HOLOCENE ACTIVITY AND PALEOEARTHQUAKES OF THE WEIXI-QIAOHOU FAULT

doi:10.3969/j.issn.0253-4967.2021.04.009

Abstract

Abstract:

The Weixi-Qiaohou Fault is located in the west boundary of Sichuan-Yunnan rhombic block, and also the north extension segment of active Red River fault zone. Strengthening the research on the late Quaternary activity of Weixi-Qiaohou Fault is of great theoretical and practical significance for further understanding the seismogeological background in northwest Yunnan and the structural deformation mechanism of the boundary of Sichuan-Yunnan block. Based on the 1︰50 000 active fault mapping and the research results of the National Natural Science Fund project, this paper mainly elaborates the latest active times of the fault and paleoseismic events along it revealed by exploration trenches at Matoushui, Shiyan, and Yushichang. Matoushui trench revealed three faults developed in late Pleistocene and Holocene pluvial fan accumulation, and the latest ages of faulted strata are(638±40)a BP and(1 335±23)a BP, respectively. The Shiyan trench revealed six faults, three in the western section and three in the eastern section. The three faults in the western section dislocated the late Pleistocene and Holocene accumulation, and the ¹⁴C ages of the latest faulted strata are(4 383±60)a BP, (4 337±52)a BP and(4 274±70)a BP, respectively; the other three faults revealed in the eastern part of the trench offset the Holocene fluvial facies accumulation, the ¹⁴C age of the latest faulted strata in the footwall of the main fault is(9 049±30)a BP, and the ¹⁴C ages of two sets of faulted sag pond deposits in the hanging wall are(1 473±41)a BP and(133±79)a BP, separately. Five active faults are revealed in Yushichang trench. Among them, the F₁ and F₂ dislocated the gray-white gravelly clay layer and the black peat soil layer. The ¹⁴C age of the gray-white gravelly clay layer is(1 490±30)a BP, and ¹⁴C ages of the upper and lower part of the black peat soil layer are(1 390±30)a BP and(1 190±30)a BP, respectively. The F₃ and F₄ faults offset the gray-white gravelly clay layer, the black peat soil layer and the brown yellow sand bearing clay, and the OSL age of brown yellow sand bearing clay is(0.6±0.2)ka. The F₅ fault dislocated the gray-white gravelly clay layer, its ¹⁴C age is(1 490±30)a BP. According to the relationship between strata and the analysis of dating data, the Yushichang trench revealed two seismic events, the first one occurred at(1 490±30)~(1 390±30)a BP, as typified by the faulting of F₅, the second paleoseismic event is represented by the faulting of F₁, F₂, F₃ and F₄.The F₁ and F₂ faulted the gray-white gravelly clay layer and the black peat soil. Fault F₃ and F₄ dislocated the gravelly clay, the peat soil and the sandy clay, and a seismic wedge is developed between fault F₃ and F₄, which is filled with the brownish yellow sandy clay. The OSL dating result of the brownish yellow sandy clay layer is(0.6±0.2)ka. Judging from the contact relationship between strata and faults, F₃ and F₄may also faulted the upper brownish yellow sandy clay layer, but the layer was eroded due to later denudation. Therefore, fault F₁, F₂, F₃ and F₄ represent the second event. Combined with the analysis of fault scarps with a height of 2~2.5m and clear valley landform in the slope near the fault, it is estimated that the time of the second paleoearthquake event is about 600 years ago, and the magnitude could reach 7. The trench at Gaichang reveals that the seismic wedge, soft sedimentary structure deformation and the medium fine sand uplift(sand vein)and other ancient seismic phenomena are well developed near the fault scarp. All these phenomena are just developed below the fault scarp. The vertical dislocation of the strata on both sides of the seismic wedge is 35cm, and ¹⁴C ages of the misinterpreted peat clay are(36 900±350)a BP and(28 330±160)a BP, respectively, so, the occurrence time of this earthquake event is estimated to be about 28 000a BP. If the fault scarp with a height of 2m was formed during this ancient earthquake, and considering the 0.35m vertical offset revealed by the trench, the magnitude of this ancient earthquake could reach 7.The Matoushui trench revealed three faults, which not only indicated the obvious activity of the faults in late Pleistocene to Holocene, but also revealed two paleoseismic events. Among them, the OSL age of the faulted sand layer by fault F₁ is(21.54±1.33)ka, which represents a paleoearthquake event of 20 000 years ago. The faulted strata by fault F₂ and F₃ are similar, which represent another earthquake event. The ¹⁴C dating results show that the age of the latest faulted strata is(638±40)Cal a BP, accordingly, it is estimated that the second earthquake time is about 600 years ago. A clear and straight fault trough with a width of several ten meters and a length of 4km is developed from Meiciping to Matoushui. Within the fault trough, there are fault scarps with different heights and good continuity, the height of which is generally 3~5m, the lowest is 2~3m, and the highest is 8~10m. Tracing south along this line, the eastern margin of Yueliangping Basin shows a fault scarp about 5m high. After that, it extends to Luoguoqing, and again appears as a straight and clear fault scarp several meters high. In addition, in the 2km long foothills between Hongxing and Luoguoping, there are huge rolling stones with diameters of 2~5m scattered everywhere, the maximum diameter of which is about 10m, implying a huge earthquake collapse occurred here. According to the length, height, width and dislocation of the rupture zone, and combined with the experience of Yiliang M≥7 earthquake and Myanmar Dongxu M7.3 earthquake, this earthquake magnitude is considered to be ≥7.

