长白山天池火山岩浆系统分析

针对外媒报道长白山天池火山在近2年内有可能喷发的言论,在长白山天池火山区布测了一条长度约为103 km的二维大地电磁测深观测剖面,对火山区深部电性结构进行探测研究.由于研究区内不明来源的电磁干扰非常强,对数据采用了远参考处理、Robust处理、Rhoplus分析、张量阻抗分解和基于一维层状介质电阻率与相位互算方法等先进处理技术,获取到一批在强干扰区质量较为可靠的电磁数据,利用数据计算分析了长白山天池火山区二维构造走向和感应矢量特征,采用NLCG二维反演技术对资料进行了二维反演解释,并将反演结果与前人探测结果进行了对比分析.探测结果表明:在天池火山口下方存在明显的直立型岩浆通道,岩浆通道在下方约5~8 km位置形成关闭;在火山口下方往北方向附近,在埋深位置约7 km深处存在一个明显的低阻异常体,电阻率小于10 Ωm,且与岩浆通道对接,推测其可能是地表浅部发育的岩浆囊;在长白山山门附近C07-C09号测点之间和C04-C05号测点之间,在埋深约7~17 km深处发现近直立型低阻带,低阻带与下方低阻体直接相连,推测低阻带内赋存有活动的岩浆;随着埋深的增加,从天池火山口南部约20 km位置往北方向,在埋深13~30 km之间壳内广泛发育明显的低阻异常体,推测其可能是活动的岩浆囊.反演结果与前人探测结果整体电性特征相似,但又局部不同.     

云南景谷M_S6.6地震震源区深部电性结构及其孕震环境

2014年10月7日云南景谷地区发生M_S6.6地震,震源机制显示此次地震为逆走滑型,地震断层面走向140°,同时余震分布显示破裂面走向也为NNW向。文中对1条横穿景谷震区,与地震破裂面垂直的大地电磁测线数据进行了由定性到定量的全面分析,通过二维非线性共轭梯度(NLCG)反演得到了震源区较为详细的地壳电性结构。结果表明:1)震源区电性结构可以分为4层:地表以下约4km为相对低阻层,主要由中、新生代盆地沉积岩组成,电阻率10~100Ω·m;地下5~10km为相对高阻层,可能由元古界变质岩系组成,电阻率1000Ω·m;15~30km为中下地壳低阻层,电阻率10Ω·m;30km以下为壳幔过渡层,电阻率值约为30Ω·m。2)景谷地震主震发生在高阻层和壳内低阻层的分界面上。3)对余震的震源深度统计发现5km和10km两个深度范围内余震较多,与电性梯度带的位置相对应。     

Analysis of Deformation Characteristic and Uplift Mechanism in Longgang Volcanoes,Jilin

吉林龙岗火山区是第四纪活动火山,具有潜在喷发危险。通过GNSS和水准资料分析区域三维地壳运动,得出2011—2019年相对欧亚板块水平运动以东南向为主,速度＜10 mm/a。敦化-密山断裂以东受日本2011年“3·11地震”影响强烈,现以拉张运动为主;近年来水准资料揭示火山区垂直运动以隆升为主。2014—2019年的InSAR资料显示隆升集中在靖宇一带。结合MT剖面反演得到的深部电性结构,龙岗火山区西侧高阻体分布在深度18 km以上,金龙顶子火山下方最浅。中部为早期喷发形成的火山,下方高阻体分布在深度40 km以上,东侧抚松一带分布在深度约20 km以上,地壳范围内的高阻结构表明岩浆已固结。高阻结构层可分辨出断层两侧电阻的差异性。高阻体下方存在大规模低阻结构,推测为中下地壳岩浆系统。金龙顶子火山深度10 km以下的低阻结构可能为岩浆通道,并与中下地壳岩浆系统相连。东侧区域岩浆平均深度约30 km,相对较浅。龙岗火山区幔源物质的上涌及间断性的向上运移引起地壳隆升及地震活动。     

