北京地区小震精定位结果初步探讨

Discussion on the Relocation of Small Earthquakes in Beijing Area

摘要:

利用双差重定位方法对北京地区(39.2°N～41.2°N,115.2°E～117.6°E)2008年10月至2020年3月期间共6463个地震事件进行重新定位,得到4776个高质量地震事件的空间位置分布。定位结果均方根走时残差显著降低,震源间的相对位置精度明显提高,地震活动的成丛性与活动断层之间的关系更加密切。根据重定位结果探讨了小震活动与活动断裂及深浅构造的关系,得到以下结论:夏垫断裂小震活动强烈,重定位后震中分布明显向断裂靠拢,呈现沿断裂走向延伸的NE向线状分布。结合人工剖面探测结果推测,该区域深度剖面上小震集中带10km深度附近较宽的丛集区主要是浅部铲形断裂活动所致,14～22km深度附近相对较窄近直立的小震丛集为深部断裂活动产生。怀—涿次级盆地北缘断裂现今小震活动活跃,该区域小震深度剖面揭示该断裂为倾向南东、上陡下缓的铲形断层。结合人工剖面探测结果,分析认为该区域深浅断裂共存和非均匀的地壳结构为中强地震发生的深部构造背景。平原区内部小震分布与第四纪断陷盆地和地层厚度分布存在较强的相关性,研究区域内部昌平—海淀小震集中区、顺义小震集中区、夏垫小震丛集区和廊坊小震丛集区分别位于马池口—沙河断陷盆地、顺义断陷盆地、大厂断陷盆地和固安—廊坊—永清断陷盆地。根据小震空间密集成带分布特征来推测未知断裂,推测怀柔南部山区小震集中区下方存在隐伏断层,宝坻至夏垫断裂段之间NW向的小震集中分布区下方存在一条走向NW或者NWW的隐伏断层。

Abstract:

In this paper,we relocated 6,463 small earthquakes which occurred from October 2008 to March 2020 in Beijing area(39.2°N～41.2°N,115.2°E～117.6°E). We obtained the spatial distribution of 4,776 relatively high precision seismic events by using the double-difference relocation algorithm. The results show that the root mean square (RMS) of travel time residuals significantly decreased,the relative position accuracy between seismic events are significantly improved,and the clustering of seismicity is closely related to active faults. Based on the results of relocation,we discussed the relationship between the activity of small earthquakes and deep and shallow structures of active faults,and came to the conclusion,in which the Xiadian fault is of strong small earthquake activity with the epicenter distribution closely to the fault in space,resulting in a NE trending linear distribution along the fault strike. Combined with the results of artificial profile detection,we speculated that the wide cluster area near the 10km depth of the small earthquake concentration zone in this area is mainly caused by the shallow listric faults activity,and the relatively narrow near vertical small earthquake cluster near 14～22km is caused by the activity of deep fault. The fault in the north margin of Huaizhuo subbasin is active in small earthquakes. The depth profile of small earthquakes in this area reveals that the fault is a listric fault inclined to southeast,steep in the upper part and gentle in the lower part. Combined with the analysis of the results of artificial profile detection,it is considered that the coexistence of deep and shallow faults and the inhomogeneous crustal structure are tectonic background of moderate strong earthquakes. There exists a strong correlation between the distribution of small earthquakes in the plain area and the distribution of Quaternary faulted basins. The Changping-Haidian small earthquake concentration area,Shunyi small earthquake concentration area,Xiadian small earthquake cluster area and Langfang small earthquake cluster area are respectively located in Machikou-Shahe fault basin,Shunyi fault basin,Dachang fault basin and Gu'an-Langfang-Yongqing fault basin. According to the spatial distribution characteristics of small earthquakes,it is inferred that there is a buried fault under the small earthquake concentration area in the southern mountainous area of Huairou,and there is a NW or NWW trending buried fault under the NW trending small earthquake concentrated distribution area between Baodi and Xiadian Fault.