ANALYSIS OF THE 1986 MT-LEWIS, CALIFORNIA, EARTHQUAKE - PRESHOCK SEQUENCE MAINSHOCK AFTERSHOCK SEQUENCE

The 1986 Mt. Lewis earthquake (M(L) = 5.7) occurred on a right-lateral fault northeast of and oblique to the Calaveras fault in a region that had not experienced significant seismicity since 1943. Data from the nearby Lawrence Livermore Seismic Network and selected U.S. Geological Survey stations are used to relocate events within 15 km of the mainshock epicenter during the period 1980-1987, using the master-event method. Beginning 17 months before the mainshock, 22 events ruptured in the depth range 5-9 km within 1.4 km and northwest of the mainshock epicenter, in an area subsequently almost devoid of aftershocks. This cluster of preshock activity is clearly separated both spatially and temporally from the background activity in the surrounding area. Composite focal mechanisms for the preshocks and for nearby aftershocks suggest that there are two slightly different focal mechanisms amongst the preshocks, one being similar to the mainshock and aftershocks and one being rotated in strike. Cross-correlations of digitally recorded short-period waveforms of 10 of the clustered preshocks (M(L) almost-equal-to 1.5-2.5) reveal that the average inter-event peak cross-correlation between seismograms is 0.62. Six nearby early aftershocks show an average inter-event peak cross-correlation between seismograms of 0.54. Only a few aftershocks have cross-correlations of 0.6 or higher, which implies that the events were slightly further apart or multiple mechanisms were active during the early aftershock period. No significant differences are observed between the spectra of the preshocks and aftershocks. The aftershock area expanded along the strike with time, ultimately defining a north-south fault plane, 11 km in length, extending from 3 to 10 km in depth. The mainshock appears to have originated at the base of the seismogenic zone and ruptured bilaterally along strike and updip. A forward modeling technique is used to model teleseismic body waveform data of the mainshock. The long-period data (P and SH) are consistent with a point source strike-slip earthquake with a teleseismic moment of 3.9 x 10(17) N m. We infer that the mainshock involved the rupture of an asperity in the central portion of the aftershock zone.