Abstract: On the basis of summarizing the circulation characteristics and mechanism of earthquakes with magnitude 7 or above in continental China, the spatial-temporal migration characteristics, mechanism and future development trend of earthquakes with magnitude above 7 in Tibetan block area are analyzed comprehensively. The results show that there are temporal clustering and spatial zoning of regional strong earthquakes and large earthquakes in continental China, and they show the characteristics of migration and circulation in time and space. In the past 100a, there are four major earthquake cluster areas that have migrated from west to east and from south to north, i.e. 1)Himalayan seismic belt and Tianshan-Baikal seismic belt; 2)Mid-north to north-south seismic belt in Tibetan block area; 3)North-south seismic belt-periphery of Assam cape; and 4)North China and Sichuan-Yunnan area. The cluster time of each area is about 20a, and a complete cycle time is about 80a. The temporal and spatial images of the migration and circulation of strong earthquakes are consistent with the motion velocity field images obtained through GPS observations in continental China. The mechanism is related to the latest tectonic activity in continental China, which is mainly affected by the continuous compression of the Indian plate to the north on the Eurasian plate, the rotation of the Tibetan plateau around the eastern Himalayan syntaxis, and the additional stress field caused by the change of the earth's rotation speed.
    Since 1900AD, the Tibetan block area has experienced three periods of high tides of earthquake activity clusters(also known as earthquake series), among which the Haiyuan-Gulang earthquake series from 1920 to 1937 mainly occurred around the active block boundary structural belt on the periphery of the Tibetan block region, with the largest earthquake occurring on the large active fault zone in the northeastern boundary belt. The Chayu-Dangxiong earthquake series from 1947 to 1976 mainly occurred around the large-scale boundary active faults of Qiangtang block, Bayankala block and eastern Himalayan syntaxis within the Tibetan block area. In the 1995-present Kunlun-Wenchuan earthquake series, 8 earthquakes with M_S7.0 or above have occurred on the boundary fault zones of the Bayankala block. Therefore, the Bayankala block has become the main area of large earthquake activity on the Tibetan plateau in the past 20a. The clustering characteristic of this kind of seismic activity shows that in a certain period of time, strong earthquake activity can occur on the boundary fault zone of the same block or closely related blocks driven by a unified dynamic mechanism, reflecting the overall movement characteristics of the block. The migration images of the main active areas of the three earthquake series reflect the current tectonic deformation process of the Tibetan block region, where the tectonic activity is gradually converging inward from the boundary tectonic belt around the block, and the compression uplift and extrusion to the south and east occurs in the plateau. This mechanism of gradual migration and repeated activities from the periphery to the middle can be explained by coupled block movement and continuous deformation model, which conforms to the dynamic model of the active tectonic block hypothesis.
    A comprehensive analysis shows that the Kunlun-Wenchuan earthquake series, which has lasted for more than 20a, is likely to come to an end. In the next 20a, the main active area of the major earthquakes with magnitude 7 on the continental China may migrate to the peripheral boundary zone of the Tibetan block. The focus is on the eastern boundary structural zone, i.e. the generalized north-south seismic belt. At the same time, attention should be paid to the earthquake-prone favorable regions such as the seismic empty sections of the major active faults in the northern Qaidam block boundary zone and other regions. For the northern region of the Tibetan block, the areas where the earthquakes of magnitude 7 or above are most likely to occur in the future will be the boundary structural zones of Qaidam active tectonic block, including Qilian-Haiyuan fault zone, the northern margin fault zone of western Qinling, the eastern Kunlun fault zone and the Altyn Tagh fault zone, etc., as well as the empty zones or empty fault segments with long elapse time of paleo-earthquake or no large historical earthquake rupture in their structural transformation zones. In future work, in-depth research on the seismogenic tectonic environment in the above areas should be strengthened, including fracture geometry, physical properties of media, fracture activity behavior, earthquake recurrence rule, strain accumulation degree, etc., and then targeted strengthening tracking monitoring and earthquake disaster prevention should be carried out.

Key words: Continental China, Tibetan block area, major active area of great earthquakes, seismic migration cycle, dynamic mechanism

摘要： 文中在总结中国大陆7级以上地震迁移循环特征和机理的基础上, 重点对青藏地块区7级以上地震的时空迁移特征、 机理及未来发展趋势进行了综合分析。 结果表明: 中国大陆区域性强震、 大地震活动存在时间上的丛集性和空间上的分区、 分带性, 并在时、 空上表现出迁移和循环的特征。 在过去的100多年, 存在由西向东、 由南向北迁移循环的4个大地震丛集区域, 每个区域的丛集时间约20a, 1个完整轮回时间约80a, 其机理与中国大陆最新构造活动主要受印度板块向N对亚欧板块的持续挤压、 青藏高原绕喜马拉雅东构造结的顺时针旋转作用以及地球自转速度变化所造成的附加应力场的影响有关。 自1900AD以来, 青藏地块区经历了3个地震活动的丛集高潮期(也称为地震系列), 即1920—1937年的海原-古浪地震系列、 1947—1976年的察隅-当雄地震系列和1995年至今的昆仑-汶川地震系列(主体活动区为巴颜喀拉地块)。 这3个地震系列主体活动区的迁移图像反映了青藏地块区构造活动从块体周缘边界构造带逐渐向内会聚、 高原内部发生挤压隆升和向SE挤出等现今构造变形过程。 分析表明, 已经持续20余年的昆仑-汶川地震系列可能趋于结束, 未来20a中国大陆7级大地震的主体活动区可能迁移到青藏地块区周缘的边界构造带上, 重点是东部边界构造带(即广义的南北地震带), 同时应关注北部柴达木地块边界带等区域的主干活动断裂中大地震空区、 空段等孕震有利部位。

关键词: 中国大陆, 青藏地块区, 大地震主体活动区, 地震迁移循环, 动力学机制