Inversion of geodetic and teleseismic body wave data for the rupture process of the 2023 M_w 6.1 Jishishan earthquake

The M_w 6.1 earthquake that occurred in Jishishan, Gansu, China, on December 18, 2023, was the strongest earthquake in the southern segment of the Lajishan Fault System in nearly a century. Obtaining the deep geometric characteristics and dynamic slip process of this earthquake is of great significance for revealing the detailed seismogenic mechanism of the hidden blind thrust fault in the southern segment of the Lajishan Fault System and understanding the expansion pattern of the northeastern margin of the Tibetan Plateau. This work jointly utilizes globally distributed teleseismic P-wave, InSAR (interferometric synthetic aperture radar), and GNSS (Global Navigation Satellite System) observation data to constrain the rupture process of this earthquake, effectively reducing the nonuniqueness of the inversion results and characterizing the dynamic rupture process of the earthquake. Results of a combined analysis of the aftershock distribution, regional tectonic framework, and spatial distribution of aftershock profiles suggest that the seismogenic fault of this earthquake is a northeast-dipping hidden blind thrust fault located within the Dajiahe Basin, with a strike of ~303°, a dip of ~52°, and a maximum slip of up to 0.8 m. The earthquake rupture was an asymmetric bilateral rupture. In the 0–2 s interval, the seismic moment release rate slowly increased, the rupture expanded at a low speed in the initial stage, and the energy release intensity was weak. In the 2–5 s interval, the rupture rate increased rapidly, reaching the peak of seismic moment release (~4.0 × 10¹⁷ N·m/s) at approximately 5 s. The rupture entered a rapid expansion phase, and the fault slip rate enhanced, which was the main phase of energy release for this earthquake. In the 5–7 s interval, the rupture rate decreased rapidly, and the slip intensity weakened significantly. The overall source rupture model indicates that this earthquake was a moderate, shallow, single main rupture event, with the main energy release concentrated in the shallow to intermediate depths. The unimodal and short-duration waveform indicates that the rupture rapidly expanded on a finite fault, consistent with the characteristics of a continuous and concentrated rupture on a blind fault, rather than a segmented rupture. The low rupture velocity and radiation efficiency of the Jishishan earthquake obtained in this study suggest that the seismogenic fault may be a hidden secondary active fault developed within the Dajiahe Subbasin inside the Qilian Mountains. The rupture morphology is simple and did not propagate to the surface. Combined with the analysis of the regional tectonic background, this study revealed that the eastern segment of the Lajishan Fault is controlled by regional northeast-directed compression and step-over structures. Immature secondary hidden faults within the eastern segment of the region have accumulated long-term tectonic strain, ultimately triggering this earthquake, indicating that hidden secondary faults within the Qilian Mountain foreland basin have the potential to generate moderate to strong earthquakes.