[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.

The ground motion from large earthquakes is often predicted based on finite-fault modeling, in which the fault plane is discretized into small independently rupturing subfaults; the radiation from all subfaults is summed at the observation point. Despite the success of the method in matching observed ground-motion characteristics, the physical interpretation of the subfaults has remained largely unclear, and a rationale for the choice of the subfault attributes has been lacking. Two key parameters-the subfault size and the maximum slip velocity on the fault-govern the amplitude of the source spectrum at intermediate and high frequencies, respectively. We determined these key source parameters, on an event-by-event basis, for all well-recorded moderate to large earthquakes in western North America (WNA) by fitting simulated to observed response spectra. We compare the values of these source parameters with those obtained previously for eastern North America (ENA) and the Michoacan, Mexico, earthquakes (a total of 26 modeled events).We find that the characteristic subevent size increases linearly with moment magnitude in an apparently deterministic manner. The subevent size relationship obtained for WNA is not statistically different from that obtained for ENA. In both regions, the subevent size follows the trend of log Dl ‫ס‬ ‫2מ‬ ‫ם‬ 0.4 M (4 Յ M Յ 8), where Dl is the subfault size in km. This trend agrees well with independent studies by Somerville et al. (1999) and Aki (1992), in which the characteristic size of the patches ("asperities" or "barriers") on earthquake faults was determined. These results indicate that large earthquakes should be viewed as a sequence of smaller events that comprise the large rupture. Interestingly, the characteristic size of these constituent small events appears to be directly related to the size of the overall rupture.The slip velocities determined for all 26 earthquakes vary in a narrow range from about 0.25 to 0.60 m/sec, with a mean of 0.40 m/sec and standard deviation of 0.09 m/sec. The slip velocities for the ENA events are distributed randomly over this range, while those for the WNA region appear to exhibit a decreasing trend with increasing magnitude. Our results indicate that a generic, region-independent earthquake source model for engineering prediction of strong ground motions can be developed.

Quantitative dynamics of a nonwetting ganglion of residual oil entrapped in a pore constriction and subjected to vibrations of the pore wall can be approximated by the equation of motion of an oscillator moving under the effect of the external pressure gradient, inertial oscillatory force, and restoring capillary force. The solution of the equation provides the conditions under which the droplet experiences forced oscillations without being mobilized or is liberated from its entrapped configuration if the acceleration of the wall exceeds an unplugging value. This solution provides a quantitative tool for estimating the parameters of vibratory fields needed to liberate entrapped, nonwetting fluids. For typical pore sizes encountered in reservoir rock, wall accelerations must exceed at least several [Formula: see text] and even much higher levels to mobilize the droplets of oil; however, in the populations of ganglia entrapped in natural porous environments, many may reside very near their mobilization thresholds and may be mobilized by extremely low accelerations as well. For given acceleration, lower seismic frequencies are more efficient in liberating the ganglia.

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