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

摘要： 在全变率(NDR)指标的基础上,结合GIS技术进一步开发了孕震构造势位(LDLEPS)分析法,以便定量评估地震危险性。在充分利用已有资料基础上绘制了华北地区LDLEPS图和地震分布图,从图中发现,孕震构造势位与地震空间分布的轮廓、位置、震级大小等是完全匹配的,即LDLEPS图可以用来预测未来震情。为此,在2006年绘制了四川、青海等地LDLEPS图,并据图对该地未来震情进行了试验性预测,后来的结果表明预测基本上是正确的。文中进一步讨论了孕震构造的形成机制: 华北幔柱亚热柱活动后期对地壳的热作用与构造再造作用而衍生出孕震的二元结构模型,即下层半塑性流变层与上层脆性层,下层动力因素控制上层孕震构造的形成与发展。根据LDLEPS图把华北地区划分出甲～己6个孕震构造异常区,其中乙异常区(京西北)地热资源丰富,是深部热物质溢出地表的表现。其地震成因是: 下层流变层的流体沿断层面涌入上层,断层复活致震,这是下层动力因素对上层间接作用的结果,其发震机制类似于水库诱发地震。乙异常区仅产生中等震级以下的地震。甲孕震构造异常区(渤海及其边缘带)是下层流变层强劲动力因素直接控制上层孕震构造的形成与发展,从而产生中频强震以下的地震。另外对探槽剖面及海相沉积物震积岩的古地震遗迹研究发现,甲区史前曾发生大量古地震事件与古海啸事件。现今该区高势位孕震构造和强震事件是过去事件的延续和重复。同时指出,华北南缘之SN地块在构造地形与震情上存在明显差异。

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