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

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 degrees/83 degrees/-162 degrees and 104 degrees/72 degrees/-7 degrees. 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 1.58x10(19) 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.