Slip on the Elmore Ranch fault during the past 330 years and its relation to slip on the Superstition Hills fault

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Abstract
The Elmore Ranch fault is a left-lateral fault that strikes northeast within the right-lateral transform boundary that strikes northwest through southern California. It lies transverse and adjacent to the segment of the Superstition Hills fault that ruptured in 1987. Rupture of the Elmore Ranch fault (MS = 6.2) preceded rupture of the Superstition Hills fault (MS = 6.6) by about 11.4 hr. The Elmore Ranch fault slipped at the surface in 1987 with left-lateral displacements of up to 130 mm. Geological data indicate that it had slipped prehistorically, sometime after about 1660 A.D., probably in a single event. Excavations at three sites enable the following comparisons: The Superstition Hills fault has also been documented to have experienced one slip event between ∼1660 and 1987. Thus these slip events on the Elmore Ranch and Superstition Hills faults may have occurred in a sequence similar to that in 1987. Neither the main fault nor the cross-fault, however, appear to have exactly duplicated their previous surficial slip. Previous slip was probably smaller on the Superstition Hills fault and larger on the Elmore Ranch fault zone than in the 1987 event. Because the temporal correlation between previous slip events is not proven, rupture sequences other than a doublet in which main fault rupture follows cross-fault rupture are possible.