关键词: GIS, 地壳运动, 地震危险性, 孕震构造势位(LDLEPS), 古地震

Abstract: The approach of latent development level of earthquake-pregnant structures in the crustal movement(LDLEPS)is a seismogeologic dynamic indicator for evaluating seismic risk quantitatively,developed based on the net deformation ratio using GIS techniques. The following concerns are addressed in this paper,including the working principle and applying conditions for LDLEPS,the drawing of LDLEPS maps,the relationship between the distribution characteristic of LDLEPS and earthquake in the North China region,and the seismogeologic analysis of the LDLEPS anomaly areas in North China. An example is presented of the use in forecasting earthquakes with LDLEPS method,and more discussions are made on the origin of formation of LDLEPS.Contrasting the spatial distribution of LDLEPS with earthquakes of 1966-1998 in the North China region in Fig 1,we found that they match and correspond completely to each other in terms of lineament,location,and size. This demonstrates that the LDLEPS method can be used in quantitative evaluation of seismic risk. To verify this conclusion,we drawn the LDLEPS maps of Sichuan and Qinghai regions and used them to forecast experimentally the earthquake trend in these regions in 2006,and the afterward outcomes have proved that the forecast is basically correct,as shown in Fig. 2 and 3.As a consequence,the 2008 M8 Wenchuan earthquake in Sichuan Province and the 2010 M7.1 Yushu earthquake in Qinghai Province just happened in the expected abnormal LDLEPS regions and times,respectively. Formation mechanism of earthquake-pregnant structures is discussed in detail in the paper. According to the geophysical exploration and seismogeologic survey data,a bilayer-structured earthquake-generation model is proposed,derived from the crustal thermal processes and tectonic reworking resulting from the late-stage evolution of sub-mantle plumes and mantle-branch structures in North China,i.e.the semi-plastic rheologic layer or lower layer in the middle crust and the rigid layer or upper layer in the upper crust,and that the dynamic of the lower layer induces the formation and development of the seismogenic structures in the upper layer,causing tectonic deformation and generating earthquakes. Based on LDLEPS maps,we divide the North China region into 6 seismogenic structure anomaly zones,as shown in Fig. 1.Of them,the anomalous seismogenic structure zone B,located in the northwestern Beijing,is rich in geothermal resources,where hot fluids upwell into the fault planes of the upper layer from the lower rheologic layer,reactivating the faults and generating the moderate and lower-magnitude earthquakes,with a causative mechanism similar to that of reservoir-induced earthquake. In the anomaly zone A of seismogenic structures(Bohai Sea area and its margins),the powerful driving force from the lower rheologic layer controls directly the generation and development of the upper seismogenic structures,producing moderate and lower-size earthquakes. Researches on palaeoseismic traces from trench profiles and seismites in marine sediments reveal that there are many strong seismic events with magnitude ≥6 and palaeotsunami events in this region in the prehistoric times. The nowadays high level of LDLEPS and strong earthquake events are the continuation and repetition of their past activities. It is also noted that there are obvious differences in structural landforms and earthquake trends between the South and North China blocks owing to difference of their tectonic background. Having the advantage of novelty,uniqueness,visibility,operability,effectiveness and computerization,the LDLEPS method can be widely applied to areas of earth science research,earthquake prediction and engineering.

Key words: GIS, crustal movement, seismic risk, latent development level of earthquake-pregnant structure(LDLEPS), paleoseismology