郯庐断裂带鲁苏皖段地壳速度结构的分段特征及其地质意义

本文采用天然地震近震走时反演地壳三维速度结构的方法获得了郯庐断裂带鲁苏皖段及附近地壳(30°N—37°N,113°E—122°E)三维速度结构.对地壳内分层速度结构的分析发现,郯庐断裂带鲁苏皖段存在速度的分段特征.郯庐断裂带鲁苏皖段浅层35.3°N以北,34.5°N—35.3°N间,33°N—34.5°N间呈现的速度分段和地表出露地层有关,与地质上安丘段、莒县—郯城段,新沂—泗洪段三个破裂单元相对应,且和各段的地震活动相呼应,表明郯庐带新沂到泗洪段可能是断裂的闭锁段.郯庐断裂带鲁苏皖段地壳速度结构自浅至深分为三段,大体位置是:南段(32.5°N—33°N以南),中段(32.5°N—33°N至35°N—35.3°N),北段(35°N—35.3°N以北).上地壳分段与苏鲁超高压变质岩带的插入有关,中、下地壳速度分段则可能和火山岩滞留有关.地壳各层速度结构不同段的速度差异反映了构造块体的速度差异,表明各构造块体在地壳下部仍有差异,郯庐带西侧速度总体高于东侧,反映了不同构造块体的形成和组成差别,也说明了该断裂带可能延伸到莫霍面.而不同深度的分段性可能反映了不同地质演化过程.

Segmentation of crustal velocity structure beneath the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone and adjacent areas and its geological implications

The Yishu segment of the Tanlu fault zone is the seismogenic structure of the 1668 Tangchen M8¹/₂ earthquake, where is the research focus of the potential strong earthquake location in the future. Geological survey shows different activity degrees along the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone, but few researches focus on the crustal velocity structure beneath this area, and the relationship of seismicity and the difference of crustal velocity structure. The 3D velocity structure beneath this segment of the Tanlu fault zone and adjacent areas (30°N—37°N,113°E—122°E) was imaged by seismic tomography.Crustal velocity structure segmentation beneath the researched area, its characters and relationship with the geological structure segmentations, and seismic levels are studied in this paper.#br#We select the earthquakes occurred from 1980 to 2011 in the research area, and relocate these events through a relative relocation technique. Based on the travel-time data of Pg, Sg, Pm, Sm,Pn, Sn, pP, and sS waves of these earthquakes,using the multi-phase travel-time inversion method, the 3D velocity structure (inversion grid is 30 km×30 km)beneath the research area is imaged by seismic tomography.#br#The analysis of the velocity distribution patterns of different depths for the research area suggests that the crustal velocity structure has segmentation. For shallow layers, different velocity segments, north of 35.3°N, 34.5°N—35.3°N, and 33°N—34.5°N, are related to exposed strata, respectively corresponding to three rupture elements of the Tanlu fault zone. They are Anqiu segment, Ju county-Tancheng segment, and Xinyi-Sihong segment, which have different earthquake activity patterns. It indicates the Xinyi-Sihong segment is the blocked segment of the Tanlu fault. The crustal velocity structure beneath the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone and adjacent areas can be roughly divided into three different segments from top to bottom. They include the south segment (south of 32.5°N—33°N), middle segment(between 32.5°N—33°N to 35°N—35.3°N)and north segment (north of 35°N—35.3°N. The segmentation of upper crust is related to insertion from the Su-Lu UHPM belt, the segmentation of middle and lower crust is related to detention of volcanics. The velocities in the west of the Tanlu fault zone is higher than that in the east. The differences of velocity distribution patterns at different depths show different tectonic blocks, which have different evolution histories and compositions, also meaning that this fault extends down to the Moho.#br#Different velocity segments at different depths beneath the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone are related to geology factors such as exposed strata, rupture units of the fault zone, Su-Lu UHPM belt, detention of volcanics or different tectonic blocks. Through analysis of the seismic activity of the different rupture umits of the Tanlu fault zone in the researched area, the consistency of velocity segments and rupture units indicate the Xinyi-Sihong segment is the blocked segment of the Tanlu fault zone, where strong earthquakes may occure in the future.