2022年四川泸定MS6.8地震前地震活动与地球物理观测异常回顾与讨论

Review and Discussion on Seismic Activity and Geophysical Observation Anomalies before the 2022 Luding MS6.8 Earthquake in Sichuan,China

摘要:

2022年9月5日四川泸定M_S6.8地震发生在2022年度全国地震重点危险区内,且震前作了较好的短期预测。本文回顾了中期(年度)和短期阶段地震活动和地球物理观测异常。①2022年度危险区确定的核心依据有川滇藏交界4级地震空区、危险区附近M_L≥3.5地震空区、跨断层形变趋势异常和重力场异常等,其中,川滇藏交界4级地震空区被2022年1月2日云南宁蒗M_S5.5地震打破具有中短期预测意义。②短期阶段,川滇藏交界4级地震空区经历了“打破—增强—平静”的演化过程,与1973年四川炉霍M_S7.6地震前高度相似,这可能与其发震构造相同、震源机制解一致和深部孕震环境相似有关。此外,还存在川滇地区震群和多个余震区准同步活动、巴塘显著震群等异常。地球物理观测方面,在2022年6月1日芦山M_S6.1和6月10日马尔康M_S6.0地震后,四川前兆异常无明显减少,而在7—8月显著增多,这可以作为强震后短期仍有可能再次发生强震的判定依据。新增异常主要分布在以三岔口(鲜水河断裂带、安宁河断裂带和龙门山断裂带交汇区,呈“Y”字形分布)为中心的300km范围内,这是震前短期地点预测的主要依据之一。③6.8级地震前形变中短期大幅度异常突出,且异常点均位于远场(距离6.8级地震震中130～300km范围内)。除礼州测距外,其余异常点均位于M_L3.5地震空区外围。形变异常出现的时间与M_L3.5地震空区打破后空区内部及边缘地震活动显著增强大体一致。④泸定6.8级地震发生在三岔口地区,该区及附近2015—2021年连续多年被确定为全国地震重点危险区,但均未发生预测地震,由此表明当前有效的强震年度(中期)时间预测依据少。

Abstract:

The Luding M_S6.8 earthquake on September 5,2022 in Sichuan Province of China is within the potential area of the annual earthquake hazard map provided by the China Earthquake Administration. In this paper,we systematically reviewed the seismicity and geophysical observation anomalies before the shock as follows. ①The medium-term forecasting is mainly based on the anomalies of seismic gap with M_S≥4.0 in Sichuan-Yunnan-Tibet border area,the M_L≥3.5 earthquake gap near the annual earthquake hazard map,trend anomalies of across-fault deformation and gravity anomaly. The gap of M_S≥4.0 earthquake was broken by the January 2,2022 Ninglang M_S5.5 earthquake in Yunnan Province,which is of significance for medium-short term prediction. ②In the short-term stage before the Luding earthquake,the seismic gap with M_S≥4.0 earthquake experienced evolution process of broken-strengthening-quiescence,which is highly similar to that of before the 1973 Luhuo M_S7.6 earthquake in Sichuan Province. This may be related to the fact that the two earthquakes have the same seismogenic structure,focal mechanism and deep seismogenic environment. In addition,there exist some anomalies,such as the quasi-synchronous activity of the earthquake swarms and the significant seismic activity in aftershock areas in Sichuan-Yunnan region,and the Batang earthquake swarm. After the Lushan M_S6.1 earthquake on June 1,2022 and the Maerkang M_S6.0 on June 10,2022,the precursor anomalies did not decrease significantly in Sichuan Province and its surrounding areas,while the anomalies increased significantly from July to August,which can be considered as a short-term basis for predicting the next earthquake. The new appeared anomalies mainly distributed within 300km of Sanchakou area(the intersection area of the Xianshuihe fault,the Anninghe fault and Longmenshan Fault zone),which is one of the main bases for the short-term location prediction before the Luding earthquake. ③At the medium-short-term stage before the Luding earthquake,the crustal deformation anomalies with large amplitude are remarkable. These anomalies are in the range of 130～300km around the epicenter of the Luding earthquake except for the Lizhou baseline,the other anomalies are located outside the seismic gap with M_L≥3.5. The occurrence time of the crustal deformation anomalies is roughly consistent with the significant enhancement of seismic activity in the gap and nearby. ④The Luding shock occurred in the Sanchakou area,which region and its vicinity has been identified as the potential area of the annual earthquake hazard map from 2015 to 2021,but there was no predicted event occurred. This suggests that the time prediction index for medium-term is not sufficient.