2022年青海门源Mw 6.6地震的发震断层及孕震构造模式

Seismic faults of the 2022 Mw 6.6 Menyuan,Qinghai earthquake and their implication for the regional seismogenic structures

2022年1月8日青海门源盆地北缘发生Mw 6.6地震,震源机制反演表明此次地震属于左旋走滑事件.震后10 d内,近600个余震被检测到,最大余震为M 5.1级.此次地震发生在祁连-海原左旋走滑断裂系统的冷龙岭段,该断裂段全长127 km,由古地震研究确定的特征地震大小在Mw 7.3~7.5.为了更为全面理解此次地震的震源机制以及当地孕震模式,我们分析了地震波形,获取了主震和17个Ms≥3.0余震的震源机制与矩心深度.利用升、降轨道SAR数据获取的像元偏移数据和同震干涉相位(interferometric synthetic aperture radar,InSAR)确定了两条地表破裂带的位置,并利用InSAR数据反演了主震的滑动模型.研究发现,此次地震破裂带对应于冷龙岭断裂西段和托莱山断裂的阶区,发震断层存在3个形变中心,最大滑动量约为4 m,出现在冷龙岭断裂上,形变中心深度为4 km.滑动模型显示释放了累计能量~1.58×1019 Nm,约合矩震级Mw 6.68,与本文利用地震学方法得到的Mw 6.58接近.结合区域活动构造特征、1986和2016年两次门源地震的位置及震源机制,推断祁连-海原断裂非对称花状构造结构可能是祁连山地区一种重要的应变卸载模式.同震滑动驱动下的库仑应力变化分析显示72.2%的余震分布符合同震触发效应.考虑到当前余震主要向东南扩展,库仑应力升高的破裂区西南部仍存在相对较高的地震风险,需要进一步关注.

The Mw 6.6 Menyuan earthquake occurred on January 8th of 2022 on the northern margin of Menyuan Basin,Qinghai.It was followed by over 600 aftershocks with magnitudes of up to M 5.1.The mainshock was located on the Lenglongling(LLLF)segment of the Qilian-Haiyuan(QLHYF)sinistral fault system,a 127-km-long active fault with a characteristic event of Mw 7.3–7.5.In this study,we used seismic and geodetic data for the characterization of this seismic event and its large aftershocks.Using the seismic data and the CAPjoint inversion method,we obtained the centroid depths and focal mechanisms of the mainshock and the 17 largest(Ms≥3.0)aftershocks.We determined that the mainshock was a strike-slip event with a moment magnitude of 6.58,centroid depth of 4 km,and the two nodal planes of 197°/83°/–162°and 104°/72°/–7°.Most of the estimated aftershocks were also strike-slip events located at depths ranging from 3–9 km at the periphery of the earthquake centroid.We analyzed the rupture directivity of the two large aftershocks(Ms 5.2 and Ms 4.8)with the azimuthal variation of source duration and found that both events ruptured 1.5–2.0 km segments along the sinistral fault plane.We also processed Sentinel-1 SAR data acquired on tracks 026,033,and 128 using an automated InSAR processing package,pSAR.The SAR-derived results that include subpixel offsets,coherence maps,and differential interferometry consistently reveal two major surface rupture segments that correlate with a step-over of Tuolaishan fault and the western segment of LLLF,respectively.We performed a geodetic inversion of the ascending and descending coseismic InSAR observations using a geodetic inversion package PSOKINV in order to determine geometric parameters and subsurface slip on the fault.During the inversion,the surface fault traces were fixed based on the observations from the subpixel offsets and coherence maps produced from the SAR data.The geodetic inversion indicates that the sinistral strike-slips on the two steep-dipping fault segments are responsible for the mainshock,having three distinct slip patterns with a maximum slip of about 4 m at a depth of 4 km,which is consistent with the seismic solutions.The geodetic moment from the slip model was 1.58×1019 Nm,corresponding to Mw 6.68,which is slightly greater than Mw 6.58 estimated from the seismic data.We also compared the seismic source solutions produced by the different seismic methods to the multiple earthquake fault segments inferred from the InSAR results obtained in this study.We found that the significant variations of the seismic solutions from P first motion polarizations,CAPjoint,GCMT and USGS W-phase are mainly due to the different periods of the seismic data used in the inversions.The solution derived from the P first motions is consistent with the fault F1 determined from the offset maps presented in this study that corresponds to the step-over of Tuolaishan fault.With longer periods,the seismic focal mechanism solutions turn out closer to the epicenter of the earthquake.This could help with the understanding of seismic solutions for other earthquakes,particularly those with significant strike variations along the rupture.Combining with regional tectonic structures and historical earthquakes,e.g.,the 1986 and 2016 Ms 6.4 thrust-slip Menyuan earthquakes,we suggest that an asymmetrical flower structure along QLHYF may have been a key model for unleashing the regional strain.The coseismic slip-derived stress analysis indicates that the earthquake significantly increased the Coulomb stress in the vicinity of the hypocenter,particularly at the western end of the earthquake rupture,where few aftershocks were observed,drawing our attention to the increased seismic risk in that region.