A study on the source parameters and rupture characteristics of the August 6, 2023, MS 5.5 earthquake in pingyuan, Shandong
摘要
Based on earthquake data from the Shandong Seismic Network from August 6, 2023, to September 14, 2023, the source physical parameters of nine ML ≥ 2.5 earthquakes in the Pingyuan seismic sequence are initially calculated based on the Brune model. Then, the earthquakes in the Pingyuan seismic sequence are located using the double-difference earthquake location algorithm (DDELA), and the focal mechanism of the mainshock is inverted via the cut-and-paste method. Finally, the source parameters of the Pingyuan earthquake and the rupture process are analyzed. The results show that the MS 5.5 Pingyuan mainshock exhibited a substantial stress drop, which was notably higher than the typical global average of 4 Mpa. This was accompanied by a large source rupture radius and a significant release of energy, indicating that this seismic event occurred under high-stress environment. This may explain the great damage observed near the epicenter and the intense surface shaking. According to the DDELA results, the focal depth of the mainshock was approximately 19.9 km, and the focal depths of the aftershocks were mainly distributed in the range of 18–24 km; the focal depths were distributed in a nearly vertical banded zone, indicating a steeply dipping fault plane. The large focal depths were attributed to the thick loess cover in the Pingyuan area, which is part of the sedimentary plain of the Yellow River. Based on a comprehensive analysis of the focal mechanism solution of the MS 5.5 Pingyuan earthquake and the spatial distribution of the precisely relocated hypocenters of the aftershocks, the MS 5.5 Pingyuan earthquake was generated by a right-lateral strike-slip fault striking NE–SW under principal compressive stress oriented in the ENE direction. The main seismogenic structure of this earthquake was the NE-trending Lingxian–Guanxian fault, which is a right-lateral strike-slip fault with a high dip angle. Moreover, the temporal variation in the stress drop in the aftershock sequence reflects the adjustment process of the stress environment in the source area. In the early stage after the MS 5.5 earthquake, due to the large release of stress by the MS 5.5 mainshock, the source area overall experienced low-stress conditions for a short period, and many aftershocks occurred in this low-stress environment. Two days after the earthquake, as the stress environment in the source area gradually adjusted, the regional stress level returned to near the normal value; that is, the various blocks in the source area reached relative equilibrium. Therefore, no subsequent major seismic event occurred.