General features of multi-parameter anomalies of six moderate earthquakes occurred near Zhangbei-Bohai fault in China during the past decades

Abstract

The great Zhangbei-Bohai fault (ZBF) is an important seismic active zone in Northeastern China, which intersects with another great fault, namely Tancheng-Lujiang fault (TLF), in the Central Bohai Sea. Most of the previous studies only investigated the earthquakes (EQs) happened in the west section of ZBF and anomalies appeared in the land area, but few researchers focused on the EQs happened in the east section of ZBF and studied the anomalies appeared above the Bohai Sea. Recently, we have investigated the disturbances on and above the Bohai Sea associated with the Zhangbei Mb5.6 EQ in 1998, whose epicenter located in the west section of ZBF, and found that there was seismicity induced gas release from submarine TLF. To study further the multi-parameter anomalies of EQs near ZBF during past decades and their relationship with ZBF and TLF, a scrutiny strategy for seismic anomaly in the sea area was proposed in this study. After comprehensive screening of EQs occurred near ZBF from 1998 to 2022 using multi-source remote sensing data, potential seismic anomalies including the anomalies of microwave brightness temperature and sea surface roughness on Bohai Sea were found correlated with 6 of the 23 investigated EQs, and there were commonly anomalies of CO and CH4 concentration in the atmosphere. All these seismic anomalies were found developing mainly along the TLF in Bohai Sea, and started usually before shocking, appeared co-seismically and in the next few days. Regional inconsistencies among the distribution of CO and CH4 anomalies could be owning to the specific underwater reservoir in the Bohai Bay Basin. Though analyzing the focal mechanism solutions of the selected EQ cases, we found that the seismic anomaly existed only when the TLF was in a stress state of tensile relaxation.