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DEPARTMENT OF GEOLOGICAL SCIENCES AND THE INSTITUTE FOR GEOPHYSICS UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TEXAS 78713
DEPARTMENT OF PETROLEUM ENGINEERING UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TEXAS 78713
BUREAU OF ECONOMIC GEOLOGY UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TEXAS 78713
Abstract
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.
Footnotes
* Present address: Marathon Oil Company, Denver Research Center, P.O. Box 269, Littleton, Colorado 80160-0269.
Present address: Union Oil Company of California, 4635 Southwest Freeway, Houston, Texas 77027.
Present address: Sohio Petroleum Company, Development Geosciences and Petrophysics, 50 Fremont Street, San Francisco, California 94105.
Present address: Frontera Exploration Services, 900 N. E. Loop 410, Suite D-303, San Antonio, Texas 78209.
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