Seismic Hazard Assessment for Active Faults of Longnan City

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    Based on active fault mapping in Longnan City,Gansu Province,we determine in this study parameters used in seismic hazard assessment for active faults.Five main fault zones are identified.The first is the Diebu-Bailongjiang fault zone,which ruptured during the M7 earthquake in 186 BC.The second is the Guanggaishan-Dieshan fault zone,which is separated into three main strands.The north strand had been found to cause a paleo earthquake,although the Pingding-Huama strand had a significantly faster slip rate.The Hanan-Qingshanwan-Daoxizi fault zone is the third.A surface rupture of approximately 3.5 km long was found in the Zhuyuanba strand.The fourth is the Lianddang-Jiangluo fault zone,which had strong activity in the Holocene.The fifth is the Wudu-Kangxian fault zone,in which four paleo-earthquakes have occurred since the Late Pleistocene.We estimated potential maximum earthquake magnitude and repeat interval time with magnitude rupture length and magnitude-frequency relationships for every fault or fault sub-area.The probability of destructive earthquake occurrence in the future 50,100,and 200 years was computed with a time-dependent seismic potential probability model and a Poisson distribution model.The methods for estimating potential maximum earthquake magnitude include four principles:(1)maximum magnitude can be an upper limit magnitude in the same seismic zone and (2)should not be smaller than the largest historical earthquake.(3)It can also be estimated by a relationship,such as M-L and G-R.Moreover,(4) it may have a reference to the maximum magnitude in a seismic zone with intensity of VIII degrees during the 200 years since last big earthquake because the lapsed time is longer than the interval time.The article gives mean maximum magnitudes,which are 7.5,7,7,7,and 6 for the Diebu-Bailongjiang,Guanggaishan-Dieshan,Liangdang-Jiangluo,Wudu-Kangxian,and Hanan-Qingshanwan-Daoxizi fault zones,respectively,with a combination of the four principles.We also used the method of b value spatial mapping to reveal high stress or high seismic hazard zones.On the basis of the seismic gaps identified,we used the relationship between magnitude and number or frequency to compute the b value of the study area with small earthquakes of the most recent 40 years.The small earthquakes were relocated with seismological methods.If the area had a lower b value,lower strain was accumulated in the area.We determined higher strain areas from the b value map;such areas are future high seismic hazard zones.We used the maximum curvature method to determine minimum magnitudes of completeness,which are minimum magnitudes of earthquakes for computing the b value.We used a 1° × 1° grid for the study area.Every node had a b value that was calculated with at least 50 selected small events.The events for computing the b value were selected by a circle with a particular radius and a node center point.The results indicate that the west Guanggaishan-Dieshan fault zone has a higher potential for seismic hazards in the future; therefore,more attention should be paid to this area.Although many methods were used to prevent error in the results,many unknown parameters remained,which caused our results to have uncertainties.

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  • Online: October 30,2014
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