沂沭断裂带及其近区地震事件地层的时空分布及意义*

沂沭断裂带纵贯山东省中部,属郯庐断裂带中段。在沂沭断裂带及其近区新元古代—新生代的沉积地层中,到目前,已识别出25个地震事件层位。这些地震事件层位的名称取自不同年代或年龄的含地震记录的岩石地层。大多数地震记录是震积岩,少部分为震火山岩,它们的时空分布支持该断裂带生成—活动与发展历史分2个阶段: 古郯庐断裂带阶段(新元古代—古生代)和中—新生代阶段。新元古代初鲁中至苏皖北部NNE向韧性剪切带的形成,沟通了秦岭大别与苏鲁洋间的NEE走向的转换断层,可能是沂沭断裂带或古郯庐断裂带的成因机制。在纵向上,古郯庐断裂带阶段形成了8个地震事件层位,其中5个地震事件层位较密集地分布于南华系至中下寒武统;中—新生代阶段形成了17个地震事件层位,其中12个层位较密集的分布于白垩系—古近系。因此,南华纪—早中寒武世、白垩纪—古近纪分别为2个发展阶段的强地震事件频繁发生时段。在这2个发展阶段,该断裂带地震活动的动力来源不同: 古郯庐断裂带阶段主要源于华北与华南板块的相向运动与碰撞;中—新生代阶段主要源于太平洋板块向欧亚大陆板块下俯冲。在横向上,有15个(占60%)地震事件层位分布在此断裂带内或由该断裂带内向两侧延伸,这体现了沂沭断裂带一直是研究区内发震构造的主体。所有地震事件地层分布于该断裂带纵中轴线两侧150~180,km以内的同沉积盆地,这证明该深大断裂带的两侧近区是强构造地震活动区。作者关于地震事件层位的时空分布的论述和图解,展示了该断裂带自形成以来的地震作用的过程与历史,清晰地勾绘出了这条长期活动地震带的影响范围,这不仅对分析此类深大活动断裂带及其附近由地震引发的软沉积物变形与地震作用具有重要意义,而且对评价此类地震带对地表和建筑物的地震破坏效应也具有重要意义。

Spatio-temporal distribution and significance of seismic event horizon in the Yishu Fault Zone and its adjacent area

The Yishu Fault Zone cut through the central part of Shandong Province and it belongs to the middle segment of the Tanlu Fault Zone. So far,25 seismic event horizons have been identified from Neoproterozoic-Cenozoic strata within the Yishu Fault Zone and its adjacent area . Names of these seismic event horizons are derived from some lithostrata with seismic records of different times or ages. Most of seismic records are seismites,but a small quantity of records are seismic volcanic rocks in these horizons. Spatio-temporal distribution features of seismic event horizons support that the history of generating,activity and development of the fault zone is divided into two stages: The Palaeo-Tanlu Fault Zone stage (from Neoproterozoic to Paleozoic) and the Mesozoic-Cenozoic stage. The formation of the ductile-shear zone with the NNE trending from the central part of Shandong to the northern part of Jiangsu-Anhui linked up the transform faults with the NEE trending between the Qinling-Dabie Ocean and the Sulu Ocean in the initial Neoproterozoic,which may be the origin mechanism of the Yishu Fault Zone or the Palaeo-Tanlu Fault Zone. In the vertical,8 seismic event horizons were formed during the Palaeo-Tanlu Fault Zone stage,therein,5 seismic event horizons densely distributed in Nanhuain to Lower-Middle Cambrian;however,17 seismic event horizons were formed in the Mesozoic-Cenozoic stage,in which 12 horizons densely distributed in Cretaceous-Paleogene. Therefore,the Nanhuain to the Early-Middle Cambrian and the Cretaceous to the Paleogene were the two time intervals of frequent respectively and strong seismic events developed during the two stage. In the two different development stages,tectonic-dynamic sources of seismic activity of the fault zone were different: The dynamic source mainly came from the opposite movement or collision between the North China Plate and the Southern China Plate in the Palaeo-Tanlu Fault Zone;but during the Mesozoic-Cenozoic stage,that mainly from the subduction of the Pacific plate toward the Eurasian plate. In the horizontal,15 seismic event horizons distributed in the fault zone or extended from the fault zone to its both sides,which reflects that the Yishu Fault Zone has been the main part of seismogenic structures in the research area. All of these seismic event strata distributed in syn-sedimentary basins within 150~180,km on both sides of the longitudinal axis of the fault zone,which proves that peripheries on the two sides of the deep fracture zone have been active areas of strong tectonic earthquakes. Expounding and diagrams on the time-space distribution of seismic event horizons have reconstructed the basic process and the history of seismic action of the Yishu Fault Zone for the first time,and clearly delineated the influence scope of the long-active seismic zone in the paper,which are important significance not only to analyze earthquake-induced soft sediment deformations and seismic action in the such large active fault zone and its vicinities,but also to evaluate seismic damage effects surface and buildings in such seismic zone.