Shallow Focal Depths of the 2022 Ms 6.8 Luding Earthquake and Its M ≥3 Aftershocks

Abstract

Abstract The accurate determination of earthquake focal depths is crucial, yet notably challenging. The 2022 Ms 6.8 Luding earthquake sequence is a typical example for which focal depths remain controversial. In this study, we conducted a comprehensive regional waveform analysis, including depth-phase waveform modeling and relative location methods, to determine the focal depths of the Ms 6.8 mainshock and its M ≥ 3 aftershocks. Specifically, we first inverted the focal mechanisms of these events using the generalized cut-and-paste method. We then determined their absolute focal depths using array-based stacking and Pn depth-phase modeling. For events without clear depth phases, we calculated their relative depths with respect to the known ones using differential travel times of Pg, Sg, and Pn phases. Our results indicate that the mainshock’s hypocenter lies at a depth of 11 km, whereas its centroid appears at a shallower depth of ∼6.5 km. All M ≥ 3 aftershocks are located within a narrow depth range of 3–7 km, in contrast to the deeper focal depths previously obtained using conventional travel-time-based location methods. This shallow seismogenic depth aligns well with the rheological behavior of the crust along the southern Xianshuihe fault, in which the brittle–ductile transition depth is estimated to be about 10km. Our results reveal that the mainshock nucleated at the base of the seismogenic zone and ruptured toward the shallower portion, consistent with the geodetically constrained mainshock slip distribution. Our study also emphasizes the necessity of caution when interpreting focal depths determined by routine travel-time-based methods, especially in regions with sparse seismic station coverage.