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

摘要： 海南琼东北地区是中国新生代以来火山活动最强烈、 持续时间最长的地区之一, 历史上曾发生1605年琼山7.5级大地震。 最新的水准、 形变测量以及InSAR资料揭示在琼山7.5级大地震震中以南的地区存在1条SN向展布的沉陷区, 沿该区小地震活动频发, 即铺前湾-冯家湾小地震密集条带, 有研究推测该小地震密集条带与深部岩浆系统的发育有关。 为了进一步探测研究琼东北火山区火山岩浆系统的深部结构、 地下介质属性、 小地震活动和形变变化与火山深部岩浆系统分布的深层关系, 2017年初在琼东北地区以雷虎岭-马鞍岭火山区为中心针对2条剖面开展了大地电磁探测, 使用相位张量分解技术对其电性结构进行分析, 并使用大地电磁三维反演技术获得了沿剖面的三维深部电性结构图像。 探测结果进一步确定了琼东北火山区下方的介质属性和低阻体的赋存范围, 并发现琼东北地区的电性结构存在明显的东西差异, 电阻率值东高西低。 此外, 在琼东北地区地壳中存在2个低阻体, 分别为海南岛西北部澄迈县区域(即雷虎岭和马鞍岭火山口西侧)下方中地壳碗状向W倾斜的低阻体和龙泉下方的低阻体; 而在海南岛东北部的铺前湾-冯家湾小地震密集条带与地面沉陷区的深部并没有低阻介质赋存, 排除了小地震是由于壳内火山岩浆活动引起的可能。

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