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Original forms and initial modifications of the Galway Lake Road scarp formed along the Emerson fault during the 28 June 1992 Landers, California, earthquake

Department of Geological and Environmental Sciences Stanford University, Stanford, California 94305-2115
Department of Geology Whittier College, Whittier, California 90608

Abstract

A remarkable scarp formed southeast of the Galway Lake Road during the M_w 7.3 Landers, California earthquake. Over the first 11 months following the earthquake, we repeated topographic surveys and ground stereoscopic photography, and interpreted the observations of original forms and initial modifications of the faulted landforms.

The complexity of the surface rupture influenced the geomorphic responses. The principal shear zone consisted of en-échelon right- and left-lateral, generally left-stepping, fault segments with cumulative offsets of 5 m right-lateral and 2.5 m vertical. Typical fault scarp profiles have different slope angles above and below the surface rupture and a small free face. The surface rupture formed steep, irregular knickpoints in channels crossing the scarp. The original channel surfaces were warped as they stepped down over tilted and rotated fracture-bounded blocks.

The landforms were modified measurably by the above-average winter 1992-1993 rainfall. Over surfaces controlled by hillslope processes only, fault scarp erosion was controlled by rainsplash and slumping. Originally sharp corners were rounded and loose sediment collected in colluvial wedges. Surface flows over simple ramp steps in channel profiles caused incision that drove knick-points back by as much as 50 cm. Over more complex portions of the principal shear zone, flow was diverted into open fractures and little modification of the scarp occurred.

The pre-earthquake topography at the site provided no clear evidence of the last similar slip event. Simple diffusion erosion models of the time required to bring the new scarp to its pre-Landers event configuration indicate that the last major rupture may have been several thousand years ago.

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