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An Examination of the Surface Rupture Gap in the Landers Earthquake Between Johnson Valley and Long Canyon Faults, San Bernardino County, California

D. J. Crane
Chevron North America, Bakersfield, CA, [email protected]

The 85-km-long surface rupture associated with the 1992 Mv7.3 Landers earthquake exhibits a 5-km surface rupture gap from south of the Johnson Valley fault to north of the Long Canyon fault. Surface mapping of the crystalline outcrop in this region reveals deformation has occurred in the form of curvilinear faults, though undisturbed by the Landers event. A seismic gap in the hypocenter data subsequent to the Landers earthquake also underlies the curvilinear fault traces to a depth of about 3 km, suggesting seismic inactivity here may be due to these faults’ oblique alignment to the main north trending stress field that triggered the 1992 event. In cross sectional profile, the surface gap contains a high density of hypocenter sites associated with the Johnson Valley fault that end abruptly along a 77° north-plunging line that projects to a depth of 12 km from the surface trace of the Pinto Mountain fault. This implies the Pinto Mountain fault under-thrusts the gap region, suggesting it contributed in part to the ground rupture gap by damping propagation of seismic activity southward. Field relationships and seismic data suggest the curvilinear fault system is the southern extension of the Johnson Valley fault associated with the Landers earthquake and terminates against the high-angle oblique slip of the Pinto Mountain fault. This study proposes that these curvilinear fault traces resulted from directional changes in regional stress orientation between the Pinto Mountain and the Johnson Valley faults. In combination with the Pinto Mountain fault, the curvilinear faults are likely responsible for the ground rupture and seismic hiatus of the Landers Earthquake.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009