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Microearthquake Location Determined by Portable Seismic Array Data in Southeast Gansu Province

GAO Jian1,ZHANG Yuan-sheng1,2,GUO Biao3,LIU Xu-zhou1,2 (1.Lanzhou Institute of Seismology,CEA,Lanzhou,Gansu 730000,China; 2.Lanzhou Base of Institute of Earthquake Prediction,CEA,Lanzhou,Gansu 730000,China; 3.State Key Laboratory of Earthquake Dynamics,Institute of Geology,CEA,Beijing 100029,China)  
In November 2009,a high-density portable seismic array consisting of 150 broadband seismographs was deployed in southeast Gansu province through the cooperation of the Lanzhou Institute of Seismology and the Institute of Geology,CEA.We obtained a large amount of microearthquakes data recorded by this array through November 2011.The data of the portable seismic array and the permanent seismic network is merged and saved in Windows Event Log(EVT) format.On the basis of the amplitude frequency characteristics of seismic waves,we design an automatic identification program of earthquake events,adjust the threshold,and clip the waveforms.We then use EDIAS/MSDP software to re-identify earthquake events,analyze seismic phases,and locate earthquakes by using the simplex location method.The initial localization of 3 243 earthquakes events in the region(32.9°-35.0° N,102.7°-107.1° E) is obtained.Finally,we relocate these earthquakes by using the grid searching algorithm.In this algorithm,the 3-D velocity model is used to obtain theoretical travel times by the ray tracing method.By computing the root mean square(RMS) residuals of theoretical and observational travel times,the corresponding point of the minimum residual is determined as the relocation point.Accurate relocation of 3014 microearthquakes recorded by three or more stations is obtained with this algorithm,which is 1 693 more than that obtained from the permanent seismic network in the same period.We apply the results to seismicity analysis in southeast Gansu province and discuss the relationship between seismicity and active faults.The seismic data recorded on three profiles across several fault zones and the spatial distribution of the faults is analyzed in detail.The results show that the relocated earthquakes are more concentrated on obvious strips or clustered around the faults.Most of these faults have comparatively strong seismicity in southeast Gansu,particularly those concentrated in the Wenxian-Wudu area,which is the transform zone of several groups of faults.However,seismic activity of the Tianshui eastern segment of the West Qinling Northern Frontal fault is relatively weak.The focal depths of the earthquake swarm are mainly distributed in the range of 5~20 km,which proves that the seismogenic layer is in the upper or middle crust in this area at an average of 11 km.As indicated by the focal depth profile,the focal depths are closely related to the faults,and earthquake belts correspond well with faulted structures.The fault zone is composed of the Diebu-Bailongjiang and Guanggaishan-Dieshan southern and northern Piedmont faults within a width of 20 km.The Tazang fault belt and the Lintan-Tanchang fault zone are wider.In depth direction,the earthquakes are obviously compact on strips near the West Qinling Northern Frontal fault,the Wenxian-Kanxian fault zone,and the Chengxian basin Northern and Southern Frontal fault.The seismic activities of these faults extend to the middle crust and upper part of the lower crust.
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