The seismogenic structure and dynamic origins of the 2023 Qingpu ML3.7 earthquake

Abstract

On June 15, 2023, a significant \({M}_{\text{L}}\)3.7 earthquake occurred in the Qingpu District of Shanghai, China. This event holds particular importance as the inland areas of Shanghai have not previously recorded earthquakes above \({M}_{\text{L}}\)3.5. To investigate the seismogenic structure of this earthquake and its relation to the tectonic stress field in the region, our study initially utilized the FOCMEC and HASH methods to invert the focal mechanism solutions for this earthquake, establishing credible centroid solution based on these results. Subsequently, leveraging previous research on the stress field in the southern Jiangsu region, we computed the corresponding maximum horizontal principal stress direction and compared it with the principal strain rate direction in the Qingpu area, as well as the fast wave polarization direction of shear wave splitting, thereby obtaining a reliable regional stress field for Qingpu area. Lastly, projecting the regional stress field onto the two nodal planes of the \({M}_{\text{L}}\)3.7 earthquake's centroid solution, we obtained relative shear and normal stress values and determined the theoretical seismogenic plane. Based on the identified seismogenic plane, this study relocates the depth of the seismic source and projects the source onto the longitudinal profile of the S-wave velocity structure of the seismogenic structure to obtain its seismogenic environment. The final results are as follows: (1) The centroid solution for the focal mechanism of the \({M}_{\text{L}}\)3.7 earthquake indicates a strike of 26.2°, a dip of 86.0°, and a rake angle of 159.8° for fault plane I, and a strike of 117.7°, a dip of 69.8°, and a rake angle of 4.2° for fault plane II, suggesting a strike-slip earthquake. (2) In the study area, the azimuth of the maximum principal stress axis is northeast (NE), with a near-horizontal plunge, while the azimuth of the minimum principal stress axis is northwest (NW), with a vertical plunge, indicating a predominantly compressional stress regime, typical of a strike-slip stress state. (3) The relative shear and normal stress values on fault plane I are 0.827 and 0.365, respectively, while on fault plane II, they are 0.871 and − 0.623, respectively, indicating that fault plane I is the theoretical seismogenic fault plane. By considering the geological structural characteristics around the earthquake location, the orientation of the major axis of isoseismal, and the fault parameters of the theoretical seismogenic plane, it is inferred that the seismogenic fault for this earthquake is the NNE-oriented Yaojiagang-Baihe fault, with the theoretical seismogenic plane being the actual seismogenic plane. (4) The earthquake was repositioned to a depth of 6.6 km, and the location was at the junction of high and low velocities of the Yaojiagang-Baihe fault, which possesses the media conditions for the accumulation of a large amount of strain energy and is susceptible to rupture and stress release. (5) The \({M}_{\text{L}}\)3.7 earthquake in Qingpu, Shanghai, occurred on the fault plane where the maximum shear stress is released under the influence of the tectonic stress field, representing a normal release of accumulated stress. The likelihood of small to moderate earthquakes occurring again in the vicinity in the short term is low. This study provides valuable insights into the seismogenic structure and dynamic origins of the \({M}_{\text{L}}\)3.7 earthquake in Qingpu, Shanghai. The research findings can serve as a reference for subsequent seismic hazard assessments and zoning for earthquake defense in the Shanghai area.