[1] Oscillations in stress, such as those created by earthquakes, can increase permeability and fluid mobility in geologic media. In natural systems, strain amplitudes as small as 10À6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitoring permeability in geothermal systems where there is abundant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution.

Numerous observations accumulated principally during the last 40 years show that seismic waves generated from earthquakes and cultural noise may alter water and oil production. In some cases wave excitation may appreciably increase the mobility of fluids. The effect of elastic waves on the permeability of saturated rock has been confirmed in numerous laboratory experiments. Two related applications have arisen from these findings. In the first application, high-power ultrasonic waves are applied for downhole cleaning of the near-wellbore in producing formations that exhibit declining production as a result of the deposition of scales and precipitants, mud penetration, etc. In many cases, ultrasound effectively removes the barriers to oil flow into the well. The ultrasonic method is reported to be successful in 40-50 percent of the cases studied. In the case of successful treatment, the effect of improved permeability may last up to several months. Whereas this method has a very local effect, a second application is used to stimulate the reservoir as a whole. Here seismic frequency waves are applied at the earth's surface by arrays of vibroseis-type sources. This method has produced promising results; however, further testing and understanding of the mechanisms are necessary.

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The matrix inversion of seismic data for slip distribution on finite faults is based on the formulation of the representation theorem as a linear inverse problem.