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Description
The MS=6.6 November 24, 1987 earthquake on the Superstition Hills fault produced a surface rupture along the entire 23.5 km length of the fault. Total maximum slip for this earthquake as of April, 1988, was about 80 cm. Offset geomorphic features along the Superstition Hills fault revealed a pre-1987 slip event and possibly as many as 4 to 5 other prehistoric earthquakes. Using electronic surveying instruments, detailed topographic maps were made of several multiply offset geomorphic features. Multiple offset data were abundant along reaches of the fault with high topographic relief and large displacement. The slip distribution of the penultimate event, clearly defined by offset rills, streams, and bush 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 1987 afterslip ceases in the near future, then the last two earthquakes were nearly identical in slip, and the Superstition Hills fault may be expected to produce characteristic earthquakes of roughly magnitude 6 1/2. On the San Andreas fault near Tejon Pass, California, Gorman Creek is deflected by a shutter ridge and four active strands of the fault. From the unique geometry of the Gorman Creek deflection, a minimum offset of 75 ± 5 m was established. The stream is deeply incised into the widespread geomorphic surface of Peace Valley. Two charcoal samples from the sediments below the geomorphic surface were dated at 4,030 +193/-191 yr BP and 2,875 +130/-113 yr BP. The youngest age of 2,875 yr BP represents a maximum age for both the surface and the downcutting of Gorman Creek. The distance of 75 ± 5 m and the age of 2,875 +130/ -113 yr BP yield a minimum late Holocene slip rate of 26.1 ± 2.8 mm/yr for the Big Bend section of the San Andreas fault. This slip rate is an minimum rate for three reasons: 1) a minimum offset distance was used; 2) the 2,875 age overestimates the true age of the offset; and 3) all active strands of the fault may not be constrained. These conditions suggest that the true slip rate may be significantly greater than 26 mm/yr. This minimum slip rate of 26 mm/yr in the Big Bend along with other slip rates for the south-central segment, suggest that the uniform-slip model best represents the Holocene behavior of the San Andreas fault in the Big Bend.
