Study On Activity In Late Quaternary And The Earthquake Risk Assessment Of The Yumu Mountain Fault Zone

Qilian Mountains is located in the middle of the north of the Tibetan Plateau.During the Collision between Indian and Eurasian plates, it is undergoing alarge-scale of continental deformation. Yumu Mountain which is between the JiudongBasin and Minle Basin is located in the north of Qilian Mountains, and it is aprominent mountain at the south side of Hexi Corridor. In the process of the TibetanPlateau uplift in the Cenozoic, the Yumu Mountain as a part of Qilian Mountains isalso undergoing a strong uplift.The active fault zone of the Yumu Mountain is in the middle of Qilian thrust beltwhich located in the northeast of Tibet Plateau and the active fault zone have twoactive faults: the Northern and the Eastern Yumu Mountain faults.Northern Yumu Mountain fault distributes in the north of the alluvial fan of theYumu mountain. From west to east, the fault successively passes by Yuanshanzi,Kushuigou, Luquanhe, Wutongquan, Shitangou, Xidakou and Liyuanhekou, and isabout60km. The overall attitude of the Northern Yumu Mountain fault is275~300°/SW∠43~61°. According to geomorphic features and tectonic activity of theNorthern Yumu Mountain fault, it is divided into three sections: the west is fromYuanshanzi to Luquanhe; the middle is from Luquanhe to Jiugequan; and the east isfrom Pailukou to Liyuanxiaokouzi.By paleo-earthquake trench in the analysis and the scarp calculations candetermine4paleo-earthquake events, and they are respectively: the event Ⅰ occurredin (4.066±0.086) ka B.P.; event Ⅱ occurred in6.28ka B.P.; event Ⅲ occurred in(8.382±0.074) ka B.P.; event Ⅳ occurred in10.29ka B.P. or earlier. Using the age ofscarp, it can be also seen with recurrence of paleo-earthquake events, and theirrecurrence interval is about2000a. Because of paleo-earthquake events in the lastelapsed time for the4ka, there is considerable risk for a future earthquake.The left-lateral strike-slip rate of west section of the Northern Yumu Mountainfault is1.69mm/a; the average vertical slip rate of middle section is1.04mm/a, thehorizontal slip rate is2.86mm/a and the left-lateral strike-slip rate is1.72mm/a; Theaverage vertical slip rate of east section is0.28mm/a, the horizon slip rate is 0.60mm/a and the left-lateral strike-slip rate is1.11mm/a. Thus, from west to eastalong the fault, vertical slip rate and left-lateral strike-slip rate shows a decreasingtrend.The Eastern Yumu Mountain fault extends NNW, with total length about110km,the strike is320~330°, the dip is SW and the dip angle is about60°. The fault startsfrom northern Western Huangjiawan, go southward through Daciyao, Xiaociyao,Heihekou, Xiwudang, Huazhaizi, to Southern Matisi then disappears gradually. Wefound an8km in length fault scarp in the west of Southern Huangjiawan and tookthree profiles to prove its activity; found a100m in length fault scarp on the newestalluvial fan in Northern Daciyaokou and took a profile.By analyzing the results of profiles, paleo-earthquake events along Eastern YumuMountain fault occurred many times since the Holocene. The age and attitude of theevents are: Event Ⅰoccurred in (2.326±0.064) ka B.P.; event Ⅱoccurred in (3.902±0.055) ka B.P.. Last event of the elapsed age is earlier than (2.326±0.064) ka. Theage of recent paleo-earthquake was (2.326±0.064) ka.Eastern Yumu Mountain fault consists of three major sections; this study mainlyconcentrates on the northern and middle sections. On the north, the thrust rate ofⅠbench on Western Huangjiawan is0.86mm/a, the horizontal slip rate is0.86mm/a;on the middle, the average thrust rate on the newest alluvial fan in NorthernDaciyaokou is0.32mm/a and the horizontal slip rate is0.21mm/a. Thus, the fault hasa faster slip rate and stronger activity on the north.The last palaeo-earthquake event caused by the Northern Yumu Mountain faultoccurred in4.066±0.086ka B. P., it doesn’t match the Biaoshi M7earthquake of180A.D.. We didn’t find the earthquake surface rupture, so M7earthquake was notcaused by the Northern Yumu Mountain fault. Investigation on the continuous lowscarp on Ⅰbench on Western Huangjiawan and the scarp on the newest alluvial fan inNorthern Daciyaokou, we believe the latest Paleo-earthquake event in the regionoccurred in (2.326±0.064) ka B.P., therefore, the event was not caused by theNorthern Yumu Mountain fault.According to the modern seismicity analysis, we found the modern earthquakeactivity is mainly concentrated in the northern section of the Eastern Yumu Mountain fault. Combined with the study of the earthquake risk in this region by thepredecessors, it is believed that there is a reasonable possibility of occurrence ofmoderate to strong earthquakes in the Eastern Yumu Mountain fault.