Fluid pressures within some oil and gas fields of South Texas have dropped to less than 20 per cent of their original values, producing earthquakes with magnitudes up to 3.9 in recent years. Differential compaction of the depressurized region may be sufficient to result in the number and size of earthquakes generated, or the faults may have been creeping prior to depressurization. In either model, the depressuring of fluids strengthens a fault and at first produces a “barrier” to slip. As strain accumulates due to compaction or the continued aseismic slip of nearby portions of the fault, stress builds up along the locked portions, eventually forming high-stress regions or “asperities.” The asperities ultimately fail and earthquakes occur. The process is repeated as long as the faults are active. As the fluid pressures continue to decrease, the barriers and subsequent asperities may increase in size and strength, resulting in increasingly large and frequent earthquakes.

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