Deep tectonics and seismogenic mechanisms of the seismic source zone of the Jishishan M_s 6.2 earthquake on December 18, 2023, at the northeast margin of the Tibetan Plateau

On December 18, 2023, an M_s 6.2 earthquake occurred in Jishishan, Gansu Province, China. This earthquake happened in the eastern region of the Qilian Orogenic Belt, which is situated at the forefront of the NE margin of the Tibetan Plateau (i.e., Qinghai–Tibet Plateau), encompassing a rhombic-shaped area that intersects the Qilian–Qaidam Basin, Alxa Block, Ordos Block, and South China Block. In this study, we analyzed the deep tectonic pattern of the Jishishan earthquake by incorporating data on the crustal thickness, velocity structure, global navigation satellite system (GNSS) strain field, and anisotropy. We discovered that the location of the earthquake was related to changes in the crustal structure. The results showed that the Jishishan M_s 6.2 earthquake occurred in a unique position, with rapid changes in the crustal thickness, V_p/V_s, phase velocity, and S-wave velocity. The epicenter of the earthquake was situated at the transition zone between high and low velocities and was in proximity to a low-velocity region. Additionally, the source area is flanked by two high-velocity anomalies from the east and west. The principal compressive strain orientation near the Lajishan Fault is primarily in the NNE and NE directions, which align with the principal compressive stress direction in this region. In some areas of the Lajishan Fault, the principal compressive strain orientations show the NNW direction, consistent with the direction of the upper crustal fast-wave polarization from local earthquakes and the phase velocity azimuthal anisotropy. These features underscore the relationship between the occurrence of the Jishishan M_s 6.2 earthquake and the deep inhomogeneous structure and deep tectonic characteristics. The NE margin of the Tibetan Plateau was thickened by crustal extension in the process of northeastward expansion, and the middle and lower crustal materials underwent structural deformation and may have been filled with salt-containing fluids during the extension process. The presence of this weak layer makes it easier for strong earthquakes to occur through the release of overlying rigid crustal stresses. However, it is unlikely that an earthquake of comparable or larger magnitude would occur in the short term (e.g., in one year) at the Jishishan east margin fault.