SEGMENTATION OF CRUSTAL VELOCITY STRUCTURE BENEATH THE SHANDONG-JIANGSU-ANHUI SEGMENT OF THE TANLU FAULT ZONE AND ADJACENT AREAS AND ITS GEOLOGICAL INTERPRETATIONS

Abstract

The Yishu segment of the Tanlu fault zone is the seismogenic structure of the 1668 Tancheng M8½ earthquake, and is also 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, the velocity characters and its relationship with the geological structure segmentation and seismic levels are studied in this paper.

We select the earthquakes occurred from 1980 to 2011 in the research area, relocate these events through a relative relocation technique. Based on the traveltime data of Pg, Sg, Pm, Sm, Pn, and Sn waves of these earthquakes, using GABWIT (Genetic Algorithms in Body Wave Inversion of Traveltime) inversion method, the 3D velocity structure (inversion grid: 30 km×30 km) beneath the research area is imaged by seismic tomography.

The analysis of the velocity image of different depths for the research area suggests that the crustal velocity structure is segmented. 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, Juxian county-Tancheng segment, and Xinyi-Sihong segment, which have different earthquake activity patterns. It indicates that Xinyi-Sihong segment is a locked segment of 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 (from 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 Sulu 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 image at different depths show different tectonic blocks, which have different evolution histories and compositions, also meaning that this fault extends down to the Moho.

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, Sulu UHPM belt, detention of volcanics or different tectonic blocks. Through analysis of the seismic activity of the different rupture units of the Tanlu fault zone in the research area, the consistency of velocity segments and rupture units indicate that Xinyi-Sihong segment is a locked segment of the Tanlu fault zone, where strong earthquake may occure in the future.