Seismic constraints and coulomb stress changes of a blind thrust fault system, 2: Northridge, California

Open-File Report 2006-1158
See related OFR 2006-1149
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Abstract

We review seismicity, surface faulting, and Coulomb stress changes associated with the 1994 Northridge, California, earthquake. All of the observed surface faulting is shallow, extending meters to tens of meters below the surface. Relocated aftershocks reveal no seismicity shallower than 2 km depth. Although many of the aftershocks lie along the thrust fault and its up-dip extension, there are also a significant number of aftershocks in the core of the gentle anticline above the thrust, and elsewhere on the up-thrown block. These aftershocks may be associated with secondary ramp thrusts or flexural slip faults at a depth of 2-4 km. The geological structures typically associated with a blind thrust fault, such as anticlinal uplift and an associated syncline, are obscured and complicated by surface thrust faults associated with the San Fernando fault that overly the Northridge structures. Thus the relationship of the geological structure and topography to the underlying thrust fault is much more complex for Northridge than it is for the 1983 Coalinga, California, earthquake. We show from a Coulomb stress analysis that secondary surface faulting, diffuse aftershocks, and triggered sequences of moderate-sized mainshocks, are expected features of moderate-sized blind thrust earthquakes.
Publication type Report
Publication Subtype USGS Numbered Series
Title Seismic constraints and coulomb stress changes of a blind thrust fault system, 2: Northridge, California
Series title Open-File Report
Series number 2006-1158
DOI 10.3133/ofr20061158
Edition Version 1.0; Revised and reprinted
Year Published 2006
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Earthquake Hazards Program, Earthquake Science Center
Description 17 p.
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