Key words: Weixi-Qiaohou Fault, Holocene activity, paleoearthquake, trench

摘要：

维西-乔后断裂位于川滇菱形块体的西边界, 晚第四纪活动迹象明显, 是红河活动断裂带的北延部分, 因此加强其活动性研究对深入认识滇西北地区地震地质背景和川滇块体边界构造变形机制具有重要的理论和实际意义。文中依据1︰5万活动断裂填图和国家自然基金项目研究成果, 重点介绍了马头水、石岩、玉狮场等探槽揭示出的断裂最新活动时代和沿线古地震事件, 结果表明, 玉狮场-平坡段为全新世活动段并存在多次古地震事件。马头水探槽揭示出的3条断层断错晚更新世-全新世洪积扇堆积, 断错的最新地层的年代为(638±40)a BP和(1 335±23)a BP。石岩探槽西段揭露的3条断层断错的最新地层的¹⁴C测年结果分别为(4 383±60)a BP、 (4 337±52)a BP和(4 274±70)a BP; 探槽东段同样揭示发育了3条断层, 主断层下盘断错的最新地层的¹⁴C年代为(9 049±30)a BP, 上盘断错的2套断塞塘堆积的¹⁴C测年结果分别为(1 473±41)a BP和(133±79)a BP。玉狮场探槽揭示出发育了5条活动断层, 其中F₁与F₂断错灰白色含砾黏土和黑色泥炭质土, 前者的¹⁴C测年结果为(1 490±30)a BP, 后者的¹⁴C测年结果分别为(1 390±30)a BP和(1 190±30)a BP; F₃与F₄断错灰白色含砾黏土、黑色泥炭质土及上部褐黄色含砂黏土, 褐黄色含砂黏土的OSL年龄为(0.6±0.2)ka; F₅断层断错的黏土层的¹⁴C年龄为(1 490±30)a。盖场探槽揭示出地震楔、软沉积构造变形和砂脉等古地震现象, 垂直错距35cm, 估计此次地震事件的时间约为28 000a BP, 震级达7级。玉狮场探槽揭示了2次地震事件, 第1次事件的时间为(1 490±30)~(1 390±30)a BP; 第2次事件的时间约为距今600a, 推测震级达7级。马头水探槽揭示出垂直错距约1m的地震事件, 发生时间约为600a BP, 据地震破裂带的长度、宽度、陡坎高度、位错量并结合宜良M≥7地震和缅甸东吁7.3级地震的经验, 认为此次地震的震级≥7级。

关键词: 维西-乔后断裂, 全新世活动, 古地震, 探槽

CLC Number:

P315.2

CHANG Zu-feng, CHANG Hao, LI Jian-lin, MAO Ze-bin, ZANG Yang. HOLOCENE ACTIVITY AND PALEOEARTHQUAKES OF THE WEIXI-QIAOHOU FAULT[J]. SEISMOLOGY AND EGOLOGY, 2021, 43(4): 881-898.

常祖峰, 常昊, 李鉴林, 毛泽斌, 臧阳. 维西-乔后断裂全新世活动与古地震[J]. 地震地质, 2021, 43(4): 881-898.

Figures/Tables 14

Fig. 1 Distribution of Weixi-Qiaohou Fault.

Fig. 2 Fault landform around the Matoushui trench and the faults revealed by the trench.

Fig. 3 The cross section of the NW-wall of Matoushui trench.

Fig. 4 Fault landforms in Matoushui area.

Fig. 5 Sketch map of fault scarps and other landforms on the terrace at Shiyan village.

Fig. 6 The fault section of N-wall of the west segment of Shiyan trench.

Fig. 7 The fault section of N-wall of the east segment of Shiyan trench.

Fig. 8 Fault landforms and the section of SE-wall of Yushichang trench.

Fig. 9 Fault scraps at Yushichang.

Fig. 10 The fault scarp landform and the fault section of SE-wall of Gaichang trench.

Fig. 11 The fault section of SE-wall of Gaichang trench.

Fig. 12 The enlarged section of seismic wedge.

Fig. 13 The enlarged map of soft sediment deformation of region a.

Fig. 14 The enlarged map of soft sediment deformation of region b.

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