Three-dimensional velocity structure and seismogenic structure of the 2023 Gansu Jishishan MS6. 2 earthquake source area

On 18 December 2023. at 23:59. the M(s)6. 2 earthquake occurred near the Jishishan Eastern Margin Fault in the southern section of the Lajishan in Jishishan County, Gansu. To investigate this earthquake's seismogenic tectonic and seismogenic environment and the relationship between the aftershock sequences and the deep structure. This paper uses the earthquake event data constructed on permanent stations, warning stations and temporarily deployed dense station arrays, using double difference tomography to carry out earthquake location and three-dimensional velocity structure studies. After relocation, the aftershock sequence spreads approximately in the SE-NW direction with a total length of about 15 km. The depth distribution of the earthquake dominance ranges from 8 similar to 14 km and the average depth is 10.4 km. There is a dislocation in the earthquake depths between the aftershock strip north of the main rupture and the aftershock sequence near the main rupture, and the two dips are approximately vertical in space. The northeast-dipping Dahejia fault is the seismogenic fault of the main rupture, and the aftershock strip north of the main rupture presumably occurred on a southwest-dipping recoil blind fault. The aftershock sequences basically occurred within the high-velocity and low Poisson's ratio anomalies, the high-velocity anomalies and the aftershock sequences are highly coincident. The non-uniformity of the velocity structure is the main deep tectonic factor controlling the pattern of this earthquake and aftershocks. This earthquake was supposed to be a typical recoil-type event that occurred in the context of extrusion tectonics, with stress accumulation and release within a high-velocity and low Poisson's ratio anomaly in the upper crust.