2016年1月21日青海门源6.4级地震的发震机制探讨

2016年01月21日青海省门源县发生6.4级地震,地震现场考察的震害分布椭圆长轴走向120°N—140°E.震后0.8 m高分遥感影像与震前高分影像的对比解译结果表明,本次地震导致大于23处较集中的崩塌滑坡,它们的空间分布表现出震中北侧多于南侧,分布点总体形态呈NNW向延展的平面特征.区域断裂几何展布和活动性质的高分遥感解译和野外考察研究表明,冷龙岭断裂水平运动分量占绝对优势,如果本次地震发生在该断裂上,不应为纯逆冲性质.震中区域活动断裂的精细研究发现在冷龙岭北侧发育一条走向约为140°的活动断裂,该断裂在高分影像上地表为北倾,该断裂与多家机构的震源机制解节面Ⅰ走向非常相近.本次地震的余震分布总体长轴方向与冷龙岭断裂相差约20°,而与最新发现的冷龙岭北侧断裂走向相近.综合以上多方面资料,认为冷龙岭北侧断裂极有可能是本次地震的发震断裂.综合余震分布在深部的展布特征,主震的震源机制解,发震断裂在地表的几何展布特征和活动性质,再结合震区附近大地电磁测深等地球物理资料,建立了发震机制模型,认为本次地震是2008年于田7.4级地震、2014年于田7.3级地震后,青藏高原块体向北东方向推挤生长过程中发生的一次地震事件.

Seismogenic mechanism of the 21 January 2016 Menyuan,Qinghai MS6.4 earthquake

On January 21th, 2016, a M_S6.4 earthquake occurred in Menyuan country, Qinghai province. Field investigations suggest that the long axis of the elliptical affected area is 120°N—140°E. Comparative interpretations of high resolution remote sensing images before and after the shock shows more than 23 sites of landslides and collapses, which are more distributed in the north of epicenter than in the south, extending in NNW overall. As the Lenglong Ling fault (LLF) is dominated by horizontal movement from high resolution remote images and field observation, it should not be responsible for this event of pure thrusting. North of the Lenglong Ling fault, there is a fault striking in 140°, which is north-dipping on the surface according to high resolution remote sensing, similar to the nodal plane I of focal mechanism solutions from deferent institutes. The long axis' direction of the overall distribution of aftershocks has also a difference of 20° with LLF, while almost parallel to the fault on its north. Thus, infer that this fault is most likely to be the seismogenic structure of the Menyuan earthquake. Integrating the characteristics of aftershock distribution at depth, main shock focal mechanism solutions, geometric distribution and activity of the seismogenic fault on surface, in combination with the magnetotelluric sounding and related geophysical data around the earthquake zone, a cartoon model of the seismogenic mechanism is established. We speculate this earthquake is one of events occurring during the pushing growth process of the Tibetan Plateau blocks in NE direction following the 2008 M_S7.3 and 2014 M_S7.4 in Yutian, Xinjiang temblors.