Seismically active wedge structure beneath the Coalinga anticline, San Joaquin basin, California

摘要

We define the subsurface geometry, kinematics, and seismotectonics of the Coalinga anticline in the San Joaquin basin, central California. Using seismic reflection data and quantitative fault-related folding techniques, we present a model of the Coalinga anticline that demonstrates that the structure is composed of a stack of imbricated structural wedges, related to two major fault ramps at depth, the deepest of which ruptured during the 1983 Coalinga (M w = 6.5) earthquake. Because of the lack of basinward deformation and the observed fold shapes, these ramps are interpreted to sole to a common upper detachment, which acts as a back thrust, forming a structural wedge. This back-thrust system generates the surface expression of the Coalinga anticline and extends to the surface as the Waltham Canyon fault and a series of related east dipping thrusts. This structural analysis helps reconcile the longstanding conflict between the southwest dipping preferred nodal plane of the 1983 main shock and the western vergence of the surface anticline. Furthermore, the seismic reflection data and our model suggest that two potentially seismogenic ramps and a major back thrust underlie the fold, rather than the single fault which has been inferred in previous studies. Using a relocated earthquake catalog, we document the three-dimensional distribution of earthquakes over a 22 year period relative to both the main fault which ruptured in the 1983 event and within the structural wedge. This analysis indicates that the majority of moment release following the 1983 event occurred within the wedge itself, compatible with a model of wedge emplacement.