURCE

The 28 June 1992 Big Bear earthquake occurred at 15:05:21 GMT and is c onsidered to be an aftershock of the earlier M(W) = 7.3 Landers earthq uake. From overall aftershock locations and long-period focal studies, rupture is generally assumed to have propagated northeast. No surface rupture was found, however, and the mainshock locations determined fr om both strong motion and TERRAscope data. are mutually consistent and do not Lie on the assumed fault plane. Further, directivity analysis of records from the TERRAscope array suggests significant short- and l ong-period energy propagating northwest along the presumed antithetic fault plane. This observation is supported by significant early afters hocks distributed along both the presumed rupture plane and the antith etic plane to the northwest. An empirical Green's function (eGf) appro ach using both the M(W) 5.2, 28 June 1992 14:43 GMT foreshock and the M(W) = 5.0 17 August 1992 aftershock produces consistent results and s uggests that the Big Bear event comprised at least two substantial sub events. From the eGf results, we infer that the second and possibly a third subevent occurred on the presumed (northeast striking) mainshock rupture surface, but that significant moment release occurred on the antithetic northwest striking surface. We present results from line-so urce fault modeling of broadband displacement recordings of the Big Be ar mainshock, which indicate that a two-fault event is necessary to pr oduce the observed waveforms. The limitations imposed by the mainshock location and directivity analysis require that the initial rupture be towards the northwest, striking 320 degrees. This was followed approx imately 4 sec later by bilateral rupture along a northeast-southwest f ault that strikes 50 degrees east of north.