SHALLOW SEISMIC STRATIGRAPHIC RECORDS OF EARTHQUAKES IN THE RADIAL SAND RIDGE FIELD OF SOUTH YELLOW SEA

Three typical submarine landslides have been found in the two shallow seismic profiles across the middle Huangshayang tidal channel in the central part of the radial sand ridge field (RSRF), South Yellow Sea (SYS), which is 34.5 km northeastwards away from the Da-Yangkou, and 17.1 km northwards away from the West Taiyangsha artificial island, Rudong County. The largest one is located on the northern side of the tidal channel,that is,on the southern slope of the Jiangjiasha sand ridge. The main scarp is up to 15 m high and more than 300 m wide. The other two are located on the southern side of the tidal channel or the northern slope of the Qie'erganzi sand ridge. These two landslides are quite close to each other, and show similar shape. They should be the same landslide on medium scale. The sedimentary dynamics in the Huangshayang tidal channel have been relatively stable in recent several decades, and there are hardly any sediments covering on the top of the displacement of the landslides, it could be deduced that they might be the records of the Ms 6.2 earthquake happened in the SYS on May 21th,1985. The epicenter of this earthquake was located at the intersection zone between the NW-SE trend Subei-Binhai fault zone and E-W trend Bencha River fault zone. As these two fault zones are close to the epicenter, two landslides were induced, and the scale of the one happened at the main fracture zone of this earthquake, that is the prior one, was much larger. The upper part of submarine sediments in the RSRF was mainly composed of sandy-silty interbedded deposits formed in tidal environments, and the lower part was mainly fluvial sand deposits. Meanwhile, the slopes of the channels were relatively steep. Shaking was easily to induce the water-bearing sands to move, even to flow because of liquefaction. The slope became instable, and the upper part of sedimentary strata began to break. Consequently, the landslides happened on the both sides of the tidal channel along the slopes of the sand ridges. In recent years, the exploitation speed of the Jiangsu coast along the SYS is vastly improving and the coastal engineering constructions are developing rapidly. Hazardous geological factors, such as potential landslides, shallow gas and soft foundation, distribute densely in the RSRF. Therefore, it is important to investigate the spatial and temporal distribution and to understand the development trends of these hazardous geological factors in the whole RSRF, in order to guarantee the safety of the coastal engineering constructions.