Study On The Characteristics Of The Fracture Activity In The West Of Helan Mountain

The study area is located in Alashan left flag, Inner Mongolia. The target fault,called the Western piedmont Fault of Helanshan Mountain and penetrating the urban area, is located 20km west from Helanshan mountain foothills. The fault strikes N, nearly paralleled with the strike of Helanshan mountain range. Three trenches revel four paleo-seismic events, occurring in 1.65-5.52 Ka 5.5-8.8Ka、8.8-11.66Ka andll. OKa-21.51Ka. The research area is full of well-developed gullies, with a low vegetation cover. As a result, it is highly optional for quantitative research of the fault activity. Slip rate of the fault is acquired by three different approaches.1) Radiocarbon age has been utilized to identify the age of deformed features so as to calculate the slip rate (1.37± 0.1mm/a, Holocene; 1.09±0.2mm/a, since upper Pleistocene) with the aid of satellite image interpretation of feature displacements.2) Through the function defining the relationship of the gully dislocation deformation angle and the age of the deformed gully, the age of the deformation (caused by the slip of the fault) can be obtained. Combining the age of deformation and the horizontal displacement, there comes the horizontal slip rate (1.38-1.55mm/a, Holocene;1.18-1.23mm/a, since upper Pleistocene) of the fault.3) The vertical slip rate of the fault is obtained through the fault scarp evolution model. In that model, the age of the scarp can be defined by the geometry of the scarp. With the age of the scarp and the height of the scarp (vertical displacement), the vertical slip rate (0.7mm/a, Holocene) can be obtained.4) Another approach to gaining the vertical slip rate of the fault is by radionuclide dating of two planation surface bounded by the fault. With the dating data and the elevation difference of the two planation surface combined, the vertical slip rate (0.64mm/a) since the upper Pleistocene can be acquired. Earthquake recurrence interval of the target fault has been obtained through-two different ways.1) Time-predictable model is based on a constant stress threshold at which rupture occurs. Given the previous slip, time until the next rupture is predictable. Based on this theory, the earthquake recurrence interval (3420-3660a) is acquired according to the slip of the previous earthquake triggered by the target fault.2) Seismic moment release rate is the quantity which defines the long term release rate of the seismic moment. The time it takes for seismic moment to be released up can be acquired by dividing the seismic moment by the seismic moment release rate. That time is the earthquake recurrence interval (3190a, M>=7.0). Additionally, the cross sections of the terraces and the dating data indicate that the cutting rate of the rivers in that area in the Holocene is 0.27mm/a.