关键词: 活动断裂, 晚第四纪地壳运动, 地震危险性, 漳州盆地

Abstract: Zhangzhou basin is the largest Quaternary fault basin in Fujian Province. Five earthquakes of magnitudes 5 1/2, 6 1/2, 6 1/4, 5.0 and 5 1/4 had successively occurred in this basin in 1067, 1185, 1445, 1549 and 1601. It is an area with highest intensity and frequency of historical earthquakes in Fujian Province. In this paper we discuss the latest tectonic motion and the seismic risk of this basin on the basis of the analysis of the rejuvenation of the basin, fault activity, neotectonic differential movement within the basin and the periodicity of strong earthquakes. We have also compared this basin with the other basins or plains in the coastal areas of Fujian Province. It is found that since the late Pleistocene the basin has got rid of the constraint of the NE-trending faults to continue its propagation toward the southeast. At that time, the NE-trending faults within the basin were dominated by strong dextral strike slip with dip-slip component, while the NW-trending faults were dominated by sinistral strike slip with dip-slip component. Elevation and subsidence have frequently occurred in this basin since late Quaternary. As a result, the early Holocene alluvial deposits overlapped the late Pleistocene alluvial beds, forming the first level terrace. The lower part of the late Pleistocene beds is still below the present river level. The elevation of some early Holocene deposits is higher than that of the middle Pleistocene deposits, indicating that during the late Pleistocene time the amplitude of subsidence was greater than the amplitude of elevation in this basin. The differential movement within the basin was very intensive. In elevated area the level of early Holocene alluvial deposits is 20m higher than the river level, while in subsided area the top of these alluvial deposits is 27 10m below the earth's surface, resulting in a height difference of 47 10m between the two. The latest historical earthquake in this basin has occurred for 400 years, which exceeds greatly the 52～260 years interval of strong earthquakes in this area. This area, therefore, is a high seismic risk area. As compared with the other basins and plains in the coastal areas of Fujian Province, this basin has some peculiarities, and that is why 5 strong earthquakes had occurred here. It is expected that a destructive earthquake may occur in this area in the future. Moreover, the historical strong earthquakes in this basin all occurred directly beneath the city, causing more severe damage than the earthquakes of the other types.

Key words: Active fault, Late Quaternary crustal movement, Seismic risk, Zhangzhou basin