张北地震前小震活动和余震序列精确定位及主震破裂方向的讨论

Accurate location of small earthquakes before Zhangbei Earthquake and aftershock sequence and discussion on the Rupture's direction of main event

1970年至1998年1月10日张北地震前, 沿NWW向发生2级以上地震24次, 其分布相对集中, 呈条带状. 对华北台网所记上述地震的到时数据全部重新查图核对, 并补充了内蒙古台网5个台的数据以改善用于定位的台站分布, 并采用主地震定位法重新定位, 除一个地震外, 其余地震更加集中, 形成一小震条带. 1998年2月中国地震局地球物理勘探中心在张北震区架设10台短周期数字化地震仪, 记录了大量微震. 作者采用P波和S波到时差重新进行了震源定位, 得到184次微震的定位结果, 连同1998年1～2月张北震区3级以上地震事件定位结果一起勾画出6.2级地震的震源破裂带. 由张北地震震中向SEE向延伸存在由强震构成的张家口-渤海地震带. 根据精确定位结果, 结合修订后主震的震源机制、宏观烈度分布和层析成像资料, 讨论了张北地震和小尺度地震带(震源破裂带)、震前的中等尺度小震条带以及大尺度的张家口-渤海强震带的关系.

Before the Zhangbei earthquake through the lst of Jan. 1970 to the 10th of Jan. 1998, 24 earthquakes with magnitude larger than M_L2.0 occurred in the NWW direction of Zhangbei earthquake, they were relatively concentrated along a belt. The arrival times of all the events are checked from the seismograms recorded at North China telemetric seismograph network. Additional data are used from other 5 stations of Inner Mongolia telemetric seismograph network to improve the distribution of stations used in location. Those earthquakes are relocated using the main event location approach. All events became more closely except one, forming a seismic belt. Many micro-earthquakes along Zhangbei seismic zone are recorded by 10 short period seismographs employed by Research Center of Exploration of Geophysics in February 1998. We relocated 184 micro-earthquakes using the arrival time difference between P and S waves. These micro-earthquakes together with the relocated earthquakes with magnitude large than 3.0 occurred along Zhangbei seismic zone in January 1998 outline a focal rupture belt of M6.2 earthquake. There is Zhangjiakou-Bohai rupture belt, where many strong earthquakes occurred in history, extended along the SEE direction of Zhangbei earthquake epicenter. In this paper, the results of accurate location together with the corrected focal mechanism and the distribution of macroscopic intensity are used to discuss the relationship between Zhangbei earthquake and small scale focal rupture belt and intermediate scale weak earthquake belt before the main earthquake and large scale Zhangjiakou-Bohai strong earthquake belt.