2016年1月21日青海门源MS6.4地震破裂方向性研究

Rupture directivity of the January 21,2016 Menyuan,Qinghai earthquake

2016年1月21日青海省门源县发生MS 6.4级地震,震中及周围地区震感明显,大量房屋损坏.虽然地震发生在人烟稀少地区但仍有9人受伤.本文开展了门源地震震源参数及破裂过程参数的研究,以期为理解门源地震的发震机理,完善震中区域震后灾害评估体系,进一步探究中等强度逆冲地震的破坏程度空间分布等提供基础信息.首先,利用青海、甘肃省固定台站和部分青海省流动台站记录的地震波形数据,拾取近震震相Pg、Pn、Sg到时,并使用HYPOINVERSE-2000和Pn/Pg相对定位方法,以2016年8月13日门源MS 4.7余震作为参考事件对主震进行重新定位.然后,结合两次地震CAP方法反演过程中的质心深度和时移信息,利用最小二乘法拟合得到门源地震沿走向和倾向的破裂方向性特征.结果显示主震沿断层走向破裂长度为9.6 km,呈现单侧破裂特征;主震质心深度和起始破裂深度分别为9 km和10 km,沿断层倾向方向上属于双侧破裂.对于门源地震的深部孕震过程,推测可能是青藏高原东北缘受到印度板块与欧亚板块碰撞和阿拉善地块与鄂尔多斯地块刚性阻挡作用下,经过长期应力积累导致地震的发生.

On January 21 2016, an MS 6.4 earthquake occurred in Menyuan County, Qinghai Province. A large number of houses were damaged and the earthquake caused numerous economic losses. Moreover, 9 people were injured although the earthquake occurred in a sparsely populated area. Menyuan is located in the northeastern margin of Tibetan Plateau with strong tectonic activity, and many researchers have worked on this earthquake. Studying the seismogenic mechanism of this earthquake helps improve the seismic assessment of the epicentral region and explore the spatial damage distribution of moderate thrust earthquakes in further. This paper has carried out a series of studies on the point source and rupture parameters of the Menyuan earthquake. First we collected seismic waveform data recorded by Qinghai, Gansu seismic network and some temporary stations in Qinghai Province, and picked the Pg, Pn, Sg phases manually. Then, we used the HYPOINVERSE-2000 and the Pn/Pg relative location method to determine the relative location between the mainshock and the MS 4.7 August 13, 2016 aftershock. We inverted the point source parameters with the CAP method, and combined the relative centroid depths and the time-shift information obtained during the CAP inversion to determine the rupture directivity of the Menyuan earthquake. The along-strike rupture directivity of the mainshock was resolved by fitting the waveform time shift with the least-square method. The results show that the along-strike rupture length is 9.6 km, suggesting unilateral rupture. The centroid depth and initial rupture depth of the mainshock are 9 km and 10 km, respectively, implying that the mainshock ruptured bilaterally along-dip. After bootstrapping tests, we found that the along-strike rupture length was about 8~12 km. The depth of the centroid and the initial rupture depth along dip were also stable, with value of 9 km and 10~11 km. We infer that the Menyuan earthquake occurred as the result of long-term stress accumulation via the collision between the Indian plate and the Eurasian plate and the blocking of Ordos and Alxa rigid block.