系统总结2020年2月—3月西藏改则3次5级以上地震发生前出现的地震活动性和地球物理观测异常，结果如下：①地震活动性：震前存在地震发生率异常；②地球物理观测：3次地震震中500 km范围内无观测台站分布，地球物理场观测能力较弱；③综合方法：识别出Wq值异常。3次改则地震均位于可可西里至唐古拉地区，其中2月21日5.0级地震位于羌塘地块北缘，而3月10日5.0级和3月12日5.1级地震位于羌塘地块南缘，且3月的2次地震相距约5 km，构成震群型序列。对3次5级以上地震序列的跟踪发现，2月21日改则5.0级地震序列主要分布在向阳湖—布若错断裂南侧发育的一条NNE走向的小断裂上，余震较少，震后40天内共发生8次ML 2.0以上地震，主震震源机制为张性破裂；3月10日5.0级和3月12日5.1级地震震源机制均为走滑型，余震序列丰富，主要分布在多条构造的延伸区域，计算得到序列h值为0.58，b值为0.88。综合分析认为，改则3次5级以上地震前地震活动异常较少，主要为具有中期意义的异常，综合方法的异常识别主要为后续强震的地点判定提供依据。
This paper systematically summarizes the observed seismicity and geophysical anomalies before three Gaize MS ≥ 5.0 earthquakes occurred from February 21, 2020, to March 12, 2020. In terms of seismicity, there is an earthquake incidence rate anomaly before earthquakes. In terms of geophysical observation, the epicenters of the three earthquakes are located in areas with weak geophysical observation ability, and there are no observation stations within 300 km of the epicenters. The comprehensive method identifies the Wq value anomaly. The three Gaize MS ≥ 5.0 earthquakes are located between Kekxili and Tangora area, the MS 5.0 earthquake occurred on February 21 is located at the northern margin of the Qiangtang block, and the MS 5.0 and MS 5.1 earthquakes occurred on March 10 and 12 are located at the southern margin of the Qiangtang block. The two MS ≥ 5.0 earthquakes that occurred in March are 5 km apart, and these two earthquakes together with aftershocks constitute an earthquake swarm. Tracking for the two groups of earthquake sequences reveals that the aftershocks of MS 5.0 earthquake on February 21 are rare and mainly distributed on a small NEE strike fault that developed in the south of Xiangyanghu-buruocuo fault. The focal mechanism of the mainshock shows a tensile rupture. The sequences of the earthquake swarm are mainly distributed in the extension regions with multiple structures. The mechanisms of the two mainshocks are both strike-slip type with abundant aftershocks. The h value and the b value of the sequence are 0.58 and 0.88, respectively. In summary, the seismic anomalies before earthquakes are rare and are mainly medium-term. The comprehensive methods mainly provide location information for the event in the future.
2020,41(3): 174-183 收稿日期：2020-06-10
DOI：10.3969/j.issn.1003-3246.2020.03.022
基金项目：中国地震局震情跟踪定向工作任务（项目编号：2020010125，2020020101，2020010103）
作者简介：王月(1990-),女,主要从事地震定位、地震视应力和地震活动性研究工作
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