A Study Of Seismogenic Structure And The Fault Activity Around The Hanzhong Basin

The Hanzhong basin is the largest Cenozoic basin in the southern Qinling range, which formed about 4～9Ma ago. It is a pull-apart basin of the Qingchuan fault which is the northeast segment of the Houshan fault in the Longmen Shan fault belt. It is about 100km in east-west, with biggest width from the south to north is about 30km. The formation of the basin is related with the tectonic transition of the Qingchuan fault, which changed from the dextral strike slip to the sinistral stike slip. There are four high-angle normal faults: the north edge fault of the Hanzhong basin, the south edge fault of the Hanzhong basin, Chaba-Linansi fault, the south edge fault of the Liangshan and one reverse-right slip fault - Qingchuan fault. Before the Wenchuan earthquake, study on the fault activity around the Hanzhong basin is relatively weak. After the earthquake, in order to meet the demand of the impact assessment, seismic review and seismic zoning review of the surrounding areas of nuclear facilities, this work chose the fault activity around the Hanzhong basin as a research object. Through the research, this work provides supportive data for the seismic zoning.According to the current situation of the research in this region, this work has carried out an investigation of the following aspects:1) Through field investigation, track the spatial distribution of faults and the geological features of these fractures, collect and analyze relevant information, combined with chronology and key survey points topography measurements, reveal the fault activity since Quaternary.2) Through the study on the Quaternary fault activity around the area, and in combination with the previous research results on the tectonic causes of this region, analyze the characteristics of the tectonic activity and the mechanism of Quaternary tectonic activity.3) Through the study on the characteristics of tectonic activity of the Hanzhong basin, combined with regional geophysical data, discuss the potential ability of earthquakes in the Hanzhong basin and review the program in the delineation of potential seismic sources. After field investigation and laboratory analysis, combined with the existing data, the situation of the faults distribution and activity around the Hanzhong basin are as follows:1) The Qingchuan fault has a number of right-lateral slip of different scale along the fault, with characteristics of reverse-right lateral and NE distribution. At the terrace of Tuguanpu, the vertical displacement reaches 18m. This fault was active in the late time of the middle Pleistocene or in the early of the late Pleistocene.2) The north edge fault of the Hanzhong basin, with its normal fault characteristic and almost EW distribution, is divided into two sections according to different activity: The western part (Mianxian-Baocheng part) cuts the T4 terrace of Baohe, with 15m of the vertical displacement. The activity era of this section is the early time of late Pleistocene. The east part (Baocheng-Youshuixiang part) cuts the diluvium of the Juyuanxiang and the T4 terrace, and the activity era of this section is the middle Pleistocene.3) The south edge fault of the Hanzhong basin, as a normal fault with NE distribution, cuts the T4 terrace of Hanjiang, its activity era is the middle Pleistocene.4) Chaba - Lin’ansi fault stretches in NE, with normal characteristic in the southwest mountain section and reverse characteristic in the basin section. The southwest mountain section cuts the T4 terrace of the Yudai river which is a tributary of Hanjiang and indicates no influence on the T3 terrace. The activity era of this section is the late time of the Middle Pleistocene to the early time of the Late Pleistocene. The activity era of the basin section presumed to be in the Middle Pleistocene, for this section has no evidence of activity in the Late Pleistocene.5) The south edge fault of Liangshan, with the normal characteristic and NE distribution, cuts the T4 terrace of the Lianshui river, and the activity era of this fault is the Middle Pleistocene.According to analysis on the activity of the faults and the sediment thickness of the basin and the distribution of the small earthquakes, this work comes to the following cognitions of the structural characteristics of the Hanzhong Basin:1) The difference of the thickness of basin sediments. The Hanzhong basin is a Cenozoic basin, and the basin sediments are mainly the Quaternary. Quaternary sediments were thin in the east and thick in the west. The sediment center of the basin lies in the south and north of the Liangshan mountains, and the Quaternary thickness reaches 700～900m. On the other side, the Quaternary thickness of the east basin is only 200～300m.2) The difference of the fault activity. For the Qingchuan fault and the west section of the north edge fault of the Hanzhong basin, the activity era is late time of the Middle Pleistocene to the early time of the Late Pleistocene. For the east section of the north edge fault of the Hanzhong basin, the south edge fault of the Hanzhong basin, Chaba - Lin’ansi fault and the south edge fault of the Liangshan, the activity era is in the Middle Pleistocene. The activity of the faults shows the characteristics that the west is more active than the east.3) The difference of the distribution of small earthquakes within the basin. Small earthquakes within the basin mainly occur in the west side of the basin, near the Yangsan area, showing planar distribution. A few of small earthquakes occured in the east side of the basin.Previous studies have shown that the Hanzhong Basin is a transition structure of the Qingchuan fault, which absorbed most right-lateral slip displacement of the Qingchuan fault (Fan Chun, 2008). According to the studies on characteristics of the tectonics activity of the Hanzhong basin, the faults around the Hanzhong basin show more intense activity in the early Pleistocene and the middle Pleistocene. In the late Pleistocene, the faults activity weakened, only the Qingchuan fault and the west section of the north edge fault of the Hanzhong basin were active, and the activity of other faults stoped. Then in the late Pleistocene and Holocene, the tectonic activity in the Hanzhong basin mainly was affected by the uplifting of the Qinling range, resulting in landform surface tilting in the Hanzhong basin and casting the landscape features in today. The activity of the faults around the Hanzhong basin was mainly due to the impact of the Qingchuan fault. The activity of four high-angle normal faults is the response to tension stress environment that is due to the movement of the dextral strike-slip fault– the Qingchuan fault. There is a great difference of the tectonics between the west side and the east side of the Hanzhong basin, which appears in the different thickness of the Quaternary sediment, the different activity of the faults around the Hanzhong basin, the different anomalies of Bouguer gravity and the different distributions of small earthquakes. This tectonic difference is maily affected by the activity of Qingchuan fault, and the effect becomes gradually weaker from west to east.Based on the activity characteristics of the Hanzhong basin, using the structural analogy, refering to active faults in eastern China and earthquake captivity of active basins and historical earthquakes of the Hanzhong Basin, the Hanzhong basin is divided into three different potential sources. Because of the more intense activity of the west basin, the largest magnitude of the potential earthquake is defines to Ms6.5. Because of the weaker activity of the east basin, the largest magnitude of the potential earthquake is estimated to be Ms6.0. The boundary of the two potential source areas in the Hanzhong Basin is the border between the different sections of the north edge fault of the Hanzhong basin. The maximum magnitude of the potential earthquakes in the Qingchuan fault area is Ms7.0.