关键词: 马鞍岭-雷虎岭火山区, 大地电磁三维成像, 电阻率结构, 小地震密集条带

Abstract: The northeastern Hainan Province is one of the areas subjected to the strongest, most frequent and longest-lasting volcanic activities in China since the Cenozoic era. Under the influence of magma and fault activities, northeastern Hainan Island has experienced many moderate and strong earthquakes in history. The Qiongshan M7.5 earthquake occurred in this region in 1605. The deformation measurement and InSAR data found a subsidence area in the south of the Qiongshan M7.5 earthquake. Small earthquakes frequently occur in this area. It has been inferred by some studies on this subsidence area, namely the Puqianwan-Fengjiawan seismic belt, that the subsidence and frequent seismic activity are related to the development of deep magma systems. Magnetotelluric methods are very sensitive to subsurface fluid, different temperature conditions, and resistivity property of the medium in the molten state. With the development of magnetotelluric three-dimensional inversion technique, using dense array magnetotelluric data in three-dimensional inversion can image the medium resistivity occurrence state and position in the volcanic area. To study the deep structure of the magma system and its relationship with seismic activity, we conducted MT observations on two profiles that cross Leihuling and Ma'anling volcanoes. Phase tensor decomposition was used to analyze the electrical structure. This paper investigates the two MT profiles using three-dimensional electromagnetic imaging technology and obtains the electrical structure of the two profiles. The result reveals the media properties and high conductivity bodies' occurrence range beneath the volcanic area in the northeastern Hainan. There are obvious differences in the electrical structure of the northeastern Hainan. The resistivity values are high in the east and low in the west. In addition, there are two high conductivity bodies in the northeast of Hainan. The high conductivity body C₁ inclines to the west and locates beneath the Chengmai County area in the northwestern Hainan Island(west of the Leihuling-Ma'anling volcanoes). Its resistivity value is less than several Ωm. This low resistive body is 40km long in WE direction and 30km wide in SN direction. Its burial depth is about 2km near the HNL1 profile and 6km near the NHNL1 profile. Its bottom reaches the depth of about 25~30km, which may be close to or through the Moho surface depth of 25~26km in this area. It is speculated that the magma eruption of Leihuling-Ma'anling volcanoes did not migrate vertically from its deep part to the surface. The high conductivity body C₂ locates beneath Longquan. The buried depth of C₂ tends to be shallower from north to south, but there is no exposed surface in the study area, nor is it connected with the shallow low-resistivity layer. It is speculated that the C₂ may be a magmatic sac trapped in the crust, but may have nothing to do with the eruption of Ma'anling-Leihuling volcanoes. The recent volcanic magma in this area comes from the lower crust and upper mantle of the ocean area to the west of Hainan Island. As magma enters the upper and middle crust, it continues to move shallowly and eastward. In this process, it should be blocked by the high resistance structure on the east side of the Changliu-Xiangou Fault and then erupt around this fault, thus forming numerous craters in this area. After the repeated eruption, deep magma channels gradually closed and volcanic activity weakened. The magma in the mid-upper crust cooled consolidated gradually, but the speed was uneven in different areas, resulting in the channels having closed down gradually in some places, and some are in the process of closing. Our results show an uneven rise and fall depth of the low resistivity body in the middle and lower crust. There is no high conductivity body in the deep part of the Puqianwan-Fengjiawan seismic belt and the subsidence area in the northeastern Hainan, which rules out the possibility that the small earthquakes are related to deep magma systems.

Key words: Ma'anling-Leihuling volcanos, magnetotellurics 3-D inversion, electrical structure, seismic belt