Abstract:

An earthquake of M_S6.5 occurred in Ludian county, Zhaotong city, Yunnan province, on August 3, 2014. Aftershock activity appears to be on two conjugated near-vertical Zhaotong-Ludian Fault and Baogunao-Xiaohe Fault, which is unusual phenomenon in aftershock distribution. Usually, most earthquakes occur on a single fault or fracture zone with several end-to-end linked faults. Therefore, the 2014 Zhaotong-Ludian earthquake sequence has attracted great attention from geoscientists, and a lot of work has been done in terms of the precise location of aftershocks, the focal mechanisms of the earthquake sequence, co-seismic slip model and the deep structure of the Earth’s crust. In order to determine the fault on which the mainshock occurred and to understand why the aftershocks occurred on the conjugate faults based on the tectonic stress point of view, we used the new developed fuzzy clustering and fault plane fitting method to calculate the geometry parameters of the 2 fault branches. The result shows that the strike and dip angle of the NNW-SSE branch(Baogunao-Xiaohe fault zone)are 336.67° and 88.41°, respectively, and the strike and dip angle of the near EW branch(Zhaotong-Ludian fault zone)are 266.10° and 86.42°, respectively. Both the Zhaotong-Ludian Fault and Baogunao-Xiaohe Fault are nearly vertical faults, which is consistent with the strike-slip stress regime in this area. In order to investigate the relative magnitude of the shear and normal stress on the two faults generated by the regional stress field, 128 focal mechanisms around the 2014 Ludian earthquake were collected, and types of focal mechanism are classified. It is found that most of the focal mechanisms in this area are strike-slip type, accounting for 61.72%, while the sum of other types of focal mechanisms is only 38.28%. By using the collected focal mechanism data, we estimated the stress tensor in this region. The determined stress tensor shows NW-SE compression and NE-SW extension, also a strike-slip stress regime. It reflects that the Qinghai-Tibet plateau was pushed and uplifted by the Indian plate, and the material flowed eastward, which was blocked by the hard blocks of the Sichuan basin and forced to turn southward, showing the extrusion of NW-SE and the tension of NE-SW. By projecting the stress field onto the two faults, it was observed that the relative shear stress on both the Baoguanya-Xiaohe Fault and the Zhaotong-Ludian Fault are large with 0.99 and 0.77, respectively, indicating that both faults are capable of seismic activity. Moreover, the Baogunao-Xiaohe Fault experiences higher relative shear stress compared to the Zhaotong-Ludian Fault, suggesting that seismic activity on the former is stronger. The observed aftershock seismicity on the Baogunao-Xiaohe Fault is stronger than that on the Zhoatong-Ludian Fault illustrated by frequency, magnitude and overall releasing seismic moment v.s. time, which validates the estimated larger shear stress on the Baogunao-Xiaohe Fault.

Key words: conjugated faults, fault structure, seismogenic mecganism, stress field

摘要：

2014年8月3日云南省昭通市鲁甸县发生 M_S6.5 地震。该事件的特殊之处在于余震活动表现在共轭的昭通-鲁甸断裂和包谷垴-小河断裂上。为了从应力角度探究地震活动发生在2条断裂上的原因及确定主震发生在哪条断裂上, 文中首先搜集了鲁甸地震周围的震源机制数据, 求解了区域构造应力场, 然后采用新发展的模糊聚类断层面拟合法获得了地震2条断层的产状参数: NNW-SSE向分支的走向和倾角分别为336.67°和88.41°, 近EW向分支的走向和倾角分别为266.10°和86.42°。之后, 将应力场投影到2条断层面上, 发现包谷垴-小河断裂和昭通-鲁甸断裂的相对剪应力都很大, 说明2条断层均可能发生地震活动。相比之下, 包谷垴-小河断裂处的相对剪应力更大, 可能预示着包谷垴-小河断裂的地震活动比昭通-鲁甸断裂更强烈。上述分析得到了2条断层上地震活动的M-T图和总体地震矩的支持。

关键词: 共轭断层, 断层结构, 发震机制, 应力场