Evidence for prehistoric earthquakes on the Superstition Hills fault from offset geomorphic features
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 6½.
Additional Information© 1989 Seismological Society of America. Manuscript received 4 August 1988. We are grateful to R. Wallace for his initial enthusiasm to pursue this study of the offset features along this fault. We thank D. Valentine at SDSU for helping with the surveying, T. Hanks at the USGS for providing us with new air photos, and P. Williams at L-DGO/C.I.T. for his discussions on afterslip. We especially thank D. Schwartz for his thoughtful review of our manuscript, which led to improvements and clarification of the presentation. This project was partially supported by USGS Grant no. 14-08- 0001-G1330.
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