结合构造应力场探究地磁转换函数异常

Combining Tectonic Stress Field to Explore the Anomaly of Geomagnetic Transfer Function

摘要:

变化的磁场是联系区域应力场和孕震区地球物理环境信息的天然媒介,而地磁短周期转换函数可以反映地下电阻率等物理信息,本研究利用有界影响估计法计算喀什地磁台地磁短周期转换函数实部Ar绝对值,并使用南天山西段及西昆仑地区的震源机制解资料反演了研究区震源机制一致性参数,其时序变化代表研究区应力场的变化。结合研究区应力场变化尝试探究地磁转换函数实部Ar绝对值异常对构造应力作用的响应。结果显示,当应力作用增强后,地磁转换函数在时间上出现同步或滞后的异常变化,表明应力在不断积累增强后,地下介质电阻率发生变化,从而引起感应磁场变化,因此得出应力变化是造成局部磁异常变化的影响因素之一,验证了地震前感应磁效应现象。通过研究8次中强震前应力场与地磁场转换函数变化,发现在6次中强震前均出现震前异常变化,两者在震前的变化有较好的一致性。研究还发现喀什地磁台地磁转换函数不同周期的实部Ar绝对值对不同方位的地震响应不同,可能与震源区地下介质各向异性差异有关。

Abstract:

The changing magnetic field is the key to connect the regional stress field and the geophysical environment information of the seismogenic area. For example,geomagnetic short-period transfer function reflects the physical information such as the underground resistivity. In order to explore the response of the absolute value anomaly of the real part Ar of the geomagnetic transfer function to the tectonic stress by combining the stress field changes in the study area,we use the bounded influence estimation method to calculate the absolute value of the real part Ar of the geomagnetic short-period transfer function at the Kashgar geomagnetic station. We also use the focal mechanism solution data of the western part of Southern Tianshan mountains and the Western Kunlun area to invert the focal consistency parameters of the study area,since the time series change represents the alteration of the stress field in the study area. The study reveals that when stress increases,the geomagnetic transfer function exhibits synchronous or lagging abnormal changes over time. This suggests that continuous accumulation and enhancement of stress causes a change in the resistivity of the underground medium,leading to alterations in the induced magnetic field. Therefore,it is speculated that stress change is one of the effective factors causing local magnetic abnormal changes,verifying the phenomenon of induced magnetic effect before earthquakes. The study further investigates changes in the stress field and magnetic transfer function before eight moderate to strong earthquakes. Pre-earthquake anomalous changes are observed in six instances,showing good consistency between the two phenomena above. The study has revealed that the response of the absolute value of the real part of Ar in various periods of the geomagnetic transfer function of the Kashgar Geomagnetic station varies depending on the azimuth of the earthquake,possibly related to the anisotropy of the subsurface medium in the seismic source region.