SEISMOTECTONIC BACKGROUND OF THE 2013 BENZILAN M5.9 EARTHQUAKE, YUNNAN PROVINCE

doi:10.3969/j.issn.0253-4967.2015.15

Abstract

Abstract:

The epicenter region of the Aug. 31 2013 M5.9 earthquake lies near the northwest boundary of the Sichuan-Yunnan rhombic block in the junction area of the three rivers geosynclinal folded system and Songpan-Ganzi geosynclinal folded system. The geological structure of this area is quite complicated, where the near-SN trending Jinshajiang Fault zone and the NW-trending Deqin-Zhongdian-Daju Fault zone converge. The Jinshajiang Fault zone is a large-scale ultra-lithosphere fault zone and an old suture line structure after the closing of the ancient Tethys Sea, which generally consists of 5-6 faults, constituting a 50km wide fault zone, with a long history of geological evolution. Since Cenozoic, the faults are mainly dominated by intense near east-west extrusion, thus, absorb partially the energy of the movement of east Tibet block towards Sichuan and Yunnan block. Therefore, the fault zone is dominated by dextral strike-slip since Pliocene.
According to the results of field geological and geomorphic investigations, the late Quaternary activity of Jinshajiang Fault zone is mainly embodied by the Zengdatong Fault, the Lifu-Riyu Fault, the Langzhong Fault and the Guxue Fault, with straight fault troughs, linear ridges, orientated facets in landforms and clear linear features in satellite imagery. These faults are obviously active in late Quaternary, and dominated by dextral strike-slip motion and partially thrust motion, with an average rate of 3.5～4.3mm/a horizontally and 0.9～1.1mm/a vertically, respectively since Holocene. Historically, the October 20, 1923, Batang M 6 (1/2) earthquake occurred near to the fault zone.
The Deqin-Zhongdian-Daju Fault is also obviously active in late Quaternary and dominated by dextral strike-slip motion and normal motion, the latest active time is from late-Pleistocene to Holocene with an average rate of 1.7～2.0mm/a horizontally and 0.6～0.7mm/a vertically, respectively. Along the fault, several string-beaded Quaternary basins are developed. The fault has played an obvious control role in the development of the Benzilan Basin, the Zhongdian Basin, the Haba Basin and the Daju Basin, and has a strict control on the boundary of the basins. Historically, several moderate-size earthquakes occurred around the fault, e.g. the 1961 Zhongdian M6.0 earthquake, the 1966 Zhongdian M5.2 earthquake and the 2013 Benzilan M5.9 earthquake, etc. It is an important NW-treading fault developed along the northwestern boundary of the Sichuan and central Yunnan rhombic block, and together with the Jinshajiang Fault, composes up the northwestern boundary of the Sichuan and central Yunnan rhombic block. The Deqin-Zhongdian-Daju Fault is a significant dextral strike-slip fault under the east-west extension of the Qinghai-Tibetan plateau and plays a role of regulating the southeast movement of the plateau substances.
The long axis of the intensity distribution of the 2013 Benzialn M5.9 earthquake is NW-directed, consistent with the strike of Deqin-Zhongdian-Daju Fault. According to the focal mechanism solutions, the M5.9 earthquake is of dextral strike-slip with normal faulting, the attitude of the NW nodal plane is consistent with the strike of the fault too, and the seismic rupture is identical to the kinematical characteristic of the fault. The linear distribution of the earthquake-induced landslides and collapse is consistent with the strike of the fault as well. Based on all these features and phenomena, we can conclude that the Deqin-Zhongdian-Daju Fault is the seismogenic fault of the Benilan M5.9 earthquake. The earthquake is closely related to the Deqin-Zhongdian-Daju Fault, but the Jinshajiang fault zone may be also the breeding structure. Based on analysis of the earthquake intensity and frequency, there is the possibility to generate larger earthquake in this area.

Key words: Jinshajiang Fault, Deqin-Zhongdian-Daju Fault, dextral strike slip, late-Quaternary

摘要：

2013年8月31日5.9级地震的震中地区位于川滇菱形块体西北边界附近, 地质构造复杂, 近SN向的金沙江断裂带与NW向的德钦-中甸-大具断裂在此交会。野外地质、地貌调查结果表明, 金沙江断裂带的曾大同断裂、里甫-日雨断裂带、郎中断裂、古学断裂等晚更新世—全新世表现出明显的活动迹象, 运动性质以右旋走滑为主, 兼有逆冲滑动分量, 其全新世右旋水平滑动速率为3.5～4.3mm/a, 垂直滑动速率为0.9～1.1mm/a。德钦-中甸-大具断裂具明显的右旋走滑兼正断性质, 最新活动时代为晚更新世—全新世, 水平滑动速率为1.7～2.0mm/a, 垂直滑动速率为0.6～0.7mm/a。该断裂与金沙江断裂带一起, 共同构成了川滇菱形块体的西北边界。它是青藏高原EW向伸展作用下的一条重要的右旋走滑断裂, 起着调节高原物质向SE运动的作用。据地震烈度等震线长轴方向、震源机制解资料和滑坡崩塌体展布位置分析认为, 2013年奔子栏M5.9地震的发生与德钦-中甸-大具断裂的活动密切相关。

关键词: 金沙江断裂, 德钦-中甸-大具断裂, 右旋走滑, 晚第四纪

CLC Number:

P315.2

CHANG Zu-feng. SEISMOTECTONIC BACKGROUND OF THE 2013 BENZILAN M5.9 EARTHQUAKE, YUNNAN PROVINCE[J]. SEISMOLOGY AND GEOLOGY, 2015, 37(1): 192-207.

常祖峰. 2013年云南奔子栏M5.9地震发生的地震地质背景[J]. 地震地质, 2015, 37(1): 192-207.

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