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Li Yongshan Li Jinzheng Lin Fuzhang (Seismological Bureau of Shanxi Province)  
In this paper, isopycnic charts were made for earthquakes of different magnitudes on the basis of seismicity data (ML≥1.0) of north China (E105°-124°; N30°-43°) obtained in last 15 years. We found that there exists a peculiar network fracture skeleton in north China that strongly controls earthquakes large and small in this region. A characteristic is suggested that, from outer points of the network to small and large belts and intersections in the network, frequency and intensity of seismicities profressively increase. 10 large earthquakes (ML≥6.0) in last 15 years are all plotted at or near the intersections in the network. In six months before the Tangshan earthquake, anomalous seismic frequency had occurred almost simultaneously on all the main seismic zones in north ,China (except anomalies occurred on the belt at, western boundary of the Oxduoce Block two months after the Tangshan earthquake). This large regional anomaly appeared only once in this region in last 15 years. These characteristics are very important for estimate of earthquake risks and for earthquake monitoring. Through forward and backward stage-by-stage scanning for seismicities that occurred in surrounding areas (400×400 Km) of the 10 strong earthquake, in 1, 2 and 3 years before occurrence of the main shocks, we found that, before the main shocks, two or more areas of relatively concentrated seismicities of small magnitude always appear in the surrounding regions of main shocks and the future main events are all located at one of these areas or near them. In addition, fracture directions of main shocks are conjugate to the main directions of pre-earthquake concentration of small earthquakes. After detailed analysis we put forward four criteria for determining locations of future main shocks. Finally some theoretical analyses and calculations are made.
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