Discussion of the seismogenic structure of the 1901 M 6¾ Nyemo earthquake
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摘要: 史料记载1901年4月26日西藏尼木发生M 6¾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MW 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。Abstract: 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 MW6.8 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.
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Key words:
- 1901 Nyemo earthquake /
- seismogenic structure /
- Yadong-Gulu rift /
- Panggang graben
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图 1 亚东-谷露裂谷构造简图
a—藏南裂谷系分布简图；b—亚东-谷露裂谷组成结构及历史地震分布、震源机制解简图
Figure 1. Tectonic sketch of the Yadong-Gulu rift. (a) Distribution of the rifts in southern Tibet. (b) Composition of the Yadong-Gulu rift and the distribution and focal mechanism of historical earthquakes
图 2 尼木地堑群几何展布特征图
PBG—帕布地堑；PEGG—彭刚地堑；PAGG—庞刚地堑；AG—安岗地堑；YG—羊易地堑；GG—格达地堑
a—尼木地堑群组成结构；b—跨尼木地堑群地质剖面图Figure 2. Geometric distribution of the Nyemo graben group. (a) Composition of the NGG. (b) Geological section cross the NGG.PBG—Pabu graben; PEGG—Penggang graben; PAGG—Panggang graben; AG—Angang graben; YG—Yangyi graben; GG—Geda graben
图 3 尼木地震等震线图(西藏自治区科学技术委员会档案馆，1982)
Figure 3. Isoseismal map of the Nyemo earthquake (The Science and Technology Committee and the Archives of Xizang Autonomous Region, 1982)
图 4 庞岗地堑几何特征展布遥感影像及解译图(a中红色箭头指示断裂位置)
Figure 4. Remote sensing image and interpretation of the geometric distribution of the Panggang graben (The red arrows in Fig. 4a show the trace of the faults)
图 5 庞刚地堑N01调查点多级河流阶地位错地貌特征
P0，P1，P2指示跨断层及阶地陡坎地形剖面位置; 图中红色箭头指示断裂位置
a，b—庞刚地堑N01点遥感影像及解译图；c—多级阶地位错照片Figure 5. Features of fault scarps of multi-stage river terraces at Site N01. (a, b) Satellite image and interpretation at Site N01 of the Panggang graben. (c) Field photo of multi-stage terrace offsets. P0, P1 and P2 show the location of the topographic section cross the faults or terraces. The red arrows show the trace of the faults.
图 8 尼木玛曲阶地野外地质照片
a—尼木玛曲五级河流阶地；b—尼木县北部T3阶地砾石层直接覆盖在基岩之上；c—T3阶地砾石统计点(砾石最大扁平面统计显示古流向为南东)
Figure 8. Field photos of the Nyemo Maqu terraces. (a) Five-stage terrace along the Nyemo Maqu River. (b) The gravel layer of T3 directly covers on the bedrock in the north of Nyemo county. (c) The position for statistics in Terrace T3, and the largest flat plane of gravel shows that the paleocurrent flowed towards southeast.
图 9 庞刚地堑晚第四纪正断层活动野外照片
a，b—剖面1尼木县雪莱村北正断层，可见断层错动晚第四纪河流阶地，断距约1.0 m；c，d—剖面2正断层观察点，可见断层错断晚第四纪冲积河流阶地，砾石层被显著错动约0.5 m；e—剖面3观察点，可见阶梯状正断层错动晚第四纪冲洪积扇
Figure 9. Filed photos showing the late Quaternary normal faulting along the Panggang graben. (a, b) Normal fault of the Section 1 in the north of Xuelai village, showing that fault dislocated the late Quaternary river terrace with a displacement of ~1.0 m. (c, d) Observation point in the normal fault of the Section 2, showing that the fault dislocated the late Quaternary alluvial river terrace with an obvious displacement of ~0.5 m on the gravel layer. (e) Observation point in the Section 3, showing the late Quaternary alluvial-proluvial fan with step-like normal fault dislocations.
表 1 历史记载以来沿尼木地堑发生的M≥6.0地震参数(吴中海等，2015)
Table 1. Seismic parameters of M≥6.0 along the NGG on record (Wu et al., 2015)
地震日期 震中位置 震级 可能的发震构造 北纬/东经 参考地点 1264 29.70°/90.60° 楚布寺 ≥6¾ 羊易地堑/安岗地堑 1901.04.21 29.50°/90.10° 尼木北 6¾ 庞岗地堑 1990.07.30 29.60°/90.25° 安岗 6.3 安岗地堑 2008.10.06 29.80°/90.30° 羊易 6.6 羊易地堑 
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