Coalinga's caveat



No one had foreseen that a shock as large as Ms 6.7 could strike at the eastern edge of the California Coast Ranges near the town of Coalinga. No faults of any age cut the gently folded sediments at the earthquake focus, and equally astonishing, no ground breakage accompanied the earthquake. A month would pass before surface rupture did occur, in a sequence of large aftershocks (see cover: large blue circles west of the yellow main shock) on a minor adjacent fault.

The May 2, 1983, earthquake, which caused $31 million in damage and took one life, alerted the seismological community to the earthquake potential of active folds. Leveling surveys demonstrated that Anticline Ridge (whose fold axis plunges southeast through the main shock) uplifted 0.5 m during the event, while adjacent Pleasant Valley (see cover: red and white squares immediately southwest of the aftershocks) subsided half that amount. The surface deformation is best explained by a fault that slipped over a depth of 4–12 km but does not reach the surface. Identification of earthquake risk has proceeded almost exclusively on the premise that faults capable of generating large shocks must extend to the surface. Surface faults that do not cut young deposits are not deemed active, even if the faults displace older beds at greater depth. At Coalinga, on the other hand, the fault, the seismic slip, and the aftershocks are confined to deeper and therefore older strata. The fault is invisible on seismic reflection profiles, but it is indisputably active. The surface fold which built Anticline Ridge appears to have formed as a consequence of repeated slip events at depth. From the pattern of earthquake deformation, R. Stein and G. King have argued that many folds mask subsurface thrust faults and that these folds may largely deform elastically in jumps, rather than through steady and aseismic ductile deformation. In retrospect, a number of large thrust earthquakes over the last 30 years have uplifted anticlines, dropped synclines, and produced little or no ground rupture, such as the 1964 Ms 7.5 Niigata shock in Japan. Thus, despite the paucity of bounding faults, the folded eastern margin of the California Coast Ranges must now be regarded as a possible target for large thrust events. The uplift rate of the folds should guide judgments of relative activity. Ominously, the Kettleman Hills anticline, which abuts the Coalinga aftershocks to the southeast, is rising at twice the rate of Anticline Ridge. (These processes will be discussed at the AGU Fall Meeting in the Tectonophysics session entitled “Mechanics of Large-Scale Faulting and Earthquakes,” scheduled for Friday morning, December 7; see abstracts in this issue for more information.)