The Amojiang fault zone and Mojiang M5.9 earthquake in 2018 in southern Yunnan Province
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摘要:
野外调查表明，阿墨江断裂Ⅲ级夷平面和T4阶地上发育断层垭口，其下地貌面则无断层地貌显示；断层构造岩胶结紧密或呈半固结状，少有断层泥发育；SEM测年显示早更新世活动，断层泥ESR年龄为549±54ka，断层上覆中—上更新统残坡积土未被切穿或发生构造变形；说明该断裂带最新活动时代为早—中更新世。2018年墨江M5.9地震等震线长轴呈与断裂带走向一致的NW方向，指示此次地震的发震构造为阿墨江断裂带西支。受青藏高原物质向东南挤出的影响，震中所在的思茅块体作S向运动。虽然阿墨江断裂带晚第四纪无明显的地表活动迹象，但因该断裂带本身构成了宽阔的构造软弱带，在近SN向挤压应力场作用下，局部地区的应力集中仍可能导致新的破裂并触发类似墨江M5.9地震的中-强地震活动。从区域上看，云南还存在构造条件与之相似的断裂带，如滇东地区的弥勒-师宗断裂带，历史上沿该断裂带曾发生11次5级以上中-强地震。因此，在区域地震危险性评价中，还应该关注第四纪活动迹象不明显，但仍有潜在发震危险性的区域性大断裂。
Abstract:Field investigation shows that fault mouths are developed on the planation surface Ⅲ and river terrace 4 in the Amojiang fault zone, and below them there is no indication of fault landforms.Structural rocks dated by SEM are Early Pleistocene in age and are cemented compactly or half consolidated, and a few fault gouges dated by ESR as 549±54ka and developed on the fault plains and Middle-Upper Pleistocene deposits covering the faults have not been displaced or deformed tectonically. All these phenomena suggest that the fault zone was mainly active in Early-Middle Pleistocene. The 2018 Mojaing M5.9 earthquake occurred on the western branch of the Amojiang fault zone, and the long azimuth of isoseismal lines of the quake was in NW direction, consistent with the strike of the fault, suggesting that the seismogenic structure of the quake should be the western branch of the fault zone. Despite of no signs of activeness in Late Quaternary along the fault zone, the Amojiang fault zone itself became a broad relative weak belt. Affected by the SE extrusion of the Tibetan Plateau matters, the Simao block where the epicenter is located moved southward. Although the Amojiang fault zone was in absence of evident ground activity during Late Quaternary, the zone itself formed broad weak structural belt. Under the circumstance of the NS-trending tectonic compression stresses, stresses concentration in local area could also give rise to new break and induce the middle-large earthquakes like the Amojiang M5.9 earthquake.Regionally, there are similar fault zones in structural conditions, such as the Mile-Shizong fault zone in southeastern Yunnan, along which there historically occurred 11 earthquakes of magnitude over M5.0. Therefore, the researchers should pay attention to the regional large faults which were not sub-surficially active in Late Quaternary, but still have potential seismic risk in the regional seismic risk assessment.
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Key words:
- Amojiang fault zone /
- Early-Middle Pleistocene /
- structural rocks /
- Magnitude 5.9 earthquake /
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表 1 研究区阶地实测年龄数据
Table 1. Dated age data for terraces in the study area
野外编号 经纬度 岩性 采样地点与层位 测试方法 年龄/ka MX3 23.042N、101.212°E 粉砂 勐先南勐先大河T1阶地砾石层之上砂层，拔河高度5~6m TL 23.49+1.99 MX5 23.013°N、101.337°E 粉砂 普义西普义河T3阶地砂砾石层，拔河高度50m TL 84.46+7.18 MX6 23.320°N、101.148°E. 粉砂 民生磨黑河T3，阶地粉砂层，拔河高度30m TL 78.49+6.67 MX7 23.320N、101.151°E 砾石夹砂士 民生磨黑河T1阶地砾石层，拔河高度6~8m TL 14.09+1.19 MX8 23.212°N、101.272°E 砂土夹砾石 把边煤矿断裂坡洪积盖层 TL 75.23+6.39. MX9 23.291°N、101.368°E 砂土夹砾石 通关西玄武岩。上覆坡洪积物 TL 285.71+24.29 注：样品测试由中国地震局地壳应力研究所热释光实验室完成 
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