【摘要】 利用双差定位方法对玉树地震序列2010年4月14日至10月31日间发生的ML≥1.0地震进行双差定位,得到1545个地震的重定位结果.综合分析地震双差定位结果和玉树地震序列中强地震震源机制解,发现玉树MS7.3地震发震构造由北西向和北东东向两条相交断层组成,主震发生在北西走向的甘孜—玉树断裂带上,5月29日的MS5.9余震序列发生在北东东走向的一条隐伏断裂上,两条断裂均接近直立.甘孜—玉树断裂是羌塘地块和巴彦喀拉地块的构造边界,由于羌塘地块和巴颜喀拉地块的差异运动使甘孜—玉树断裂强耦合段应力高度积累,在应变能超过岩石强度时破裂失稳发生了MS7.3地震.主震断层的左旋滑动导致北东东向断层的正应力减小,库伦应力增加,45天后触发了MS5.9余震序列的活动.更多还原

【Abstract】 The object of this paper is to investigate the seismogenic fault structures and stress field where MS7.3 Yushu earthquake sequence occurred in April 2010,and then tentatively discuss the triggering mechanism for the Yushu MS7.3mainshock and MS5.9strong 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 ML≥1.0earthquakes among Yushu earthquake sequence from April14 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 with NW trending and NEE trending respectively.The MS7.3mainshock occurred in the north-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 MS5.9aftershock on May 29 occurred in a wide NEE rectangular strip,which extended approximately 40 kilometers with a width of 18 kilometers,indicating apossible 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 8to 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 MS5.9aftershock 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 MS5.9aftershock 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 MS7.3 Yushu earthquake consists of two NW trending and NEE trending faults.The mainshock occurred in NW Ganzi-Yushu fault,while MS5.9aftershock 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 MS7.3earthquake 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 MS5.9aftershock sequence was triggered by the mainshock.更多还原