Abstract: The 2014 Jinggu M6.6 earthquake attacked the Jinggu area where few historical earthquakes had occurred and little study has been conducted on active tectonics. The lack of detailed field investigation on active faults and seismicity restricts the assessment of seismic risk of this area and leads to divergent view points with respect to the seismotectonics of this earthquake, so relevant research needs to be strengthened urgently. In particular, some studies suggest that this earthquake triggered the activity of the NE-trending faults which have not yet been studied. By the approaches of remote sensing image interpretation, structural geomorphology investigation and trench excavation, we studied the late Quaternary activity of the faults in the epicenter area, which are the eastern margin fault of Yongping Basin and the Yixiang-Zhaojiacun Fault, and drew the conclusions as follows:
(1)The eastern margin fault of Yongping Basin originates around the Naguai village in the southeastern margin of Yongping Basin,extending northward across the Qiandong, Tianfang, and ending in the north of Tiantou. The fault is about 43km long, striking near SN. The linear characteristic of the fault is obvious in remote sensing images. Structural geomorphological phenomena, such as fault troughs, linear ridges and gully dislocations, have developed along the faults. There are several dextral-dislocated gullies near Naguai village, with displacements of 300m, 220m, 146m, 120m and 73m, respectively, indicating that the fault is a dextral strike-slip fault with long-term activity. In order to further study the activity of the fault, a trench was excavated in the fault trough, the Naguai trench. The trench reveals many faults, and the youngest strata offseted by the faults are Holocene, with ¹⁴C ages of(1 197±51)a and(1 900±35)a, respectively. All those suggest that it is a Holocene active fault.
(2)The Yixiang-Zhaojiacun Fault starts at the southeast of the Jinggu Basin, passes through Xiangyan, Yixiang, Chahe, and terminates at the Zhaojiacun. The total length of the fault is about 60km, and is a large-scale NE-trending fault in the Wuliangshan fault zone. Four gullies are synchronously sinistrally dislocated at Yixiang village, with the displacements of 340m, 260m, 240m and 240m, indicating that the fault is a long-term active sinistral strike-slip fault. A trench was excavated in a fault trough in Yixiang village. The trench reveals a small sag pond and a fault. The fault offsets several strata with clear dislocation and linear characteristic. The thickness of strata between the two walls of fault does not match, and the gravels are oriented along fault plane. The offset strata have the ¹⁴C age of(2 296±56)a, (3 009±51)a, and(4 924±45)a, respectively, and another two strata have the OSL age of(1.8±0.1)ka, (8.6±0.5)ka respectively, by which we constrained the latest paleoearthquake between(1.8±0.1)ka(OSL-Y01)and(378±48)a BP(CY-07). This again provides further evidence that the fault is a Holocene fault with long-term activity.
(3)Based on the distribution of aftershocks and the predecessor research results, the 2014 Jinggu M6.6 earthquake and the M5.8, M5.9 strong aftershocks are regarded as being caused by the eastern margin fault of Yongping Basin, which is part of the Wuliangshan fault zone. The seismogenic mechanism is that the stress has been locked, concentrated and accumulated to give rise to the quakes in the wedge-shaped area near the intersection of the SN and NE striking faults, which is similar to the seismogenic mechanism in the southwest of Yunnan Province.

Key words: Jinggu earthquake, the eastern margin fault of Yongping Basin, the Yixiang-Zhaojiacun Fault, Holocene, seismogenic structure and seismogenic mechanism

摘要： 2014年景谷6.6级地震发生在历史地震活动稀少的景谷地区，该地区的活动构造研究程度薄弱，制约了人们对该区地震危险性的认识，也造成了对发震构造认识的分歧。文中通过遥感影像解译、构造地貌调查及探槽开挖，对震中区永平盆地东缘断裂和益香-赵家村断裂的活动性进行了研究，并得到以下认识：1）近SN向的永平盆地东缘断裂南起于永平盆地东南缘的那丙一带，向N经迁东、田房，终止于田头北，全长约43km，主要表现为右旋走滑的运动性质。那拐探槽揭示出多条断层发育，断裂断错的最新地层为全新世地层，其¹⁴C年龄为（1 197±51）a和（1 900±35）a，充分表明该断裂为全新世活动断裂。2）益香-赵家村断裂是无量山断裂带中规模较大的NE向断层，北端始自景谷盆地东南缘，向SW经过香盐、益香、岔河，至赵家村，全长约60km。在益香村处可见4条同步左旋位错的冲沟，位错量分别为340m、260m、240m和240m，显示该断裂是1条长期活动的左旋走滑断裂。益香探槽揭示该断裂断错了多套全新世地层，被断错地层的¹⁴C年龄分别为（2 296±56）a、（3 009±51）a和（4 924±45）a，另外2套被断错地层的OSL年龄为（1.8±0.1）ka和（8.6±0.5）ka，最新一次地震活动时间在（1.8±0.1）ka BP（OSL-Y01）-（378±48）a BP（CY-07）之间，同样表明为全新世活动断裂。3）据余震分布情况并结合前人成果分析认为，2014年景谷6.6级地震及5.8、5.9级强余震的发震构造为永平盆地东缘断裂，归属于无量山断裂带。其孕震机制为近SN向和NE向断裂交会的楔形区地应力闭锁积累，这与滇西南地区的孕震机制类似。

关键词: 景谷地震, 永平盆地东缘断裂, 益香-赵家村断裂, 全新世, 发震构造与孕震机制