Active Fault Exploration and Seismic Hazard Assessment in Fuzhou City

摘要

It has been proven by a number of earthquake case studies that an active fault-induced earthquake beneath a city can be devastating. It is an urgent issue for seismic hazard reduction to explore the distribution of active faults beneath the urban area and identify the seismic source and the risks underneath. As a pilot project of active fault exploration in China, the project, entitled “Active fault exploration and seismic hazard assessment in Fuzhou City", started in early 2001 and passed the check before acceptance of China Earthquake Administration in August 2004. The project was aimed to solve a series of scientific issues such as fault location, dating, movement nature, deep settings, seismic risk and hazard, preparedness of earthquake prevention and disaster reduction, and etc. by means of exploration and assessment of active faults by stages, i.e., the preliminary survey and identification of active faults in target area, the exploration of deep seismotectonic settings, the risk evaluation of active seismogenic faults, the construction of geographic information system of active faults, and so on. A lot of exploration methods were employed in the project such as the detection of absorbed mercury, free mercury and radon in soil, the geological radar, multi-channel DC electrical method, tsansient electromagnetic method, shallow seismic refraction and reflection, effect contrast of explored sources, and various sounding experiments, to establish the buried Quaternary standard section of the Fuzhou basin. By summing up, the above explorations and experiments have achieved the following results and conclusions: (1)The results of the synthetic pilot project of active fault exploration in Fuzhou City demonstrate that, on the basis of sufficient collection, sorting out and analysis of geological, geophysical and borehole data, the best method for active fault exploration (location) and seismic risk assessment (dating and characterizing) in urban area is the combination of various approaches, that is, the possible location of active fault determined by using geochemical exploration as a guide “scout", the shallow seismic sounding as the main tool, the electromagnetic method as a supplement, establishing the standard Quaternary profile or stratigraphic sequence from drilling and various geophysical parameters from borehole logs as methods to correct and verify the data above. And in addition, the method also includes the field surveys on fault exposures, trenching, paleoearthquake investigation, dating and comparison of lithology, strata sequence, absolute or relative ages of the cores on the two sides of buried faults. (2)The Fuzhou basin locates under the regional seismotectonic settings which have the potential of moderate earthquake. Comparatively, the region is less affected by the “Taiwan dynamic Antenna"; (3)The activity of the major faults in Fuzhou basin is weak in general. All the six identified target faults are not Holocene faults, among which the Bayi Reservoir-Shanggan fault and the Minhou-Nanyu fault are dormant at least since the mid Epipleistocene time, and the rest are dormant since the Epipleistocene time; (4)In terms of deep-seated structures beneath the basin, there is no evidence indicating the possible occurrence of the underneath strong destructive earthquakes. The adjacent Changle-Zhao’an fault zone is the potential seismic source which may possibly affect Fuzhou City; (5)There exists potential of moderate-strong earthquake on the major faults of the region, but the probability is low; (6)The seismic hazards are weak in the region and the surface earthquake fractures are not likely to occur; (7)The first geographic information system of active faults is developed with functions of information query and display, data management, analysis and processing, etc.