SOUTHERN SURFACE RUPTURE ASSOCIATED WITH THE M-7.3 1992 LANDERS, CALIFORNIA, EARTHQUAKE

Although most evidence suggests that the 28 June 1992 M 7.3 Landers ea rthquake ruptured unilaterally north, significant surface rupture was mapped on the Eureka Peak and Burnt Mountain faults, to the south of t he Landers epicenter. An eyewitness account reports that surface ruptu re occurred on the northern Eureka Peak fault within approximately 35 sec of the mainshock initiation. Array analysis of the Landers mainsho ck provides evidence in support of this report; a significant southern subevent in the early mainshock coda. I also analyze dense array reco rdings of a M 5.6 aftershock that occurred 3 min after the mainshock a t 34-degrees 7.65'N, 116-degrees 23.82'W and show that there is strong evidence that this event was also associated with significant rupture on the Eureka Peak fault. This analysis thus suggests that the Eureka Peak fault rupture was not caused by direct bilateral mainshock ruptu re but instead was initially triggered less than a minute after the ma inshock and reruptured by the M 5.6 aftershock. Results for the evolut ion of the Landers sequence suggest that mainshock subevents may in so me cases be accurately described as aftershocks (i.e., disjoint trigge red events) that occur within the duration of mainshock strong ground motion.