Seismic Constraints and Coulomb Stress Changes of a Blind Thrust Fault System, 1: Coalinga and Kettleman Hills, California

This report reviews the seismicity and surface ruptures associated with the
1982-1985 earthquake sequence in the Coalinga region in California, and the
role of Coulomb stress in triggering the mainshock sequence and aftershocks.
The 1982-1985 New Idria, Coalinga, and Kettleman Hills earthquakes struck on a
series of west-dipping, en echelon blind thrust faults. Each earthquake was
accompanied by uplift of a Quaternary anticline atop the fault, and each was
accompanied by a vigorous aftershock sequence. Aftershocks were widely
dispersed, and are seen above and below the thrust fault, as well as along the
up-dip and down-dip projection of the main thrust fault. For the Coalinga and
Kettleman Hills earthquakes, high-angle reverse faults in the core of the
anticlines are evident in seismic reflection profiles, and many of these faults
are associated with small aftershocks. The shallowest aftershocks extended to
within 3-4 km of the ground surface. There is no compelling evidence for
aftershocks associated with flexural slip faulting. No secondary surface
rupture was found on any of the anticlines. In contrast, the 1983 Nunez rupture
struck on a high-angle reverse fault 10 km west of the Coalinga epicenter, and
over a 40-80-day period, up to 1 m of oblique surface slip occurred. The slip
on this Holocene fault likely extended from the ground surface to a depth of
8-10 km. We argue that both the Nunez and Kettleman earthquakes were triggered
by stresses imparted by the Coalinga mainshock, which was the largest of the
four events in the sequence.

[Summary provided by the USGS.]