Source Mechanism and Rupture Directivity of Small Earthquakes in the Changning Region, China, Using Dense Array Data

Abstract Integrating focal mechanism solutions with rupture directivity analysis enables high-resolution characterization of subsurface fault geometry and earthquake rupture processes. However, resolving these parameters for small-magnitude earthquakes remains challenging due to small rupture sizes, short durations, and low signal-to-noise ratio. Here, we utilized a dense array of nodal seismometers in the Changning region, Sichuan basin, China, to study the focal mechanism and rupture directivity of aftershocks following the 2019 Ms 6.0 earthquake. Using PhaseNet+ and SKHASH, we first enhance the focal mechanism catalog (1<ML<4). Then, applying the spectral ratio method with empirical Green’s functions, we observe azimuth-dependent corner frequencies of two ML∼3 aftershocks by spectral fitting to the Brune’s model, which are consistent with unilateral rupture. Our results reveal that the two earthquakes occurred at an unidentified conjugate fault and ruptured toward N60° E unilaterally, which significantly differs from the northwestward rupture of the Ms 6.0 mainshock. Furthermore, we obtain a rupture speed of approximately 0.6 times the shear-wave velocity. We also apply the spectral decomposition method to compute stress drops (ML>1), and their spatial variability reflects a long-term interplay between fluid injection and faults in the salt-mining area. These findings illuminate a complex fault system beneath the Changning anticline and highlight the importance of high-resolution seismic arrays in resolving rupture processes of small-magnitude events.