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根据55个微震和4次爆炸的P波到时对震中、速度和台站延迟进行联合反演,这些事件分布在加利福尼亚北方以Geysers地热区为中心的一个90×100km~2的范围内。得到的一维层状模型表明,速度随深度增加:从地表的4.4km/s增加到深8km及其以下的5.9km/s。用这个新的速度模型我们重新定位了在1975年5月和1982年2月之间发生的7215个地震。重新定位表明,靠近蒸汽田开采井有一清晰的空间震群,这在较早的研究中是不明显的。我们根据地震活动的时空图象推测,Geysers的地震活动性是由地热开采活动诱发的。由于1979年到1980年电力开采增加近70％,靠近新开采区地震活动已经有所增加。地震活动也向现开采区的西北延伸,虽然这个区域没有现役的蒸汽井。对月地震数、月蒸汽开采量以及月注入量之间的统计互相关进行了计算。就这些井的研究,我们没有发现注入和地震活动性之间一致的相关图象。同样,我们几乎没有发现蒸汽开采量和地震活动性之间的相关性,而这些蒸汽井已开采了七年多,两个机制对于Geysers的诱发地震活动仍然是似是而非的。大量的蒸汽开采引起的体积收缩可能破坏应力场并可能导致因区域应力场已接近破裂的储集岩的断裂。另一方面,区域构造应力导致的无震形变可能因断层面摩擦系数的增加(如溶解硅的出溶进入断层面沉淀下来),转变为粘滑(有震)形变。没有一个机制能够说明开采和地震活动速率之间存在着很强的关系,尽管如此,我们预计Geysers的地震活动将持续下去并会出现在新的开采区。
A combined inversion of epicenter, velocity and station delay based on 55 microseismic and 4 burst P-wave arrivals occurred in a range of 90 × 100 km ~ 2 centered on Geysers geothermal area in northern California. The obtained one-dimensional layered model shows that the velocity increases with depth from 4.4 km / s at the surface to 5.9 km / s at depth 8 km and below. With this new velocity model we repositioned 7,215 earthquakes between May 1975 and February 1982. Repositioning showed that there was a clear spatial swarm of swarms near the steam fields, which was not apparent in earlier studies. According to the temporal and spatial images of seismic activity, we speculate that the seismicity of Geysers is induced by geothermal mining activity. Seismic activity near new mining areas has increased due to a nearly 70% increase in power generation from 1979 to 1980. Seismic activity also extends to the northwest of the current mining area, although there are no active steam wells in the area. The statistical cross-correlation between the number of monthly earthquakes, monthly steam production, and monthly infusions was calculated. In the study of these wells, we found no consistent correlation between injection and seismicity. Similarly, we found little correlation between steam production and seismicity, and these wells have been mined more than seven years ago, and the two mechanisms are still plausible to Geysers’ induced seismics. The volumetric shrinkage caused by the large amount of steam exploitation may destroy the stress field and may lead to the fracture of the reservoir rock which is close to the rupture due to the regional stress field. On the other hand, the earthquake-induced deformation induced by regional tectonic stress may change into stick-slip (earthquake) deformation due to the increase of the friction coefficient of the fault surface (eg, the dissolution of dissolved silicon precipitates into the fault plane). There is no mechanism to show that there is a strong relationship between the rate of mining and seismic activity, but we expect Geysers seismic activity to continue and to appear in new mining areas.