怀—涿地区深部孕震环境研究与涿鹿地震发震机制的讨论

怀—涿地区是我国首都圈重要地震活动区之一,为探析北京地区深部孕震环境及发震机制,布设并完成了NWW向横跨怀—涿盆地的大地电磁测深数据采集、处理与反演工作,并对已有的区域重磁数据进行了三维聚焦反演.基于以上二维电性结构剖面和重磁三维结构,结合区域地质、地震探测和水化学资料,取得以下认识:(1)怀—涿盆地中下地壳存在低密度、低磁性和高导性的同源物质异常体,该异常平面上呈NEE向椭圆形分布,在温泉屯一带埋藏最浅,约12 km.而盆地边缘岩体地球物理特征表现为高密度、中高磁性和高电阻率.震中多发生在盆岭交汇处,而震源位置则处在物性结构梯度带上,表明中下地壳不同物质的电性、密度、磁性及温度的差异是怀一涿地区深部重要的孕震环境.(2)怀—涿地区中下地壳高导异常体可能为上涌的地幔物质与沿断裂带进入地壳内部的地表水共同的反映.(3)在怀—涿盆地西北缘和东南缘形成的两大震区与盆地内部高温流体的运移有着密切关系,即两大震区为同一高温流体在中下地壳与围岩相互作用的结果.(4)本次涿鹿地震发生可能的机制为:在NEE向构造应力作用下,地下高温流体向NNW方向缓慢迁移,当遇到大海陀岩体阻挡后,转而从岩石结构稳定性较差的老君山亚口向东部延—矾盆地迁移,在高温流体迁移的过程中促使杏园村—万窑村一带的老君山体发生高倾角右旋走滑型断裂.

The deep seismogenic environment beneath the Huailai-Zhuolu area and the mechanism of the Zhuolu M4.3 earthquake on 6 September 2014

The Huailai-Zhuolu region is one of the important seismically active areas in the capital circle around Beijing of China. To study the seismogenic mechanism and seismogenic environment of this region, we have completed the magnetotelluric sounding data collection, processing and inversion in NWW direction across the Huailai-Zhuolu basin. We also finished a 3D focusing inversion of existing gravity and magnetic data of this region. Based on the 2D electrical structure and 3D gravity and magnetic structure from above work, in combination with regional geology, seismic exploration and hydrochemical data, we obtained the following perceptions:(1) There exist abnormal bodies of low density, low magnetic, and high conductivity in the middle and lower crust below the Huailai-Zhuolu basin. In planar view, these anomalies trend in NEE as an elliptical distribution, which are the shallowest buried at 12 km depth in the Wenquantun area. The rocks of the basin edge are characterized by high density, medium to high magnetic and high resistivity. The epicenters tend to occur in the intersecting areas of basins and ranges, and seismic sources are located in the gradient zones of physical properties, implying that the differences in electricity, magnetism, density and temperature of middle and lower crust, what's more, the magnetic and temperature characterize the seismogenic setting of this region. (2) The high-conductivity anomalies in the middle and lower crust of the study area may be resulted from joint reaction of the upwelling mantle material and the water in the earth's crust which came from the surface along the fault zones. (3) The genesis of the two major seismic zones located in the northwest edge and the southeast edge of the basin might have a close relation with the migration of internal high-temperature fluid, which means the same heat fluid in the middle and lower crust interacting with surrounding rocks. (4) The possible mechanism of the 2014 Zhuolu M4.3 earthquake is as follows:Under the NEE directed tectonic stress, the underground heat fluid migrated slowly in the NNW direction. When it was subjected to the impediments of the Dahaituo rock mass, the fluid diverted from the Laojun mountain mouth with a poor stability of structure to the Yanfan basin in the east. During this process of high temperature fluid migration, the Laojun mountain mass was ruptured as right-lateral strike-slip faulting around the Xingyuan village to Wanyao village.