关键词: 活动断裂, 布格重力异常, 小波多尺度分析, 6级以上地震

Abstract:

There are many first-order intersecting tectonic units and different strike faults developed widely in Henan Province, and many historical earthquakes with magnitude 6 and above occurred, which have brought great losses to people’s lives and property. In order to effectively reduce the risk of earthquake disaster in Henan Province and understand the deep seismogenic environment, we have carried out a systematic study on the deep structural characteristics of these active faults. Firstly, based on the high-precision Bouguer gravity anomaly data of Henan Province and its adjacent areas, we obtained the characteristics of gravity anomaly fields at different spatial scales in the study area by using the multi-scale wavelet analysis method. Then the detailed characteristics of different orders wavelets of Bouguer gravity anomaly field in the study area and its relationship with regional structure were analyzed. We found that within 14km of the crust, the regional tectonic activity has an obvious control effect on the trend of gravity anomaly zone. The trend of gravity anomaly zones is obviously different in different tectonic units in the study area. In the north of Henan, the trend of gravity anomaly zones is NE, which is consistent with the regional tectonic trend. The horizontal density difference is obvious. In the south of North China depression and Qinling-Dabie uplift area, the trend of gravity anomaly zones is NW, NWW and EW. In the differential uplift area of western Henan, the trend of gravity anomaly zones is NE. At the 27km depth of the crust, most gravity anomalies are in a clumpy shape, and the consistency between the trend of the gravity anomaly and the regional structure decreases, indicating the differences in regional tectonic stress effect and formation process at different depths of the crust. For example, under the northward compression from Qinling-Dabie uplift, the crust structure in the south of North China depression is different, and the difference gradually decreases from shallow to deep. At the same time, with the increasing of depth, the boundary between Qinling-Dabie uplift and southern North China depression moves to the Pingdingshan and Luohe. Our results show that the regional deep faults have an obvious control over the distribution of gravity anomalies, and the linear transition zone of gravity anomalies often corresponds to the deep faults. In order to obtain the distribution characteristics of active faults in Henan Province and adjacent areas, we analyzed the wavelet multi-scale decomposition of Bouguer gravity anomaly and identified 38 faults. Based on the seismic and geological results, we interpreted the 38 faults, including10 shallow faults in the upper crust with a depth of less than 8km, 15 faults at the bottom of the upper crust with a depth of 12~14km and 13 faults in the lower crust with a depth of 27km. In the study area, the deep faults control the boundary of the first-order tectonic units, such as Liaocheng-Lankao Fault, Tangxi Fault, Xinxiang-Shangqiu Fault, etc., and many moderately strong earthquakes occurred in these faults in history. At last, we analyzed the deep tectonic environment of historical earthquakes with magnitude 6 and above in Henan Province. The results show that the historical earthquakes with magnitude 6 in Xuchang locate near the boundary zone of second-order tectonic units. Other historical earthquakes with M6.0 locate below the secondary uplift or depression controlled by deep and large faults in the crust, such as Puyang earthquake which locates in the Dongpu depression. It can be concluded that the intersections of gravity anomalies zones with different trends, the deep seated fault-controlled intra-crust low gravity anomaly areas, and the intersections of deep seated fault with different strikes are the deep tectonic background and favorable locations for generating earthquakes with magnitude 6 and above in Henan Province. The results of analysis of the characteristics of major deep active faults in Henan Province expanded our understanding of the tectonic environment of the study area and provided a geophysical basis for earthquake prevention and disaster reduction in Henan Province in the future.

Key words: active faults, Bouguer gravity anomaly, multi-scale wavelet analysis, earthquake of magnitude 6 or above