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Seismotectonics of the Northern Elsinore fault zone, southern California

Alan G. Hull, & Craig Nicholson

Published 1992, SCEC Contribution #1575

Earthquake hypocenters and focal mechanisms, within and adjacent to the Elsinore-Temecula trough, are used to determine the contemporary pattern of subsurface deformation and fault geometry along the northern Elsinore fault zone. Instrumentally-recorded earthquakes with magnitudes ML ≥ 1.5 (1979 to 1989) were relocated using a velocity model derived from previous seismic refraction and velocity inversion studies in the Peninsular Ranges. Single-event focal mechanisms were determined for all events with at least 10 P-wave first-motions. Our preferred hypocentral locations, combined with gravity modeling and additional geological information, suggest considerable curvature and oblique motion along the Glen Ivy North fault. The fault at depth curves westwards away from the more linear surface trace, and changes dip from near-vertical at Lake Elsinore to ~70°S and then 70°-75°SW along the base of the Santa Ana Mountains. Along the Wildomar and Willard faults, earthquakes tend to cluster, and focal mechanisms exhibit a mixed pattern of strike- and oblique-slip motion, particularly near the extrapolated intersection with the E-W oriented Murrieta Hot Springs fault. Distinct aseismic zones extending to depths of 10 km correlate with the two known areas of active subsidence in the Elsinore-Temecula trough. Average P-axis from earthquake focal mechanisms for the 74 events analyzed, both on and off the fault zone, trends N01°W±17°—or about 45°-55° from the various NW-striking segments of the northern Elsinore fault zone. The observed pattern of oblique-slip and non-vertical dips along active segments of the northern Elsinore fault zone is consistent with their young age (~2.5 Ma), their small cumulative horizontal offset (< 15 km), and with a model in which oblique strain has yet to be partitioned between vertical strike-slip and subparallel low-angle dip-slip faults, as is observed along segments of the San Andreas fault zone in central California.

Hull, A. G., & Nicholson, C. (1992). Seismotectonics of the Northern Elsinore fault zone, southern California. Bulletin of the Seismological Society of America, 82, 800-818.