西藏尼木1901年M 6?地震的发震构造探讨

史料记载1901年4月26日西藏尼木发生M 6?级地震，其发震构造尚未有报道，对其发震构造的厘定有助于理解尼木地堑群的地震复发规律，科学评价周边地区的未来强震危险性。遥感解译与地质调查表明，尼木地堑群内部的庞刚地堑西边界断裂长约30 km，走向近北西—北北西，以彭刚玛曲为界分为南北两段。北段断裂地貌线性特征显著，陡坎发育，断错了多级冰碛物及河流阶地。位移恢复结果显示，河流阶地垂直断距T0约1.0 m，T1约2.6 m，T2约5.0 m。南段断裂沿虾庆曲展布，地貌线性特征显著，陡坎发育，断错了多期冲洪积扇体。尼木县城北部发现断裂错动T2阶地剖面，显示该断裂延伸至尼木县城北部。根据位移-震级经验公式计算，庞刚地堑西边界断裂最新一次地震的矩震级约为M_W 6.8，这与尼木地震比较吻合。遥感解译、地质调查与震级表明，庞刚地堑可能为1901年尼木地震的发震构造。结合历史地震记录分析认为，尼木地堑群中各个地堑具有独立发生中强地震的能力，其地震复发模式及其与亚东-谷露裂谷南北两段的地震活动差异等尚需进一步研究。 

Discussion of the seismogenic structure of the 1901 M 6¾ Nyemo earthquake

According to historical records, an M 6¾ earthquake occurred in Nyemo, Tibet on April 21, 1901 without any reports on its seismogenic structure yet. To sort out its seismogenic structure helps to understand the earthquake recurrence pattern of the Nyemo graben group (NGG) and to scientifically evaluate the future earthquake risk of surrounding areas. According to our results from the remote sensing interpretation and geological survey, a 30 km-long fault with a trend of nearly NW-NNW at the western boundary of the Panggang graben (PG), one of the NGG, is divided into north segment and south segment by the Penggang River. The north segment is characterized by significant linear features, with fault scarps developed, offsetting multistage river terraces and moraines. The recovered displacements show that the vertical offsets of Terrace T0, T1 and T2 are ~1.0 m, ~2.6 m and ~5.0 m, respectively. The south segment with obvious linear features develops along the Xiaqing River and extends to the north Nyemo County, with fault scarps developed, offsetting multi-stage alluvial fans. We found the offset Terrance T2 profile in northern Nyemo, indicating the extension of the fault. Through the magnitude recovery, we calculated the moment magnitude of the latest earthquake as about M_W6.8 along the PG, which is consistent with the Nyemo earthquake. Inferred from the results of the satellite image interpretation, geological survey and magnitude recovery, the PG is likely the seismogenic structure of the Nyemo earthquake. Also based on historical seismic records, we consider that each graben in the NGG could independently generate moderate-strong earthquakes, and the earthquake recurrence pattern and the seismicity difference between the NGG and the north and south segments of the Yadong-Gulu rift need further study.