Temporary stress regime reversal immediately after the complete release of the localized compressive stress by the 2021 Luxian Ms 6.0 earthquake, Sichuan Basin

Aftershocks can provide fundamental information of the seismogenic structure and the stress condition of the source region to reveal the seismogenic mechanism of earthquakes without clear surface rupture. On 16 September 2021, a destructive Ms 6.0 earthquake broke the seismic quiescence of the Luxian county in the inner Sichuan Basin, China. Here, we explore the seismogenic structure and the postseismic stress condition of the Luxian earthquake through characterizing sedimentary faults in the spatiotemporal distribution of aftershocks, through the focal mechanism solutions and further stress inversion, and through the coseismic rupture model based on synthetic aperture radar interferometry (InSAR) data. Our results show that aftershocks are concentrated at two clusters in the broad Yujiasi syncline. Seismicity of the northern cluster reveals that the main shock ruptured an east-southeast striking fault with a dip angle of - 45 degrees to the southwest (F0), and activated a gently dipping (- 18 degrees) fault (F1) below F0 and a nearly vertical shallow fault (F2) on the northern side of F0. These three faults outline the edges of a thrust wedge in the sedimentary cover. The northern cluster occurred on a conjugate fault system within which steep faults with different dipping direction interlaced downward forming 'V' shapes. The main reverse rupture on the east-southeast striking F0 fault is inconsistent with the background northwest-southeast tectonic stress field. Combined with the morphology of anticlines, we suggest that the main shock was resulted from the localized NE-SW compressive stress concentration due to the inhomogeneous deformation of the sedimentary cover. Aftershocks with magnitude > 2.0 are characterized by the dominant normal slip. Coulomb stress change (ACFS) results show that normal slip is enhanced by the main rupture. However, the amplitude of ACFS (a few bars) is not large enough to reverse the dominant slip direction of faults. Postseismic stress inversion results show a temporary stress regime reversal from the nearly north-south subhorizontal compressive stress regime before the main shock to the normal slip favored stress regime with a subvertical maximum principal stress (sigma 1). We suggest that the Luxian Ms 6.0 earthquake as an unusual event completely released the localized horizontal compressive stress and hence caused a temporary reversal of the localized stress regime. (c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons .org /licenses /by-nc -nd /4 .0/).

Automated summary

The Luxian earthquake in the inner Sichuan Basin, China revealed the seismogenic structure and mechanism through the analysis of aftershocks. The main shock ruptured fault systems within a thrust wedge in the sedimentary cover, and the localized stress concentration resulted from the inhomogeneous deformation of the cover. The aftershocks showed enhanced normal slip, but not enough to reverse the dominant slip direction of faults. The postseismic stress inversion revealed a temporary stress regime reversal from horizontal compression to favored normal slip.