Characteristics of earthquake sequences and seismogenic background around Jiaokou Reservoir, eastern China

Abstract

The Jiaokou Reservoir area is one of the areas with relatively frequent seismicity in Zhejiang, eastern China. We made an analysis and summary of seismicity near Jiaokou Reservoir from 1982 to 2022 using data such as seismic waveforms from stations, earthquake catalogs, and seismic observation reports. Firstly, we analyzed spatiotemporal variations of historical seismicity and b-values of earthquake sequences around Jiaokou Reservoir in different periods. Then, we used the HypoDD method to relocate events after 2008, and used CAP and HASH methods to obtain focal mechanisms of events with M_L ≥ 3.0 after 2008. Finally, we analyzed the characteristics of earthquake distribution, stress field, seismogenic structures, and seismogenic mechanisms in the region. The main views are as follows. (1) The relocation results indicate that the narrow-band distribution characteristics of earthquake sequences after 2008 around Jiaokou Reservoir are apparent, with a strike of roughly NW-SE 110° and a high dip angle of roughly SW 70-80°. (2) The focal mechanisms show that these events are basically left-lateral strike-slip ruptures. Combined with relocation results, we infer that the seismogenic fault is close to a nodal plane with a strike of 99°, dip of SW 73°, and rake of −5°. (3) P-axis directions of Jiaokou events are NE-SW, and T-axis directions are NW-SE, with relatively small dip angles, which show the near-horizontal compression in NE-SW and near-horizontal tension in NW-SE. The two directions of tension and compression axes are consistent with the stress field directions of eastern China. (4) Combined with the tectonic features of fault zones around Jiaokou Reservoir area, it is inferred that the main seismogenesis structures of events after 2008 may be minor strike-slip faults with a strike of NW-SE (about 99-110°) and a high dip angle of SW (about 70-80°), which are located in Changxing-Fenghua fault system. (5) Under the influence of water infiltration and corrosion in Jiaokou Reservoir, the fracture strength of underground rock decreases, the fault plane softens, and the friction strength decreases. Under the effect of reservoir water and far-field tectonic loading, the stress on the faults of Changxing-Fenghua fault system is more likely to reach a critical and unstable state, resulting in the occurrence of earthquakes around the reservoir.