Earthquake Risk Assessment On Main Active Faults Of Linfen City

Linfen region is located in the central of strong neotectonic activity Fen-Wei fault basin, destructive earthquakes have occurred many times in history.It has breeding and tectonic conditions of strong earthquake.Paper based on the methods such as establish the quaternary standard stratigraphic section, the reflection wave seismic exploration, drilling joint profile, exploratory trench and large section, acquired a large number of basic data in the active fault exploration work of linfen. To study the main seismogenic fault include Luo Yunshan piedmont faults and Guo Jiazhuang faults for precise positioning and segmentation, as far as possible to master faults geometry, active characteristics of secondary paragraph. Adopts evaluation method suited to the characteristics of the structure in the study area to evaluate the potential maximum earthquake magnitude, the recurrence interval and earthquake probabilities. Get the main achievements include:(1) In the study area in blunt diluvium is late pleistocene to holocene stratum, middle pleistocene lacustrine formation is less exposed. According to remote sensing image, DEM image analysis and field geological mapping work, comprehensive map out the geomorphic features in the study area and the quaternary stratigraphic distribution. By area of quaternary controlling borehole data and reference to previous research results, a comprehensive analysis of research in the fourth stratigraphic classification scheme is as follows: Holocene(Q4) 0.0 ~ 0.4 m, thick 0.4 m; late pleistocene(Q3) 0.4 ~ 80.8 m, thick 80.4 m; middle pleistocene(Q2), 80.8 ~ 245 m, expose thick 164.2 m..(2) Luo Yunshan piedmont faults control development of linfen basin and the main tectonic framework. According to the the geometry of fracture structure, the nature of the activities and time division, divided into seven sections from north to south : Luo Yun village segment(F1), Wan An segment(F2), Kang village segment(F3), Tu Men segment(F4), Long Ci segment(F5), Wei village segment(F6) and Fan Jiauzhuang ~ Xi Weikou segment(F7).(3)Luo Yun village segment(F1) is characterized by three secondary fault, the latest activity time is the early late pleistocene, vertical displacement rate of 0.075 mm/a. Wan An segment(F2) faulting intensity from northeast to southwest become weak gradually, the latest activity time should be mid later late pleistocene, vertical activity rate is 0.1 mm/a. Kang village segment(F3) T2 continuous terraces, since late pleistocene activity is not strong; Tu Men segment(F4) the latest activity occured between 8~ 12 ka, the average slip velocity of 0.06 mm/a as the early holocene active faults. Long Ci segment(F5) since the late late pleistocene happened 4 earthquakes incidents: in the latest incident dislocation 3 ka acient soil under surface 2.5 m, may be associated with 7? level earthquake in 649 AD, the holocene activity rate of 0.41 mm/a. Wei village segment(F6) occurred three times incidents in the late pleistocene, the activity rate is 0.017 mm/a. Fan Jiauzhuang ~ Xi Weikou segment(F7) the latest events before present 17ka±, activity rate in 0.13 mm/a ~ 0.22 mm/a.(4) Luo Yunshan piedmont faults right twist occurs mainly in the north of Wei village, fault right hand twist volume decreases south along the fracture, the Fan Jiauzhuang ~ Xi Weikou segment(F7) is completely normal fault.(5) Luo Yunshan piedmont faults occured three events since 17 ka ago, the average interval about 7ka. E1 event may be associated with earthquakes in 649 AD, the vertical displacement of the 2.9 m. E2 events before present 8.14~10ka, the vertical displacement of the 2.2 m to 3.0 m. E3 event before present 16.4~18.48 ka, vertical displacement of 3.0 ~ 3.5 m. Since the late pleistocene, the activity is a continuous distribution of fracture, the overall activity besides Luo Yun village segment(F1), but the latest activities only in Long Ci segment(F5), which the strongest part of faulting.(6) Guo Jiazhuang faults divid Gan Ting and Long Ci Settlement center of Linfen sag, divided into south and north two segments, northern(Bei Lu segment) 9 km long, southern(Guo Jiazhuang segment) 16 km long.(7)Drilling Guo Jiazhuang reveal range, the formation of different layers have different degree of fault, Slip increases to 1-1.5 m with depth deepening. Faulted late pleistocene stratum the age of at least 66.6 ka. Combining previous data and field geological survey, the fault of the latest activity is in the holocene.(8) The regional history and current earthquake time distribution with three active period and every active period there are 10 ~ 30 years, interval about 300 years earthquake activity climax period.Since 1970, the medium and small earthquakes with M ≥4.7 historical earthquakes are mainly distributed in the Taiyuan basin and Linfen basin, shows that the characteristics of the uneven distribution of regional seismic spatial. The earthquake mainly distributed in the surface 5-24 km and belong to light source. Therefore, the seismic active period is not over since 1813 in region. As the main regional earthquake distribution area, Linfen faulted basin is also the main area of activity light source earthquake in one hundred years.(9) Tectonic stress field and focal mechanism solutions show that maximum principal stress axes(8 ° 237 °), the minimum principal stress axes(144 °, 21 °), the intermediate principal stress axes(346 °, 346 °) in study area. Results same with the tectonic stress field of north China, shanxi region. The advantage of medium-strong earthquakes rupture direction of NNE and NWW, focal rupture of strike-slip type and properties of seismic fracture for high Angle normal fault.(10) According to the fracture relative dangerous section of the judgment standard, determine the Luo Yunshan piedmont faults except the southern tip of Fan Jiauzhuang ~ Xi Weikou segment(F7), the rest of the 90 km is the relative risk section. Guo Jiazhuang faults on the entire length of 25 km long part is relatively dangerous section.(11) On the basis of Deng Qidong, Long Feng summarizes north China experience relations(seismic intensity- rupture scale), WC experience relationship(seismic intensity, the surface rupture length and the area), the experience relationship between the earthquake intensity and the seismic moment. Luo Yunshan piedmont faults relatively dangerous section of the potential maximum earthquake magnitude is 7.78, 7.50, 7.67. Respectively to endow them with different weights, comprehensive evaluation Luo Yunshan piedmont faults relatively dangerous section of the potential maximum earthquake magnitude is Ms=7.68. Guo Jiazhuang faults relative dangerous section of the potential maximum earthquake magnitude is Ms=7.05.(12) Luo Yunshan piedmont faults earthquake interval 7 ka, the latest incident death time is 1366 years. Guo Jiazhuang faults interval seismic events as 1800 years, the latest incident death time is 320 years. For Poisson model and Brown time process model, comprehensive conclusion Luo Yunshan piedmont faults relatively dangerous section of the potential maximum earthquake(Ms=7.68) in the next 50 a, 100 a and 100 a seismogenic probability of 0.009, 0.018 and 0.033. Guo Jiazhuang faults relatively dangerous section of the potential maximum earthquake(Ms=7.05) in the next 50 a, 100 a and 100 a seismogenic probability of 0.033, 0.066 and 0.128.