2019年夏河MS5.7地震前应力应变变化特征研究

Characteristics of the stress-strain variation before the 2019 Xiahe M 5.7 earthquake

利用CMONOC的GPS观测资料和中小地震震源机制解资料,通过GPS速度场分析、两站间基线时序分析、震中区域的应变时序分析和震源机制一致性分析,研究了2019年夏河MS 5.7地震前的应力应变累积特征.结果表明,震中处于运动速率和方向差异性较为显著的应变能易积累地区;跨发震断裂的去趋势基线长度和基线方位角时序反映了震前发震断裂的左旋走滑速率有所减缓;多站组合的应变参数结果反映出,震前震源区NS向运动幅度增强,有助于此次地震的发生,且2016年后拉张和走滑运动有所减缓;同时,震中附近区域震源机制一致性较高,反映了震源区的高应力水平.综合分析认为夏河MS 5.7地震前,震源区存在应力应变累积增强的背景性异常变化特征.

The stress-strain accumulation characteristics before the Xiahe MS 5.7 earthquake occurred in 2019 are studied based on the GPS observation data of CMONOC and the focal mechanism solution data of small and medium earthquakes. The comprehensive analysis method including the GPS velocity field, baseline time series between two stations, strain time series in epicenter region, and focal mechanism consistency are used. The results show that the near-field velocity value of the epicenter is smaller than the far-field velocity value. It is located at the edge of the Gansu-Qinghai vortex and the Yunnan-Tibet vortex, and in a strain energy accumulated area where has an obvious difference in crustal movement rate and direction. The time series curves of the detrended baseline length and baseline azimuth angle across the seismogenic fault showed a significant trend to slow down before the earthquake, which reflected that the left-lateral strike-slip rate of the seismogenic faults slowed down before the earthquake. Besides, the time series of strain parameters results of multi-station combination show that the linear strain rate in NS direction increases, the tensile strain rate decreases and the first shear strain rate decreases before the earthquake, which reflected that the increase of the NS-directional movement in the focal area before the earthquake has a beneficial to the occurrence of the earthquake, and the tension and strike-slip movement slowed down after 2016. Moreover, the north segment of the East Kunlun fault(Maqu-Maqin section) had a small-scale and short-term spatial orientation change before the MS 5.7 Xiahe earthquake, which showed that highly consistent of focal mechanisms and low parameter values, the parameter values were in a low-value state. It reflected the process of tectonic stress enhancement and the high-stress level before the earthquake. The comprehensive analysis shows that there was a background abnormal change characteristic of stress and strain accumulation enhancement in the focal area before the earthquake, which contributed to the occurrence of the earthquake.