Abstract

Co-seismic flexural-slip folding of a small anticline along the Eastern Elmore Ranch fault associated with the Superstition Hills earthquake sequence demonstrates that variations in left-lateral slip can be explained locally by folding. Surface rupture on the eastern branch of the Elmore Ranch fault was greatest to the northeast of the fold but decreased to zero across the southern limb. A kilometer long surface-slip gap exists just southwest of the fold, being bounded on the northeast with 75 mm of slip and to the southwest with about 60 mm of slip. Lateral slip continues southwesterly to the faults intersection with the Superstition Hills fault.

The gentle-to-openly folded, moderately plunging, upright asymmetric anticline experienced left-lateral slip on the northern limb and right-oblique slip on the southern limb with maximum documented lateral slip along bedding planes of 10 mm. Slip values were greatest on the limbs and decreased to zero at the fold hinge, which is consistent with the flexural-slip folding process. Secondary left-lateral faults ruptured both limbs of the anticline and bedding plane slip was not observed hingeward of these faults.

These data suggest that left-lateral strain associated with seismogenic fault rupture was continuous in the subsurface but was accommodated locally by folding at the surface.

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