Evidence for Prehistoric Earthquakes on the Superstition Hills Fault from Offset Geomorphic Features by Scott

Offset geomorphic features along the Superstition Hills fault show evidence for at least one slip event prior to the 1987 surface rupture, and possibly as many as four to five earlier prehistoric earthquakes. We documented several geomorphic features that appeared offset by multiple events by making detailed topographic maps. Offset features were abundant along reaches of the fault with high topographic relief and large displacement. Slip distribution for the penultimate event, as recorded by offset rills, streams, and shrub-coppice dunes, is very similar to the slip distribution from the 1987 earthquake through April 1988. This similarity may prove to be fortuitous if afterslip from the 1987 event continues to increase the total slip for this earthquake. But if afterslip associated with the 1987 event ceases in the near future, then the past two earthquakes were nearly identical in slip, and the Superstition Hills fault may be expected to produce characteristic earthquakes of roughly magnitude 4 . INTRODUCTION Paleoseismic studies of strike-slip faults have utilized offset streams and other features to determine the slip per event, length of rupture, and magnitude of prehistoric earthquakes (Wallace, 1968; Sieh, 1978; Sieh and Jahns, 1984; Rockwell and Pinault, 1986; Zhang et al., 1987). Tectonically offset geomorphic features have enabled these and other workers to characterize a particular fault's prehistoric activity and assess its future seismic potential. Sieh (1978) measured stream offsets to show the slip distribution for the 1857 M > 8 earthquake on the San Andreas fault in south-central California. Rockwell and Pinault (1986), grouping offsets of similar amounts, determined the slip per event and estimated the magnitudes of prehistoric earthquakes on a segment of the Elsinore fault in the Coyote Mountains of southern California. The Superstition Hills fault ruptured in the 24 November 1987 Superstition Hills earthquake (Ms = 6.6) (Sharp et al., 1989). Displacements from the 1987 earthquake, as well as apparent offsets that appear to represent prehistoric slip events, have been recorded by offset rills, streams, and shrub-coppice dunes (mounds of aeolian sand accumulated at the base of a bush) along the Superstition Hills fault. Surface rupture from the November 1987 earthquake provided calibration for studying slip from past events, using offset geomorphic features. The concept of similar amounts of slip per event or "characteristic" earthquakes was first implied by Wallace (1970) when he presented the idea of average recurrence intervals. The quotient of slip per event and the long-term slip rate, or recurrence interval, assumes that slip in an earthquake will be repeated by a similar amount in future events. Schwartz and Coppersmith (1984) have shown that some individual faults and fault segments produce repeated similar size earthquakes. Their charac* Present Address: Lindvall, Richter & Associates, 825 Colorado Blvd., Los Angeles, California 90041.