Abstract
The Zigui Basin in the Three Gorges area is a syncline running in the north-south axial direction; the sediments in the basin are mainly late Triassic-late Jurassic sandy and argillaceous rocks. Since the Three Gorges Project began undergoing impoundment in May 2003, nearly 20,000 mini-earthquakes have occurred in the Zigui–Badong region, including five earthquakes with magnitudes greater than Ms 5.0. Herein, a 3D gravity inversion method was introduced to investigate the density structure of the Zigui Basin and its adjacent areas. A comprehensive analysis of the geological structure of the area was carried out, based on focal mechanism solutions of six moderate–strong earthquakes in the area. The Zigui Basin is a low–density area from the periphery, and the sediment in the east is thicker but less dense than in the west. The shallow part of the Zigui Basin is a weak bottom layer prone to slippage, and the deep part (5–10 km) could comprise lithological strata such as limestone, sandstone, and shale, which easily undergo dissolution by leaking reservoir water. Under the action of external forces such as long-term infiltration and unloading of reservoir water, fissures will expand and squeeze then earthquakes are likely induced in the deep strata. Furthermore, new buried faults in the western and southeastern edges of the core area of the Zigui Basin could also trigger an earthquake under long–term pressurization and reservoir water penetration.
Highlights
The Zigui Basin in the Three Gorges area is located in the thin–skinned structural zone of Chongqing City, Hunan and Hubei Province, with the Yangtze Plate to the south, the Qinling Orogenic Belt to the north, the Sichuan Basin in the west, and the Huangling Anticline in the east
4–6 earthquakes occurred intensively in this local region, and it can be considered that the long–term accumulation of tectonic stress in this region is related to the continuous deposition of the Zigui Basin and the east-west material migration movement extrusion
According to the principle of clustering of small earthquakes, it could not be ruled out that there may be one or several NE and NEE small buried faults in the western core area of Zigui Basin, and one NE buried fault exists in the southeast margin of the basin, which was induced by the long–term infiltration of reservoir water and the effect of loading–unloading
Summary
The Zigui Basin in the Three Gorges area is located in the thin–skinned structural zone of Chongqing City, Hunan and Hubei Province, with the Yangtze Plate to the south, the Qinling Orogenic Belt to the north, the Sichuan Basin in the west, and the Huangling Anticline in the east. While the distribution and changes in the density of the Earth’s crust can be considered to reflect the underground material migration, no study has been carried out to analyze the evolution of underground material migration against the background of geological formations and, to analyze the relationship between regional faults and seismic tectonics coupling with natural seismic events (Li et al, 1987; Li et al, 2010; Wang K. et al, 2013; Wang Q. et al, 2013; Zhang et al, 2017). J1x-T3s: Lower Jurassic (Xiangxi Formation)Upper Triassic (Shazhenxi Formation), J1-2n: Middle Jurassic (Niejiashan Formation), J2x: Lower Jurassic (Shaximiao Formation), J3s: Upper Jurassic (Suining Formation), J2s: Upper Jurassic (Shaximiao Formation), J3p: Jurassic (Penglai Town Formation), T1j: Lower Triassic (Jialingjiang Formation), T2: Middle Triassic, Pre-ε: Precambrian, O: Ordovician, S: Siluric, P: Permian, D+C: Devonian-Carboniferous, K1s: Cretaceous (Shimen Formation), T1d: Lower Triassic (Daye Formation), ε: Cambrian]
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