鄂尔多斯活动地块边界带15000年以来强震活动与现今大地震空区

Strong earthquake activities around the Ordos active block in past 15000 years and its implications

在青藏高原和太平洋构造域的共同作用下,鄂尔多斯活动地块及周缘边界带的断裂构造存在明显的运动性质差异,形成了明确的分区分段特征.本文基于探槽古地震记录和历史地震资料,通过多探槽中揭露的多类证据综合限定各断裂带上的强震事件,获得了鄂尔多斯活动地块周缘边界带主要断裂带上15000年以来180次强震事件,重建了地块边界带不同构造区强震活动的时间序列,将千年尺度的地震记录延长到万年尺度.在此基础上,建立了鄂尔多斯活动地块各不同构造区近15000年以来强震活动时空分布图像.鄂尔多斯活动地块各边界带长时间尺度的强震活动特征显示:地块边界带各分区内存在差异性的活动期和平静期,北、东北、东、西边界均存在不同程度的地震丛集和迁移规律;围绕鄂尔多斯活动地块周缘的边界带内15000年以来强震分布具有较为明显的丛集发生特征,平静期持续时间(约200~500年)不等;近5000年以来,各边界带上强震活动频次有明显增强的趋势,部分区段存在未来发生强震的风险;综合近3000年以来历史地震、最新强震破裂图像等判定鄂尔多斯活动地块边界带仍存在7个明显的大地震空区,即强震危险区.

The active block theory was introduced into intracontinental earthquake research that strong continental earthquakes are controlled by the movement and deformation of active block.This theory is very important for understanding strong earthquake occurrence and related mechanisms far away from the plate boundaries,and also provides conceptual framework for short-and long-term prediction of strong earthquakes on the continents.The boundary zones of active blocks composed of different types of active structures is the potential risk areas where strong earthquakes occur,which have been proved by the occurrences of modern earthquakes in recent years.With respects to this correlation between the occurrence of strong earthquake among different active tectonics,a comprehensive study is needed to focus on the faults related to strong earthquake but also the associated active block.Here we summarized long-term strong earthquake activities in the boundary zones and characteristics of the active blocks,and provides more complete estimates on future risk areas around the Ordos block.The Ordos Block,situated in the central part of the mainland of China,represents a typical active block surrounded by diverse active tectonic zones.As one of the most significant seismically active regions in the mainland of China,the Ordos Block has experienced over 50 strong earthquakes with magnitudes(M)≥6.5 according to historical records.Notably,the vicinity of the block has experienced over 5 large earthquakes with M≥8.Due to the dynamic effect of the Tibetan Plateau and the Pacific Plate,the Ordos block has significant strong earthquake,leading to distinct fault systems with varying characteristics of movement,zoning,and segmentation along its boundary zones.In total,we recovered 180 strong earthquake events from the main fault zones around the Ordos active block and recovered the different structural zones in the boundary zones.Based on paleoearthquake and historical earthquake records,we reconstructed time series of strong earthquake occurrence in the past 15000 years.Our time series of seismic activities extended strong earthquake research from thousand-year to ten-thousand-year scale that enabled us to obtain better spatial and temporal image of strong earthquake activities from different tectonic areas around the Ordos active block.Our results showed that there are differential active and quiet periods in each zone of the block boundary,with varying clusters of strong earthquakes in the north,northeast,east and west boundaries,quiet periods varied from~200 to 500 years in the past15000 years.In the past 5000 years,the frequency of strong earthquake has been significantly increased in each boundary zone,indicating high risk of strong earthquake in these segments.Based on historical earthquake records and our latest images,we recognized 7 high risk areas in the boundary zone,namely,Wuhai-Linhe,Tongxin-Azouqi,Tianshui-Baoji,Yuncheng basin,Taiyuan basin,northeast Shanxi basin and Hohhot-Daihai basin around the Ordos active block.