Geohazards in the Yellow Sea and East China Sea

ABSTRACT Shallow submarine geology in the Yellow and East China seas is dicta ted mostly by the proximity of the Yellow and Yangtze Rivers and by the late Quaternary history of the area. Most serious potential geological hazards (buried channels throughout the area, shallow gas in areas with high rates of modern sediment accumulation, and possible lateral mass displacement of both sediments) stem from these two factors. Both tectonic and growth faults are present locally, but neither appear to present a major engineering problem. Sediment movement due to currents from tides and storms is potentially a serious problem, as is the local influence of shallow or exposed bedrock in the East China Sea. INTRODUCTION The Yellow Sea and adjacent East China Sea occupy a broad epicontinental shelf off eastern -and northeastern China. Water depths reach 100 m about 350 to 500 km seaward of the present-day shoreline (Fig. 1). South of the East China Sea is the Okinawa Trough, where depths reach 2000 m. Another distinguishing feature of the Yellow Sea and East China Sea is the proximity to two of the world's largest rivers - the Yellow River (Huanghe) and the Yangtze River (Changjiang). These two rivers rank 2nd and 4 th in the world in terms of sediment transported to the sea, 1.1 and 0.5 billion tons of sediment annually (Milliman and Meade, 19B3); combined they transport about 7 times the amount presently carried by the Mississippi River. Both seas are underlain by sedimentary basins containing thick sequences of Cenozoic and Mesozoic sediments. In the Yellow Sea Basin, located in the southern Yellow Sea, Cenozoic sediments reach thicknesses of 1500 m (Fig. 2), while Mesozoic sediments surpass 4000 m in thickness (Zhang, 1983). The East China Sea Basin (termed the Taiwan Basin by Emery et al., 1969) is an epicontinental rift-depression basin located between the Fukien-Reinan Massif to the north and the Ryuku Folded Zone to the south. The thickest sedimentary sequence (5 to 8 km according to Li, 1984) lies landward of the Taiwan-Sinzi Folded Zone. The East China Sea Basin contains mostly Cenozoic sediment (Fig. 2), but locally with more than 5000 m of Mesozoic sediment. In both basins the best petroleum sources appear to be in lacustrine to transitional Paleogene beds (Li, 1984). Frazieretal. (1976) also mention petroleum potential in the continental/ lacustrine Mesozoic strata in the southeastern part of the Yellow Sea. The shallow waters and thick sedimentary sequences make the Yellow Sea and East China Sea interesting areas for the exploration of petroleum, and activity in these two basins has increased markedly in recent years. While the "hottest" prospects for China offshore oil continue to be the Pearl River and Yingge basins in the South China Sea (e.g., Li, 1984), a major oil discovery recently was made in the East China Sea Basin (North Taiwan Basin) (Fletcher and Soeparjadi, 1984).