Seismicity and geometry of a 110‐km‐long blind thrust fault 2. Synthesis of the 1982–1985 California Earthquake Sequence

Abstract

During 1982–1985, three 5.4 ≤ Mw ≤ 6.5 earthquakes migrated 65 km along the north half of a string of Quaternary folds at the east front of the California Coast Ranges. There is no surface fault associated with these earthquakes, but the fold chain, oriented parallel to the San Andreas fault 30 km to the west, takes up contraction normal to the San Andreas fault. Identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth. Because of excellent seismic, geodetic, and geologic data, this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics. We relocate seismicity and compute focal mechanisms in a three‐dimensional velocity space and model the vertical deformation associated with the 1983 M = 6.5 Coalinga earthquake. The aftershock zones abut at echelon offsets in the fold axes, and the mainshocks display reverse slip perpendicular to the axes, suggesting that the folds conceal a contiguous, segmented thrust fault. Background seismicity concentrates at bends and breaks in the fold chain, sites that may correspond to tears and ramps in the thrust fault at depth. Seismic reflection profiles reveal thrust and reverse faults dipping toward the San Andreas fault at depths of 5–10 km with several kilometers of cumulative slip and high‐angle reverse faults in the anticlines with several hundred meters cumulative slip. Coseismic fold uplift accompanied the Coalinga and Kettleman Hills North Dome earthquakes, suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts. The north half of the fold chain has been the site of several 6 ≤ M ≤ 6.5 earthquakes since 1885. Thus Kettleman Hills Middle Dome, the next fold segment to the south, has an elevated seismic potential. The aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip. Aftershocks occur 5–7 km from the fault, regions where, on the basis of a boundary element model, the shear strain increase caused by the fault slip exceeds ∼20 ppm (equivalent to about 0.7 MPa). We argue that the broad aftershock zone is a product of high, sustained off‐fault stress caused by repeated displacement on faults that do not cut the Earth's surface. The fault tip stresses lead to the formation of secondary faults, which can become sites of aftershocks and postseismic creep.