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

Abstract

[1] 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.

Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone

“…The 1983 M W 6.5 Coalinga earthquake, for instance, had maximum surface uplift of 0.5 m from a combined rupture length of 20-35 km with primary and secondary slip on several shallow (<30°) ramp thrusts and bedding-plane reverse faults above a 10 km main shock depth (Eaton, 1990;Stein and Ekström, 1992). However, the maximum surface uplift was not on a fault scarp but occurred directly above an actively growing fault-bend fold at depth, where no beddingplane reverse faults were observed at the surface (Guzofski et al, 2007).…”

“…Bedding-plane faults have ruptured coseismically and produced surface scarps in conjunction with large earthquakes on deeper structures (Lensen and Otway, 1971;Philip and Meghraoui, 1983;Hull, 1990;Treiman, 1995;Yeats, 2000). While in some cases bedding-plane faults above thrust wedges and passive roof duplexes root in thrust ramps or roof thrusts (Banks and Warburton, 1986;Guzofski et al, 2007), in other cases bedding-plane reverse faults root in the axial plane region of folds (Shaw and Suppe, 1994;Ishiyama et al, 2004).…”

“…Bedding-plane slip at shallow to intermediate (3-12 km depth) levels is implicated for earthquakes in many convergent and obliquely convergent structural settings in addition to the Seattle fault zone (Philip and Meghraoui, 1983;Yeats, 1986;McInelly and Kelsey, 1990;Tsutsumi and Yeats, 1999;Guzofski et al, 2007). Bedding-plane faults have ruptured coseismically and produced surface scarps in conjunction with large earthquakes on deeper structures (Lensen and Otway, 1971;Philip and Meghraoui, 1983;Hull, 1990;Treiman, 1995;Yeats, 2000).…”

“…Folds related to such faults often are manifest at the surface (Wentworth and Zoback, 1990;Stein and Ekström, 1992;Guzofski et al, 2007) or in near-surface boreholes (Dolan et al, 2003), and such folds aid in describing the character (recurrence interval, magnitude, and coseismic slip amounts) of blind, master-ramp earthquakes. Blind faults pose another potential seismic hazard through coseismic folding (Chen et al, 2007).…”

Recent deformation of Quaternary sediments as inferred from GPR images and shallow P-wave velocity tomograms: Northwest Canterbury Plains, New Zealand

“…Buried or blind faults that have the potential to rupture as large earthquakes make the construction of reliable seismic hazard and risk maps extremely difficult in numerous regions worldwide (Ambraseys, 1981;Calais et al, 2010;Dolan et al, 2003;Feng et al, 2010;Guzofski et al, 2007;Jackson, 2006;Leon et al, 2009;Lettis et al, 1997;MonaLisa et al, 2007;Shyu et al, 2005;Talebian et al, 2004). Since ruptures on such faults may cause dislocation or folding of the overlying strata, their presence at some locations may be deduced from detailed geomorphological studies and/or paleoseismological and geophysical investigations of the shallow sedimentary units.…”