以精定位背景地震活动性研究中国东部中强地震活动区隐伏发震构造

Most regions of the eastern China are characterized by moderate seismicity, especially the land area of its southern part,which belong to the southern China block. Few studies have been carried on to the seismotectonics for this region since most active faults are buried under the Quaternary sediments with low to moderately historical earthquake activity. However,many large earthquakes in the world that occurred at the unexpected tectonic regime without surface or known active faulting and large historic earthquake documentation alarm us that it should pay more attention to this region as where the south segment of the Tancheng-lujiang fault zone locates and possesses a potential large earthquake hazard. The key point to properly evaluate the seismic risk for this region is to reveal the buried active faults.It drives us to propose this project to investigate the buried active faults and the potential seismic hazard using relocated background seismicity. It is a hot issue in the world to study active tectonics by using the seismological approachs, while few such studies have been done in China. Most regional seismic networks have accumulated a large amount of catalogue records, and have the digital records of the seismic waveforms for more than 10 years, which makes it possible to carry detailed investigation on the active faults using these data. We plan to perform double-difference relocation for the background seismicity and the 3-D velocity structure inversion using tomoFDD of Zhang (2006) basing on the catalog data and waveform cross-correlation. With the high precision resolutions of the hypocenters, we will reveal the buried active faults and try to carry on a series of studies to depict the structure and motion sense of the inferred active faults in details. We will try to obtain the geometric parameters using 3-D structure modeling technique and analysis, and to infer the motion sense of the faults by calculating the focal mechanism solutions for the microearthquakes located on the fault planes, and to calculate the deep slip rates of the faults using repeating earthquakes. The slip feature will be investigated by calculating the sum moment release. The deep tectonic regime will be analyzed by combination investigation of the inferred active faults and velocity structure variation. Through all the above studies, we will get better understand the buried seismogenic faults and the possibility of the occurrence of the large earthquake, as well as its deep tectonic regime of this region. If this project gets support, the results from this research will help us to get proper evaluation of the seismic hazard for this region, and provide a new way to investigate the active fault for other tectonic regions in China.

中国东部大部分区域以中强地震活动为主，以其活动断裂大多隐伏于第四纪沉积物之下以及历史地震活动水平不高而成为我国活动构造研究中的薄弱区，发震构造的揭示是正确评价这一地区潜在地震危险性的问题关键。本项申请拟充分利用我国区域地震台网积累记录的大量地震观测数据，尤其是近十年来的数字波形数据，以先进的波形处理技术、地震定位方法与层析成像技术对研究区现今背景地震活动进行重新定位与三维速度结构反演，揭示研究区存在的隐伏发震断裂，探索获取其几何学、运动学参数。拟以精定位震源分布精细刻画主要隐伏发震断裂的几何结构面与深部构造变形样式；以震源机制解分析推断断裂的运动学性质；使用波形互相关寻找并识别出断裂面上可能存在的重复地震，计算断裂的深部滑动速率；探索研究断裂的现今滑动习性与地震危险性；研究活动断裂与速度结构的关系，分析其深部构造环境，探讨中强地震活动区发震构造条件，为长期地震危险性评价提供重要的基础数据。

中国东部中强地震活动区断裂构造以隐伏活动性质为主，由于采用常规构造研究方法的探测成本较高，已探明的活动断裂极为有限，对正确评价这一地区长期地震危险性带来了极大困难。本课题通过对该地区1990-2016年记录到的地震重新定位和层析成像等工作，揭示出一些以往工作所未发现的隐伏活动断裂，在断裂深部几何结构面刻画、断裂活动性质推断，发震构造判定等方面取得了实质性进展。取得的重要成果概括如下： 1）获得了50000余地震重新定位的震源参数，重新定位结果发现在四川盆地存在我国稳定大陆板内的最大震源深度，达43km，该结果对约束四川盆地地壳流变强度结构，及理解其对高原物质东移的阻挡作用具有重要意义。2）以精定位与地震层析成像在东南沿海南澳一带揭示出3条NEE向斜切区域主构造带的隐伏活动断裂，刻画出的结构面大致呈 65°-85°倾向南东，综合断层面解显示断裂具有逆冲或走滑兼逆冲活动性质，这3条断裂可能为早期历史大震所形成的新生地震断层。在霍山一带揭示出一条NE向断裂及与其平行的一条分支断裂，结构面呈70°-80°倾向北西，震源机制解显示其具有右旋走滑活动性质。3）以精定位背景地震活动对郯庐断裂带中南段现今活动习性的研究发现，昌邑-大店断裂与安丘-莒县断裂以高角度相背而倾，呈右阶斜列形式参与了1668年郯城Ms8.5地震破裂，该段仍未愈合闭锁。安丘以北段与中南段转折处的泗洪-嘉山段现今可能已闭锁，其未来地震危险性值得重视。4）对浙江删溪水库地震开展的研究发现，两次震群序列虽然沿穿过库区的双溪-焦溪垟断裂破裂面分布，但该断裂并非为发震构造，而未来存在被激发活动的危险。5）以震源机制解反演的构造应力场结果表明，最大主压应力在研究区28°以北以北东-北东东向水平挤压为主，在28°以南以北西向挤压为主。 以往探测发现的断裂多以正断性质为主，与现今构造应力场不相匹配，不可能成为发震构造，而通过精定位揭示出的走滑兼逆冲性质断裂与现今应力场相一致，成为东南部地区的发震构造。