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

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.

Acticle consists of 17 pages, including 14 figures.

[Summary taken from USGS Open-File Report at
http://pubs.usgs.gov/of/2006/1158/of2006-1158.pdf