2021年5月22日青海玛多M_(S)7.4地震发震构造分析

Analysis of seismogenic structure of Madoi,Qinghai M_(S)7.4 earthquake on May 22,2021

精确的余震序列定位及震源机制反演能够提供强震破裂尺度、发震断层面和区域应力场等信息,为震后应急决策和分析发震构造提供科学依据.本研究采用双差定位方法对2021年5月22日青海玛多M_(S)7.4地震序列进行精定位,得到震后9天内共1055个事件的精定位结果;同时,利用青海、西藏、四川和甘肃台网记录的波形数据,采用近震全波形矩张量反演方法得到了玛多M_(S)7.4地震15次中等余震(M_(S)≥4.0)震源机制解,并进一步反演得到震源区构造应力场.地震定位结果显示,玛多主震位于玛多—甘德断裂与甘德南缘断裂之间,发震断层面较为陡立,余震序列在时间上呈现出不对称的双侧破裂模式,且沿主破裂面的两端均表现出分支破裂特征,说明本次地震触发了分支断层;震源机制结果显示15次中等余震包含12次走滑型和3次逆冲型地震,暗示主断层破裂受到局部异常结构的影响;另外,应力场反演表明震源区为近EW向挤压特征,与该区域最大水平主压应力优势取向一致.结合上述结果以及周边地质构造背景,我们认为玛多地震发震构造为位于巴颜喀拉地块内部一条NWW向的高倾角左旋走滑断裂,主破裂触发了东西两端分支断层活动,断层面的非均匀性控制了余震序列时空分布的差异性.

High-precise relocation of aftershock sequence and detailed focal mechanism inversion could reveal the rupture properties of larger earthquakes,seismogenic structure and regional stress field,which provides the scientific evidence for decision-making regarding post-earthquake emergency management.In this study,the double-difference earthquake location algorithm was employed to delicately describe the spatial and temporary characteristics of Madoi,Qinghai M_(S)7.4 earthquake sequence within 9 days after the earthquake,a total of 1055 earthquakes were determined with greatly improved location accuracy.The focal mechanism of moderate aftershocks with M_(S)≥4.0 were inverted from the local and region waveforms recorded by the Qinghai,Tibet,Sichuan and Gansu regional seismic stations with the method of full-wave moment tensor inversion,and then the regional stress field of focal region was estimated by fault plane solutions of these aftershocks.The accuracy of relocation of results show the Madoi sequence lied between the Madoi-Gande fault and the southern margin of Gande fault,which appeared to be a steep seismogenic zone and an asymmetrical bilateral rupture in spatial-temporal evolution.Clear bifurcation geometry features were recognized at the both ends of surface trace of aftershocks,indicating the mainshock occurred in the complex tectonic system and triggered its branch faults.Most of moderate aftershocks,12 out of 15 earthquakes,were of strike-slip faulting following the tectonic area,3 thrust-type earthquakes mainly concentrated in the eastern tip of the main tectonic line which probably indicates the segmented character of the fault.The stress field inversion suggests the focal region is undergoing the compression in the EW direction,and being compatible with the dominant stress orientation in Tibetan Plateau.We deduced the Madoi mainshock ruptured on a left-lateral strike-slip fault with NWW-striking in the Bayan Har block,and it activated faults near the eastern and western tip of seismogenic fault and involved multiple fault branches.The heterogeneity of rupture plane controls the variability of spatial and temporal distribution of aftershocks.