The Research Of NE-SW Trending Active Faults, In The Coastal Area Of Zhejiang Province

The coastal area of Zhejiang Province is located in transition zone between the lower Yangtze-South Yellow Sea and the southeast coast seismic zone. Hangzhou Bay, Ningbo, Zhoushan, Jiaojiang and Wenzhou not only have occurred Ms> 4.0 earthquakes in the history, but still have small earthquakes occurred today. Systematic fault activity studies in the coastal area of Zhejiang Province can not only provide advices for geological exploration works, but also have guiding significance to urban construction in this region. This paper presents a great deal of research work for the activity studies of 14 north-east trending faults and their branches in the southeast coastal area of Zhejiang' Province, by gathering information on previous studies, combined with field reconnaissance and sample dating. The author achieves some results as follows:1. Field investigations, shallow seismic exploration, aerial remote sensing and the thermoluminescence (TL) and electron spin resonance (ESR) ages of fault gouges from the bedrock outcrop show that the activities of the north-east-trending faults in Zhejiang coastal area have different features in different segments:Northern section of Zhenhai-Wenzhou fault shows a obvious activity, whereas the middle section is of a weak activity and the southern section doesn't show any activities. The northern and southern sections of Lishui-Fenghua fault are active, whereas the middle sections are not. Xiaofeng-Sanmenwan fault have activity in the western and middle section, but are inactive in the eastern section.2. Yuyao-Lishui fault is the most eye-catching fault in the southeast of Zhejiang. By analyzing the nowadays earthquake distribution of Zhejiang Province, a clear linear seismic zone can be found along the Yuyao-Lishui fault, indicating the earthquakes are controlled by this fault. We suggest that this fault should be carefully studied as an active fault.3. Fault outcrops show that the fault in front of Yuwang Shan cuts off the Pleistocene weathered residual layer and is covered by Holocene strata. The geological features, trough and drilling studies suggest that the latest activity of its western branch was in the middle Pleistocene, whereas its eastern branch was still active in the early stage of late Pleistocene.4. The north-east trending faults of Zhejiang coastal area and the East China Sea formed under the same geodynamic background. They control the development of the coastal Zhejiang and East China Sea Cenozoic basin. These faults not only showed strong extension activities during new tectonic movement period formed fault basins, but also had some tectonic inversion, making basin raise and denudation. The start-up active time of these NE-trending faults gradually get newer from west to east.5. The interactions between the NE-trending fault and the NW-trending have important controls on the earthquake in the study area. The MS 4.0 earthquake at Shenzhou in 1998 just occurred at the intersection of the northeast Yuyao-Lishui fault and the northwest Xiaofeng-Sanmenwan fault. The NW-trending faults have important controls on the northeast-trending faults, it tends to control the activity of the NE-trending fault at different segments.