THE 1992 LANDERS EARTHQUAKE SEQUENCE - SEISMOLOGICAL OBSERVATIONS

The (M(W)6.1, 7.3, 6.2) 1992 Landers earthquakes began on April 23 wit h the M(W)6.1 1992 Joshua Tree preshock and form the most substantial earthquake sequence to occur in California in the last 40 years. This sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long. The f aulting was predominantly strike slip and three main events in the seq uence had unilateral rupture to the north away from the San Andreas fa ult. The M(W)6.1 Joshua Tree preshock at 33-degrees-N58' and 116-degre es-W19' on 0451 UT April 23 was preceded by a lightly clustered foresh ock sequence (M less-than-or-equal-to 4.6) beginning 2 hours before th e mainshock and followed by a large aftershock sequence with more than 6000 aftershocks. The aftershocks extended along a northerly trend fr om about 10 km north of the San Andreas fault, northwest of Indio, to the east-striking Pinto Mountain fault. The M(W)7.3 Landers mainshock occurred at 34-degrees-N13' and 116-degrees-W26' at 1158 UT, June 28, 1992, and was preceded for 12 hours by 25 small M less-than-or-equal-t o 3 earthquakes at the mainshock epicenter. The distribution of more t han 20,000 aftershocks, analyzed in this study, and short-period focal mechanisms illuminate a complex sequence of faulting. The aftershocks extend 60 km to the north of the mainshock epicenter along a system o f -at least five different surficial faults, and 40 km to the south, c rossing the Pinto Mountain fault through the Joshua Tree aftershock zo ne towards the San Andreas fault near Indio. The rupture initiated in the depth range of 3-6 km, similar to previous M approximately 5 earth quakes in the region, although the maximum depth of aftershocks is abo ut 15 km. The mainshock focal mechanism showed right-lateral strike-sl ip faulting with a strike of N10-degrees-W on an almost vertical fault . The rupture formed an arclike zone well defined by both surficial fa ulting and aftershocks, with more westerly faulting to the north. This change in strike is accomplished by jumping across dilational jogs co nnecting surficial faults with strikes rotated progressively to the we st. A 20-km-long linear cluster of aftershocks occurred 10-20 km north of Barstow, or 30-40 km north of the end of the mainshock rupture. Th e most prominent off-fault aftershock cluster occurred 30 km to the we st of the Landers mainshock. The largest aftershock was within this cl uster, the M(W)6.2 Big Bear aftershock occurring at 34-degrees-N10' an d 116-degrees-W49' at 1505 UT June 28. It exhibited left-lateral strik e-slip faulting on a northeast striking and steeply dipping plane. The Big Bear aftershocks form a linear trend extending 20 km to the north east with a scattered distribution to the north. The Landers mainshock occurred near the southernmost extent of the Eastern California Shear Zone, an 80-km-wide, more than 400-km-long zone of deformation. This zone extends into the Death Valley region and accommodates about 10 to 20% of the plate motion between the Pacific and North American plates . The Joshua Tree preshock, its aftershocks, and Landers aftershocks f orm a previously missing link that connects the Eastern California She ar Zone to the southern San Andreas fault.