Active Fault Exploration and Seismic Hazard Assessment in Fuzhou City

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 ：

Research on the Effects of the Contents of Radioactive Elements on Granularity and Climate Change:An Example of SZK1 and SZK2 Drill Holes Strata in the Fuzhou Basin

Based on the content of radioactive elements (U, Th, K) of strata in two drill holes in the Fuzhou basin, and combined with the result of spore_pollen analysis, the relationship between radioactivity and lithology and deposit environments is discussed and the results show that the content of radioactive substances is related to the granularity and lithology in sediment, and it is higher in argillaceous sediment (e.g. silt and clay), lower in sand sediment and in the middle in gravels between the above two kinds of sediment. The content of radioactive substances is also related to paleoclimate. A warm and humid environment is propitious to the deposition of radioactive substances, while a cool and dry climate is just the reverse.

Division of Buried Late Quaternary Strata of the Fuzhou Basin

Based on detailed logging of fourteen drill holes, combined with the results of other researchers, the preliminary summarization on sedimentary characteristics of the late Pleistocene of the Fuzhou basin is done. The deposition of Fuzhou basin began from about 56.5ka BP and the sediments are composed of gravel, sand, clay and silt, which show a depositional sequence with granularity becoming smaller from the lower to the upper and a big lateral variation of lithology. There are three silt layers in late Quaternary strata of Fuzhou basin. The first and second layers were developed during the middle and late Holocene, the deposition time is about 7.86ka BP to 1.44 ka BP and they are the results of ＂Changle Transgression＂. The third was developed during the late of late Pleistocene and the deposition time is from 44ka BP to 20ka BP and it is the result of ＂Fuzhou Transgression＂. The buried late Quaternary sediments in Fuzhou basin can be divided into upper Pleistocene series and Holocene series, which include four formations from old to new. They are the Longhai formation （ Q3p 1）, Dongshan formation （Qhd）, Changle formation （Qhc） and Jiangtian formation （Qhj）, respectively. The Longhai formation can be divided into three members and belongs to the Pleistocene series, other three formations belong to the Holocene series.

Rupture segmentation and slip partitioning of the mid-eastern part of the Kunlun Fault,north Tibetan Plateau

The Kunlun Fault, an active fault on the border between the Bayan Har and Kunlun-Qaidam blocks, is one of the major left lateral strike-slip faults in the Tibetan Plateau. Previous research has not reached a consensus on agreeable slip rates along much of its length and the slip rate gradient along the eastern part, both of which play critical roles in a range of models for the eastward extrusion and thickened crust of the Tibetan Plateau. New slip rates have been determined at sites along the eastern part of the Kunlun Fault by dating deposits and measuring atop displaced fluvial terrace risers. Field investigations and interpretation of satellite images reveal geometrical features of the fault and the late Quaternary offset, new earthquake ruptures and surface-rupturing segmentation, from which long-term slip rates and earthquake recurrence intervals on the fault are estimated. The tectonic geomorphology method has determined that the long-term horizontal slip rates on the Tuosuohu, Maqin and Ma- qu segments from west to east are 11.2±1, 9.3±2, and 4.9±1.3 mm/a while their vertical slip rates are 1.2±0.2, 0.7±0.1, and 0.3 mm/a in the late Quaternary. Results indicate that the slip rates regularly decrease along the eastern -300 km of the fault from 〉10 to 〈5 mm/a. This is consistent with the decrease in the gradient such that at the slip rate break point is at the triple point intersection with the transverse fault, which in turn is transformed to the Awancang Fault. The vector decomposition for this tectonic transformation shows that the western and eastern branches of the Awancang Fault fit the slip-partitioning mode. The slip rate of the southwestern wall is 4.6 mm/a relative to the northeastern wall and the slip direction is 112.1°. The mid-eastern part of the Kunlun Fault can be divided into three independent segments by the A＇nyemaqen double restraining bend and the Xigongzhou intersection zone, which compose the surface rupture segmentation indicators for themselves as well as the ending point of the 1937 M7.5 Tuosuohu earthquake. The average recurrence interval of the characteristic earthquakes are estimated to be 500-1000 a, respectively. The latest earthquake ruptures occurred in AD 1937 on the western Tuosuohu segment, as compared to -514-534 a BP on the Maqin segment, and -1055 to 1524 a BP on the Maqu segment. This may indicate a unidirectional migration for surface rupturing earthquakes along the mid-eastern Kunlun Fault related to stress triggered between these segments. Meanwhile, the long-term slip rate is obtained through the single event offset and the recurrence interval, which turn out to be the same results as those determined by the offset tectonic geomorphology method, i.e., the decreasing gradient corresponds to the geometrical bending and the fault＇s intersection with the transverse fault. Therefore, the falling slip rate gradient of the mid-eastern Kunlun Fault is mainly caused by eastward extension of the fault and its intersection with the transverse fault.