2. Geological setting
The ECSSB was developed on the broad continental shelf of the southeast Eurasian continental segment; to its west is the Zhemin upwarping folded zone, and it is separated from the Okinawa Trough, Ryukyu island arc, and Ryukyu Trench by the Diaoyu Island upwarping folded zone in the east. The north is bordered by a ray from the Yangtze River to Jizhou Island, and the north is bordered by a ray from the Guangdong to Taiwan provinces. The basin is approximately 1,400 km long from north to south and 250–300 km wide from east to west, with a total area of approximately 26.7 × 104 km2. Influenced by the subduction of the Philippine plate to the Eurasia plate during the Eocene to Oligocene period, the ECSSB is in a state of back-arc spreading. Simultaneously, seismic data showed apparent extension fracture activity near the NE direction, which resulted in the formation of a structural framework with nearly east-west zoning and a nearly north-south block in the plane. In addition, the ECSSB can be divided into three secondary building units: western geotectogene, central uplift belt, and eastern geotectogene from west to east. Among them, the eastern geotectogene can be classified into Diaobei Sag, Xihu Sag, and Fujiang Sag from south to north (Zhu et al., 2012; Ju et al., 2016; Chen et al., 2017; Hao et al., 2018; Zhu et al., 2019; Su et al., 2020) (Fig. 1).