Earthquake Risk Assessment On Active Faults Of Lanzhou City

The research area lies in the north-eastern rim of the Tibetan Plateau. The neotectonic activities are strong in this region, and several moderate-strong earthquakes had occurred in the history. This region possesses earthquake-gestating and earthquake-generating structure condition.This paper basises on the project"Assessment of Earthquake Risk on Active Faults of Lanzhou City". In the paper, our research objects include the Maxianshan northern margin fault and Xinglongshan northern margin fault in the south of Lanzhou city,Zhuanglang river fault and Baiyin-baiyangshu river fault in the north of Lanzhou, and the folding deformation zone of Lanzhou basin. The structures mentioned before have different structural mechanism. So, we use appropriate methods for their earthquake risk assessment respectively.The major results that we got showed in the following text.1,The Maxianshan north marginal fault is a left-lateral reverse strike-slip fault during late Pleistocene to Holocene periods, and it primarily constrains seismicity of Lanzhou city. From east to west,it can be divided into three segments:Neiguanying segment,Maxianshan segment,Qidaoliang segment and Wusushan segment.(1)The maximum magnitude of potential earthquakes on Neiguanying segment is MS6.5, and its average recurrence interval is 710-1200a. It will bring little influence to Lanzhou city in the future.(2)The maximum magnitude of potential earthquakes on Maxianshan segment is MS7.1±0.2, and the average recurrence interval of its characteristic earthquakes is 2750-3120a. Its probability of earthquake-generating is big in 200a, potential seicmic hazard is comparatively high.(3)The maximum magnitude of potential earthquakes on Qidaoliang segment is MS6.5. The geomorphic features of its new activity are not legible, and overlaps with Wusushan segment about 20km long. We should pay attention to the moderate earthquakes occurring on it in the future.(4)Wusushan segment is the causative structure of earthquake MS7.0 in 1125a. The maximum magnitude of potential earthquakes on this segment is MS7.0±0.2, and the average recurrence interval of its characteristic earthquakes is 2250-3590a. The last earthquake occurred in 1125a, and the elapsed time is just 882a.The cumulated probability and conditional probility is low in 200 years. It is unlikely that characteristic earthquake would occur on this segment in 200 years.2,Xinglongshan northern margin fault is late Pleistocene active fault, and is a sub-fault of Maxianshan north marginal fault. The maximum magnitude of potential earthquakes on it is MS6.5. Its average recurrence interval of corresponding earthquakes can make reference to the value 710-1200a.3,Since late quarternary, tectonic deformation of Lanzhou basin is inherit-slighted fold deformation. It lies in north of Maxianshan northern margin fault which primarily controls the structural deformation in our study area . The structural deformation of the basin is more weaker than the southern faults. The maximum magnitude of potential earthquakes in the basin is MS6.0, and its average recurrence interval can make reference to the value 250-430a. The strong earthquake would hardly occur in the basin , but we can't eliminate the probability that moderate earthquakes would occur in the future.4,Zhuanglang river fault and Baiyin Baiyangshu river fault are major late quarternary active faults in the north of Lanzhou city. The seismicities on them would impact on Lanzhou city in a certain extent. In this paper, we make reference to the method that Wen Xue-Ze(2007)developed to assess the maximum magnitudes of potential earthquakes in sub-areas of moderately and weakly active faults in eastern China mainland, and formed empirical relationship between the maximum magnitudes Mmax and the at/b values of the sub-areas'frequency-magnitude relationships in our study area. Zhuanglang river fault's activity is fold-reversed fault deformation since late Quarternary. Its maximum magnitude of potential earthquakes is MS6.9 assessed by using the empirical relationship, and recurrence interval of relevant earthquakes is 3130a. Baiyin Baiyangshu river fault's activities are in the metaphase and afternoon of the late Pleistocene. Seismicities on this fault are weak. The maximum magnitude of potential earthquakes on it is MS6.5, and the average recurrence interval is 3960a.