Abstract: Several earthquakes above M_S6.0 occurred in Jiashi, Xinjiang region in China in history. A new M_S6.5 event occurred in this area on Jan. 19, 2020, for which a ‘virtual scientific investigation’ was carried out by China Earthquake Administration in a short time after the earthquake. In this ‘virtual investigation’, a vital important question is that which fault controls the occurrence process of this event, and what is the correlation between this event with other previous earthquakes. To understand the solutions of these questions well, the seismologists analyzed different types of monitoring data, the source parameters, focal mechanisms, seismic waves, InSAR data, regional tectonics, seismic activity including the aftershock sequence, abnormalities of CH4 and GPS TEC, etc. Some conclusions can be drawn on the features of this earthquake and the potential of large aftershocks based on analysis of these kinds of data.
Statistical seismology tools may provide a significant constraint in the case that some earthquakes cannot be described very well through traditional approaches. Concerning the seismicity in the Jiashi area, whether this earthquake is independent background seismicity, or has a certain triggering relationship to the other previous events, is a main question to be well answered during this research. So, to explore the features of the background events and the triggering ability of the events in this area, we used the spatial-temporal epidemic type aftershock sequence(ETAS)model to fit the seismicity using the earthquake events from Jan. 1, 1970 to Jun. 1, 2020 to obtain the spatial and temporal distribution of total seismicity rate, background seismicity rate and clustering seismicity rate. Then the stochastical declustering method based on ETAS model was used to separate all the events into background events and clustering events. The result shows that the clustering seismicity has a main contribution to the total seismicity in this region. The north and south part of the study area show different features of background and triggering seismicity. The north part shows a more homogenous spatial distribution of background seismicity, while the south part shows a high level of triggering or clustering seismicity. Through the calculation of the ETAS algorithm, this M_S6.5 event has a 99%probability of being a triggered event, in which the main contribution is from an M_S5.3 event that occurred 1 day before this event. On the other hand, we find that among all of the events which have contribution to others, an M_L4.1 event that occurred on Apr. 21, 2020 has the highest ability to ‘disturb’ the other events, the probability reaches 0.505, but this needs to be confirmed by other methods. As generally recognized by seismologists, this M_S6.5 event and other previous large earthquakes are mainly influenced by the western Himalayan syntaxis in this region.
As everyone knows, to find the statistical solutions for these questions based on the stochastic or statistical theory, we need focus on the analysis of large group events, rather than a single one. Some algorithms and methods of statistical seismology seem to provide us an opportunity to analyze the group features of seismicity in the study region. Through the analysis using spatial-temporal ETAS model, we can use the statistical methods to describe the spatial and temporal behavior of the background and triggered events, and obtain some special information which cannot be obtained effectively with other traditional tools in some cases. In addition, the traditional ETAS models have been rapidly expanded and developed in recent years, such as 3D-ETAS model incorporating focal depth information, and the finite element ETAS model incorporating the element of spatial morphology of the seismogenic faults. In this view, we can suggest that statistical seismology approaches may have a chance to supply a significant balance point between the pure scientific research and the earthquake consultation work in the future, especially between the conventional seismic research and operational work in China.

Key words: Jiashi region of Xinjiang, the M_S6.5 earthquake, spatial-temporal ETAS model, background seismicity, triggered seismicity

摘要： 新疆伽师地区历史上发生过多次M_S6.0以上地震, 2020年1月19日该地区再次发生了M_S6.5地震。 这些地震属于独立的背景地震活动, 还是与已发生的其他地震存在一定的相互触发关系?探索与此问题相关的背景地震活动和触发地震活动特征, 对深化这一区域地震活动特征的认识具有重要意义。 为此, 文中利用时-空ETAS模型对该区1970年1月1日以来的地震序列进行分析, 拟合得到了较为稳定可靠的模型参数, 给出了区域背景地震活动水平、 表示地震活动丛集特征的丛集率指标及包含背景及丛集活动的总活动水平等结果, 并利用随机除丛方法将背景地震活动与丛集活动剥离开来, 得到了每个地震事件作为背景事件或触发余震事件的概率。 结果显示, 研究区北部与南部的背景地震活动和触发丛集活动存在明显差异, 丛集地震活动为总体地震活动的主要成分; 计算结果表明, 此次M_S6.5地震被触发的可能性达99%, 其中2020年1月18日M_S5.3地震对其触发贡献最大, 达94%; 在对其他地震事件具有触发贡献的个体中, 2020年4月21日M_L4.1地震 “干预”其他事件发生过程的平均能力相对较高, 达0.505, 但需要通过其他手段进一步佐证。 文中工作是中国地震局开展的针对此次地震的虚拟科考工作的一部分, 所得结果可以为后续深入研究该地震提供帮助。

关键词: 新疆伽师地区, M_S6.5地震, 时-空ETAS模型, 背景地震活动, 触发地震活动