2017年8月8日九寨沟M_S7.0地震序列分析

A study on the Jiuzhaigou M_S7.0 earthquake sequence,August 8,2017,in Sichuan Province

2017年8月8日四川九寨沟发生M_S7.0地震,该地震发生在巴颜喀喇块体的东北边界,震中区域构造条件复杂,是巴颜喀喇块体北侧左旋走滑环境向东侧逆冲挤压环境过渡的位置,附近地区历史强震较多。九寨沟地震是一次主-余型地震,余震活动水平较弱,主震发生后短时间内ML≥4.0余震的"等待时间"存在异常,震后较长时间余震活动恢复到正常状态,序列h值、余震视应力等符合主-余型序列特征。序列b值为0.84,G-R关系推测序列最大余震的震级约为ML5.4(M_S5.0),8月9日发生的M_S4.8地震是目前该序列的最强余震。通过与1970年以来附近地区7级左右地震序列的对比认为,九寨沟地震与1976年松潘-平武2次7.2级地震序列在余震空间位置、发震构造和震源机制等方面存在较大差异,因此,不具备发育为震群型序列的条件。九寨沟地震主震视应力为0.36~0.38MPa,属于应力下调模型,序列余震的平均视应力水平接近龙门山断裂带附近中小地震的平均背景水平。

On August 8,2017,an earthquake occurred near Jiuzhaigou County,Sichuan Province with M_S7.0. The earthquake happened near the northeast boundary of the Bayan Har fault block where the geological structure is very complicated. The earthquake is located at the transitional position from the north boundary of the Bayan Har fault block with a left-lateral tectonic environment to the east boundary with a strike-slip tectonic environment and many strong earthquakes happened near this field in history. The Jiuzhaigou earthquake sequence belongs to mainshock-aftershock type with an extremely low seismic activity of the aftershocks. Although the time intervals between the aftershocks with M_L≥ 4.0 is abnormal in a short time after the mainshock,the seismic activities of the aftershocks returned to normal soon after that,and the h value of the sequence and the apparent stress of the aftershocks show that the characteristics of the sequence is in accordance with the mainshock-aftershock type. The b value of the sequence is0.84,the magnitude of the largest aftershock deduced from the G-R relationship is about ML5.4（ M_S5.0） and the M_S4.8 earthquake on August 9 is the largest aftershock of the sequence up to now. With the comparison between the Jiuzhaigou earthquake sequence and the other sequences with the magnitude about M_S7.0 near the field since 1970,we find that there are many apparent differences between the Jiuzhaigou sequence and the two M_S7.2 earthquakes near the SongpanPingwu area in 1976 in the spatial position of the aftershocks,the seismogenic structure and the focal mechanism of the mainshocks. Therefore,the conditions were not sufficient for the Jiuzhaigou earthquake to develop into a multiple mainshock sequence. The apparent stress of the mainshock of the Jiuzhaigou sequence is 0.36 ~ 0.38 MPa. It shows that the earthquake belongs to the undershoot model. The average apparent stresses of the aftershocks are close to the background average stresses of the moderate and minor earthquakes near the Longmenshan fault zone.