Research On Seismic Hazard Evaluation Of Active Faults On The Notrh Boundary Of The Hetao Fault-depression Zone

Research on seismic hazard evaluation of active faults has evolved fromtraditional qualitative analysis to quantitative evaluation on the basis of probability.Real-time model method is a mid-long term seismic hazard evaluation which isderived from theory of characteristic earthquake and probability, and built ongeological research findings and observations of crustal deformation. The research onseismic hazard evaluation of active faults is consisted by three parts: earthquakerecurrence model, earthquake interval, and density distribution of earthquake events.The Hetao fault-depression zone is on the north boundary of Ordos block, ofwhich the north margin is consisted of four strong active faults since Late Pleistocene:Langshan piedmont fault, Se’ertengshan piedmont fault, Wulashan piedmont fault andDaqingshan piedmont fault.In the past decade, a new technology of crustal deformation observation calledGPS, short for Global Positioning System, has been rapidly developed, and adequateGPS data have been accumulated. On the basis of horizontal velocity data fromproject Crustal Movement Observation Network of China, a well-distributed grid ofGPS velocity on the Hetao fault-depression zone has been constructed by Kriginginterpolation. Then the GPS velocity rate is changed into strain rate, which is finallytransformed into seismic moment rate by the best formula proposed by Savageconverted form Kostrov’s moment rate calculation formula. The average earthquakerecurrence intervals of active faults are calculated with the moment rate.83samples of paleoearthquakes and historical earthquakes are collected in theHetao fault-depression zone and its neighboring area, and then constructed intoearthquake recurrence distribution. By the method of maximum likelihood estimation,the parameters of the lognormal distribution are determined, which are: μ=-0.02254,σ=0.2216. Besides the lognormal distribution, Poisson distribution model and BPTmodel, short for Brownian Passage Time model, are also considered in the paper. Thecoefficient of variation of BPT model is set as0.5. The event recurrence interval of each fault is determined by the characteristicmagnitude of that fault. Due to its lacking of historical record about earthquakes overmagnitude7and the uncertainty of empirical rupture-magnitude relationship,magnitude7,7.5, and8are all considered as characteristic magnitudes of each fault,and seismic hazard of each fault is evaluated separately according to the threemagnitudes and the recurrence intervals determined by them. In the end, recurrenceinterval and elapse time related to each characteristic magnitude are substituted intothe three selected probability-distribution models. The probabilities of occurrence ofstrong earthquake in the future30,50and100years are calculated, based on whichthe seismic hazard evaluation are made:①The seismic hazard of Langshan piedmont fault is relatively high. Within thenext100years, earthquakes of magnitude7are likely to strike, while the occurrenceof strong earthquakes of magnitude7.5and above is with relatively low possibility.②The possibility of an earthquake occurring in the next century of Se’ertengshanpiedmont fault cannot be judged; the possibility of a Ms7.5earthquake is relativelyhigh, and the likelihood of a Ms8earthquake cannot be completely excluded.③Thelikelihood of a earthquake of magnitude7and above occurring in Wulashan piedmontfault in the next100years is relatively high, while a earthquake of magnitude8isunlikely to happen.④Daqingshan piedmont fault is with high probability of strikingof a Ms7earthquake in the next century. An earthquake of magnitude7.5is likely tooccur, while an earthquake of magnitude8is also unlikely to happen.