Southern surface rupture associated with the 1992 M7.4 Landers Earthquake: Did it all happen during the mainshock?

Approximately three minutes after the magnitude 7.4 Landers mainshock on 28 June 1992, a M5.7 aftershock occurred south of the mainshock epicenter, with a location of 34° 7.65′N, 116° 23.82′W. This aftershock was recorded on an array of portable digital seismic instruments deployed in Morongo Valley, 21 km southwest of the event. Although peak accelerations are found to differ by approximately 50% at stations with similar general site conditions within 500 m of each other, there is good coherence of arrivals across the array for frequencies ≤1 Hz. We use the recordings to determine the apparent phase velocity and azimuth of propagation across the array, and show that the event clearly ruptured to the south, with a rupture length of ∼11 km and a rupture velocity of approximately 3.0 km/s. Our results suggest that at least some of the mapped surface rupture south of the town of Yucca Valley (∼11 km in extent; maximum displacement of 20 cm) may have been associated with this aftershock. If this is the case, then the fault that produced the southern end of the Landers mainshock (the Johnson Valley fault) need not continue at depth across the active left-lateral, east-west trending Pinto Mountain fault.