High-Rate GNSS-Based Rapid Determination of Coseismic Deformation and Source Characteristics for the 2023 M6.2 Jishishan Earthquake

An M6.2 earthquake struck Jishishan County, Gansu, on December 18, 2023, with its epicenter located in the arc-shaped tectonic belt formed by the Lajishan-Jishishan Fault. Continuous high-rate global navigational satellite system (GNSS) data were utilized to simulate real-time data resolution, enabling the rapid determination of coseismic static and dynamic deformation caused by the earthquake and the estimation of empirical magnitude. Far-field body waves served as constraints for the source rupture process, facilitating the analysis of potential seismogenic fault structures. GNSS stations within 30 km of the epicenter exhibited significant coseismic responses: horizontal peak displacement and velocity reached approximately 6.3 cm and 6.1 cm/s, respectively. Additionally, quasi-real-time differential positioning and post-event precise point positioning results were consistent throughout the source process. Vertical velocity, calculated via epoch-by-epoch differential velocity determination, showed clear coseismic signals, with peak values increasing to 2.6 cm/s. The empirical magnitude, based on displacement, was 5.99, while the magnitude derived from the velocity waveform amplitude was 6.05, both consistent with the moment magnitude. The dynamic displacement distribution preliminarily suggests directional effects of northward rupture propagation, aligning with subsequent aftershock occurrences. Finite fault inversion results, based on the two nodal planes of the focal mechanism, indicate that asperity ruptures concentrated at the hypocenter played a major role. These ruptures propagated from the hypocenter to shallow regions and northward, lasting approximately 10 seconds. Although the coseismic deformation determined by sparse high-rate GNSS cannot constrain the specific fault dip angle, the relationship between rupture propagation direction from the seismic source model and aftershock distribution suggests a northeast-dipping fault. Moreover, seismic source models representing single faults as geometric structures can only simulate permanent formations. In contrast, the conjugate fault model, which aligns with aftershock distributions, more accurately explains high-rate GNSS displacement waveforms. Considering both regional tectonics and geological survey results, the seismogenic fault is believed to be a local northeast-dipping blind thrust fault. Northward rupture propagation may have caused the movement of conjugate faults. This study is an effective case of using high-rate GNSS for rapid earthquake response, providing a reference basis for understanding the seismic activity patterns and earthquake disaster prevention in the region.