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导致古森林沉没于海的福建深沪湾古地震研究

RESEARCH OF PALEO-EARTHQUAKE THAT CAUSED THE SUBSIDENCE OF PALEO-FOREST ON THE SEAFLOOR IN SHENHUWAN BAY, FUJIAN PROVINCE

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【作者】 徐起浩冯炎基施建生

【Author】 Xu Qihao 1) Feng Yanji 2) Shi Jiansheng 3) 1) Seismological Bureau of Guangdong Province, Guangzhou 510070 2) Guangzhou Institute of Geography, Guangzhou 510070 3) Committee of Sciences and Technology of Jinjiang City, Fujian Provin

【机构】 广东省地震局!广州5100702广州地理研究所!广州5100703福建省晋江市科学技术委员会!晋江362200

【摘要】 福建深沪湾海底古森林是强古地震遗迹。研究结果表明 ,全新世早期该处海底古森林生长在由松散的晚更新世含砾砂粘土构成的低凹洼地内。距今 70 0 0a左右的强古地震导致古森林沉没于海并形成泻湖环境 ,约距今 2 0 0 0a的又一次强古地震使海底古森林沉没到更深的海底 ,推测这两次古地震的震级都超过 7级。

【Abstract】 The submerged paleo forest in Shenhuwan Bay, Fujian Province, is accepted to be the vestige of strong paleo earthquake. The fossil trees were formerly growing in low lying land filled with late Pleistocene gravel bearing sandy clay during Holocene. A strong earthquake occurred 7000 years B.P. had caused the subsidence of the trees onto the seafloor and the development of lagoon environment, while another strong earthquake occurred 2,000 years B.P. had given rise to the further subsidence of the trees to a greater depth. The evidences of these two events are as follows: 1. Earthquake landform composed of loose sediments was buried by littoral facies deposits on the seafloor, and its remnants were left on the beach. This may indicate that the land was subsided rapidly below the sea level during the earthquake, while several island like highlands composed of red and yellow gravel bearing sandy clays of late Pleistocene age remained above the sea level after the earthquake. After long period erosion by sea water, red brown and yellow land spots were left on tidal zone or shoal, while isolated blocky highlands surrounded by marine and eolian deposits were remained on the coast. 2. Many paleo trees had killed simultaneously, some of which were collapsed and broken. Some broken trunks laid down on the sea floor, and some broken branches were still connected with the trunks. All these may indicate a sudden subsidence caused by strong earthquake, which had given rise to the rapid burying of the paleo trees. 3. Evidence from seismic facies sediments. The ZK 1 and ZK 2 drill holes on the shoal have revealed that the Holocene sediments contains a 20~30m thick bed of carbonized, half carbonized and un carbonized broken wood chips with a little amount of gravels. This bed has not been eroded and transported by the sea water, containing marine facies sands of fine to medium size and small yellow clay balls. The formation of this bed seems to closely related to the sudden subsidence caused by strong earthquake, which had given rise to the transgression of the sea and the transportation of fine to medium grained sands into the pre existing slope wash and alluvium. 4. Evidence from the rate of crustal movement. The Shenhuwan paleo earthquake occurred 7,000 years ago. According to estimation, the rate of sedimentation during the period of 7,000~6,000a B.P. was 11 times as high as that during the period of 6,800~2,000a B.P. The rate of crustal movement during the former period was 25 times as high as that during the latter period. 5. Four sag ponds possibly produced by earthquake are distributed along the fault in the coastal area on the southwest margin of the Shenhuwan graben. The earthquake of late Holocene occurred about 2,000 years ago. This event can be confirmed by the facts that the elevation of littoral deposits at Shenhuwan Bay at that time differed from the sea level for about 4m, and that elovian sand deposits of continental facies were remained in the marine strata on the seafloor. According to the estimation, the subsidence of Shenhuwan Bay caused by these two earthquakes reached up to 3m, and the magnitudes of the two events are postulated to be above 7.

【基金】 地震科学联合基金资助 (1970 96)
  • 【文献出处】 地震地质 ,Seismology and Geology , 编辑部邮箱 ,2001年03期
  • 【分类号】P316
  • 【被引频次】15
  • 【下载频次】82
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