Source mechanisms for Wilmington Oil Field, California, subsidence earthquakes

Abstract

An unusual set of man-made “earthquakes” was generated in the Wilmington Oil Field, California, during the exploitation of this field. The Wilmington Oil Field was the classic example of an elliptically shaped subsidence bowl produced by the extensive withdrawal of the underlying oil. This surface subsidence produced horizontal shear stresses relieved several times by damaging sudden horizontal movements on very shallow slippage planes. Damaging shocks occurred in 1947, 1949, 1951, 1954, 1955, and 1961. These shocks produced seismograms, primarily composed of surface waves, which present an interesting opportunity to study seismic source mechanisms inasmuch as the focal depth, amount of slip, and source dimensions are known. Rayleigh-wave amplitude spectra for these events reveal seismic moments ranging from 5 × 10²² to 1.7 × 10²³ dyne-cm, in good agreement with moments inferred from field observations. A key feature of the subsidence shocks is that very low rupture velocities (0.1 to 0.3 km/sec) were involved. The inferred stress drops indicate that the subsidence shocks must have involved a significant fraction of the available strain energy.