Analysis of the crustal movement characteristics and uplift mechanism of Longgang volcanoes, Jilin, based on geodesy and MT technology

The Longgang volcanic field is one of the most active volcanoes in modern China. It is situated in the central region of the Longgang mountains on the west slopes of Changbaishan in Jilin. The present three-dimensional crustal movement velocity fields in the different periods were obtained by processing Global Navigation Satellite System (GNSS) observations from 2010 to 2020 and data for the leveling profiles from 1970s to 2010s in the area. The horizontal velocities of GNSS stations were larger in the eastern region than in the western regions, indicating that the eastern region is mainly tensile. The results of the continuous surface strain rates showed that a volcanic field is located in the expansion area. The results of the first-order leveling data for the Changfu profile and the Danfu profile showed that the vertical movements were predominantly uplift, with rates of 0.55~1.83 mm/a for most of the sites. The area with relatively higher uplifting rates of more than 1.0 mm/a was Fusong-Xianrenqiao-Laoshandui, which is rich in geothermal resources and prone to earthquakes. At the adjacent area, Meitong, the rates of the second-class leveling profile were relatively small, at 0.23~0.77 mm/a. The deep electrical structure of the volcanic area was obtained through the three-dimensional inversion of 99 broadband magnetotelluric soundings. The clear structures of the low resistivity in the middle and lower crusts corresponded to the uplift area, which is suggested to be a magmatic system. The position of the magma was relatively shallow. The electrical boundary zone at the northeast end of the Hunjiang fault was speculated to be the northern extension of the fault. The low-resistivity body had the largest scale and extended downward to the mantle. The shallowest low-resistivity body was approximately 10 km below the youngest Jinlongdingzi volcano, and the above high-resistivity structure was considered to be the retreat and consolidation magma after the overflow eruption ended. A comprehensive analysis showed that the ongoing inflation, uplift, and seismic activities of the crust in the volcanic area were related to the upwelling of mantle materials and fault movements caused by intermittent magma migration.