Deep tectonics and seismogenic mechanisms of seismic source zone of Jishishan Ms6.2 earthquake on 18 December 2023 in NE margin of Tibetan Plateau

On December 18, 2023, a Ms 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 the South China block. In this study, we analyze the deep tectonic pattern of the Jishishan earthquake by incorporating data from crustal thickness, velocity structure, GNSS strain field, and anisotropy. It was discovered that the earthquake's location was related to changes in the crustal structure. The results showed that the Jishishan Ms 6.2 earthquake occurred in a unique position with rapid changes in crustal thickness, Vp/Vs, phase velocity, and S wave velocity. The earthquake’s epicenter is situated at the transition zone between high and low velocities and is in proximity to a low-velocity region. Additionally, the source area is flanked by two high-velocity anomalies from the east and west. The principle compressive strain orientation near Lajishan fault is primarily in the NNE and NE directions, which align with the principle compressive stress direction in this region. In some areas of Lajishan fault, the principle compressive strain orientations show the NNW direction, consistent with the direction of the upper crustal fast-wave polarization from local earthquake and the phase velocity azimuthal anisotropy. These features underscore the relationship between the occurrence of the Jishishan Ms 6.2 earthquake and the deep inhomogeneous structure and deep tectonic characteristics. The NE margin of Tibetan Plateau was thickened by crustal extension in the process of north-eastward expansion, and the middle and lower crustal materials experienced structural deformation and may be filled with salt-containing fluids during the extension process. The presence of this weak layer makes it easier for strong earthquakes to occur due to the release of the 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 eastern margin fault.