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摘要
The object of this paper is to investigate the seismogenic fault structures and stress field where M(S)7. 3 Yushu earthquake sequence occurred in April 2010, and then tentatively discuss the triggering mechanism for the Yushu M(S)7. 3 mainshock and M(S)5. 9 strong aftershock sequences. We adopted double difference algorithm for earthquake hypocenter relocation by using different seismic velocity models, and chose the outcome with the maximal proportion of relocated earthquakes in total events as our final result. With the combination of focal mechanism solutions and earthquake relocation, the fault structures and stress field for Yushu earthquake sequences were comprehensively analyzed. After the relocation of M-L >= 1. 0 earthquakes among Yushu earthquake sequence from April 14 to October 31, 2010, we obtained precise locations of 1545 earthquakes. Relocation results show that Yushu earthquake sequence was mainly composed of two intersecting seismic strips west trending seismic zone along Ganzi-Yushu fault. This seismic strip extended for about 80 kilometers with 300 degree trending and indicated a high-angle fault plane that is close to upright. The M(S)5. 9 aftershock on May 29 occurred in a wide NEE rectangular strip, which extended approximately 40 kilometers with a width of 18 kilometers, indicating a possible buried fault. The relocated Yushu earthquake sequences occurred from ground surface to around 15 kilometer in depth, but were mainly concentrated in the depth range of 8 to 12 kilometers. Combined analysis of double difference and focal mechanism leads to the conclusion that the NW fault plane in the focal mechanism solution for mainshock is most likely the seismogenic fault of mainshock sequence. The mainshock fault is a NW trending, nearly vertical, and sinistral strike-slip fault. The seismogenic fault plane for M(S)5. 9 aftershock is an NEE, almost upright sinistral strike-slip fault. The maximum principal stress direction in mainshock seismic zone is consistent with the regional principal stress direction, while the maximum principal stress direction in the M(S)5. 9 aftershock zone equals to the direction after 34 degrees counterclockwise rotation of the maximum principal stress in the main quake zone, which might be attributable to local stress accumulation and adjustment after the mainshock. The seismogenic structure for M(S)7. 3 Yushu earthquake consists of two NW trending and NEE trending faults. The mainshock occurred in NW Ganzi-Yushu fault, while M(S)5. 9 aftershock possibly took place in an NEE trending buried fault. Both of these faults appear to be upright. Ganzi-Yushu fault is the tectonic boundary between Qiangtang Block and Bayan Har Block. Due to differential block movements of Qiangtang Block and Bayan Har block, the segment within Ganzi-Yushu Fault with strong coupling tends to accumulate stress dramatically, and when accumulated strain energy exceeded the rupture strength of rock, the M(S)7. 3 earthquake took place. The left-lateral slip of Ganzi-Yushu fault induced by the mainshock resulted in a significant reduction of normal stress, leading to the increase of Coulomb stress. After 45 days, the M(S)5. 9 aftershock sequence was triggered by the mainshock.
- 出版日期2015-6
- 单位中国地震局地震预测研究所; 江苏省地震局
