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本文把较大地震前观测到的地震波振幅异常变化与模拟实验的结果及理论分析加以对比,借助于地壳介质的变化对振幅比前兆异常特征提出了某些物理解释。较大地震前观测到的地震波振幅比时间变化呈“低值——上升到稳定高值——发震——回到正常值”的形态;主震危险区位于异常区内,多数在比值梯度大的地方,临震时比值骤增,各台比值峰值出现时间由震中距,方位角不同而有序。岩石压力实验中压力达岩石破坏强度50—60%以后,P波振幅随压力增加而下降,幅度较大;A_S/A_P随加压而下降,达破坏强度90%以后,迅速回升;若加载达破坏强度之95%时充水稳压,P波振幅在各方向均减小。在理论上把孕震过程中介质的变化分别用单相和多相介质模拟,多相介质中考虑以气和水为孔隙流体,给出本构关系,求出速度和垂直向位移。理论工作表明:A_S/A_P在孔隙介质为气时比为水时小;P波振幅变化大;孔隙度大,振幅比变小。介质中应力集中程度达岩石破坏强度60%前后,可分别用单相和多相来描述介质。两相中,微裂隙张开,以气体为主,A_S/A_P变小,呈负异常,达90%时,A_S/A_P上升,围压大于孔隙压,孔隙流体以水为主,且水进速率大于孔隙度增大速率;临震时予滑,应力部分解除,孔隙部分闭合,孔隙饱和,振幅比出现峰值,至于空间分布则要考虑应力积累与调整部位介质强度不同,可能造成孔隙度及饱和度不同所致。由于本文的物理解释中综合了实际观测,模拟实验和理论分析的结果,又把应力状态与介质变化联系起来,故此种解释可能具有成因观点。
In this paper, the abnormal changes of seismic wave amplitude observed before larger earthquakes are compared with those of simulation experiments and theoretical analysis. Some physical explanations of the characteristics of amplitude anomaly precursors are proposed by means of the changes of crustal medium. The amplitude of the seismic wave observed before the larger earthquake is in the form of “low value-rising to stable high value-seismogenic-returning to normal value” over time; the main shock hazard region is located in the abnormal region, Large places, the ratio of sudden increases in the epicenter, the ratio of peak value of each epicenter by the epicenter, azimuth and orderly. After the rock pressure reaches to 50-60% of the rock failure strength, the amplitude of P wave decreases with the increase of pressure, and the amplitude of A-wave decreases with the increase of pressure. After the failure strength reaches 90%, the amplitude of P- 95% of the destruction of the water-filled regulator, P-wave amplitude decreases in all directions. In theory, the change of medium in seismogenic process is simulated by single-phase and multi-phase media respectively. Considering the gas and water as the pore fluid in multiphase media, the constitutive relation is given and the velocity and vertical displacement are obtained. The theoretical work shows that: A_S / A_P is smaller when the gas in the pore medium is water; the P-wave amplitude is larger; the porosity is larger and the amplitude ratio is smaller. Before and after the stress concentration in the medium reaches 60% of rock failure strength, the medium can be described by single phase and multiphase respectively. In the two phases, the micro-cracks are open and the gas is dominant. The A_S / A_P becomes smaller and shows a negative anomaly. At 90%, A_S / A_P increases, and the confining pressure is greater than the pore pressure. The pore fluid is mainly water, The rate of increase is greater than the rate of porosity; Pro-slip to the slippery when the stress part of the release of the pore part of the closed pore saturation, the peak amplitude ratio, as the spatial distribution should be considered stress accumulation and adjustment of parts of different dielectric strength may result in porosity and Saturated due to different. As the physical interpretation of this article combines the results of actual observations, simulation experiments and theoretical analysis, and again the stress state and the change of the medium, so the explanation may have a genetic point of view.