强地震前兆异常特征与深部流体作用探讨

根据华北几次强震前后地下流体和重力异常特征,结合大地测深资料探讨了地震发生前后深部介质的变化和地壳浅层应力场变化特征。地震发生前震中区地下水位异常主要表现为下降型,气体异常呈上升型,重力和形变也出现异常;在外围地区地震前地下水位异常以上升型为主。 海城地震和大同–阳高地震前地下水位上升、下降异常有方向性分布的趋势,呈 X 形分布,这意味着地壳浅层应力场是压扭型的,并存在垂向应力的作用。在孕震过程中,地球深部流体向上运移,使得震源区及其附近的岩石膨胀、密度降低,震中区地壳浅层应力场呈拉张型,造成地下水储容积增大,地下水位出现了下降型异常。 在特定的构造单元内,地下流体、重力异常的空间分布特征能较好地反映地球深部物质运动和地壳浅层应力场特征。

Characteristics of earthquake precursors and its geological significance

Based on distribution characteristics of anomalies of underground fluids and gravity in the North China, the features of deep seated material and stress field in the shallow crust before and after main earthquakes are discussed combining the geophysical prospecting data. The water level anomalies close to the epicenter of the strong earthquakes belong to the decreasing type, but the gas anomalies are the increasing type before the main shocks in the continental rift valley of eastern China and Shanxi graben system. The water level anomalies far away from the epicenter, however, are usually the increasing type. There are the orientation trends of the increasing type and decreasing type of anomalies of water level before Haicheng and Datong earthquakes. Deep seated material (especially fluids) rises during earthquake generation in the epicentral area, which results in expansion of the shallow crust, subsidence of water level and gas escape from strata. The stress field of the shallow crust in the epicentral area seems to be expansive with the action of upward stress, but compressive far away the epicenter. The stress field in the shallow crust was the compression shearing type in the epicentral area of Haicheng and Datong earthquakes. The distribution characteristics of seismic precursors can reflect the state of stress field of the shallow crust in the certain tectonic region and have a close relation with the seated material movement.