Abstract

The 1983 Coalinga M 6.5, the 1985 Kettleman North Dome M 6.1, and the 1987 Whittier Narrows M 5.9 earthquake sequences each occurred beneath major producing oil fields and caused an aftershock sequence whose epicentral extent coincides approximately with that of the overlying oil field. Despite this coincidence, a causal relationship between these sequences and oil production has been discounted because the earthquakes were located at depths of the order of 10 km, whereas oil production is confined to the topmost several km. Moreover, because all three oil fields, Coalinga, Kettleman, North Dome, and Montebello, are situated on anticlines that have undergone recent uplift as a consequence of horizontal tectonic compression of the crust, natural earthquakes are not unexpected. A mechanical connection between oil production and the earthquake sequence is suggested, however, by the observation that in each case the total seismic deformation was just that required to offset the force imbalance caused by oil production. Specifically, for each of the three sequences ΣM0 = 2 μΔmγ/ρc, where ΣM0 is the sum of the seismic moments, μ is the modulus of rigidity, Δm is the mass of the fluid removed due to oil production, γ is the fraction of the upper crust that is seismogenic, and ρc is the average upper crustal density. In essence, net extraction of oil and water reduces slightly the average density of the upper crust, causing an isostatic imbalance. The ductile lower crust deforms in response to this imbalance, thus increasing the load on the seismogenic layer, which fails seismically to thicken the crust so as to restore static equilibrium locally. Accordingly, earthquakes near the base of the upper crust may be an expected outcome of major oil production from growing anticlines, irrespective of the depths of the producing formations.

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