长白山火山区地壳S波速度结构的背景噪声成像

利用探测深俯冲的中国东北地震台阵NECsaids的60个流动台与固定地震台2010年7月至2014年12月的垂向连续波形数据,采用地震背景噪声成像方法获得了研究区6~40 s周期的瑞雷波相速度分布,并通过相速度频散反演得到了研究区下方0~50 km的三维S波速度结构.结果表明:研究区下方地壳S波速度结构存在明显的横向和纵向不均匀性,浅部速度结构与浅表地质构造单元有较好的对应,深部速度结构较好地反映了区域火山活动及深部热物质作用的结构特征;在长白山火山下方9~30 km深度范围内存在明显低速区并有向下延伸的趋势,推测可能为长白山火山地壳岩浆囊;在龙岗火山下方12~30 km深度范围内发现较弱的低速区,可能代表火山喷发后的残留物,而在镜泊湖火山下方没有明显的低速异常,说明镜泊湖火山地壳内可能不存在部分熔融的岩浆物质.     

阿尔山火山群深部“拱桥式”岩浆系统及其稳定性分析

阿尔山火山群自更新世至近代持续活动,具有潜在喷发的危险,因此加强对阿尔山火山群岩浆系统的研究,评价火山群的活动性具有重要的意义。在获得阿尔山火山群地区大地电磁测深数据的基础上,对其进行标准化处理后经二维非线性共轭梯度反演,获得二维电性结构成像,发现阿尔山地区岩石圈内部存在大规模的"拱桥式"中、低阻异常(电阻率320Ω·m),并在伊尔施镇西侧和柴河镇东侧分别存在明显的高导异常(电阻率40Ω·m);前者体积相对较小,埋藏深度在40~60km,后者体积较大,埋藏深度在60~90km,甚至更深。结合地质、地球化学资料,推断"拱桥式"异常为新生代来自两侧盆地软流圈的玄武质岩浆运移的通道,其所包含的2处高导异常,极可能为未冷凝或正在聚集的岩浆房,因此阿尔山岩浆系统由"拱桥式"通道和来自软流圈的玄武质岩浆组成,火山群具有统一的岩浆系统。进一步分析表明,"拱桥式"通道物质熔融百分比≥0.5%,岩石圈结构总体趋于稳定,其所包含的2处岩浆房熔融程度为2.5%~11.5%,颗粒边界可能已被溶体全部润湿,岩石的流动强度相对较低,岩石圈结构稳定性较差。区域地震分布、温泉的形成与阿尔山火山群岩浆系统也具有一定的相关性。综合迹象表明阿尔山群正处于休眠期,并非死火山,有再次喷发的可能,应密切监测。     

琼东北马鞍岭-雷虎岭火山区深部岩浆系统大地电磁三维探测

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In SAR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。     

聚类分析算法在大地电磁三维解释中的应用

为了提高对大地电磁三维反演结果的分析和解释能力,完成构造界面识别、异常构造刻画等地质和地球物理解释,本文提出使用非监督的聚类方法分析大地电磁反演结果.根据三维反演模型电阻率值的分布和各种聚类分析方法的特点,选择使用K均值聚类方法对反演模型电阻率值进行聚类分析.在K均值聚类分析过程中,本文采用了RS指数指导选择聚类数目, Kaufman法进行中心初始化.通过对东北地区大地电磁数据三维反演结果使用K均值聚类分析方法,得到了东北地区电性岩石圈的厚度估计,结果表明东北地区岩石圈底部聚类电阻率大约为339Ω·m,其中松辽盆地岩石圈最薄,约为60 km;大兴安岭地区最厚,约为150 km;佳木斯地体厚度约为100 km;而长白山地区岩石圈厚度不易确定,可能受新生代构造活动影响,电阻率明显减小.聚类方法能够有效地帮助对大地电磁三维反演结果中的地质构造进行识别和归类.     

华北地区上地幔及过渡带电性结构研究

采用远参考道和Robust技术,处理了华北地区14个地磁台站资料,得到了相干度超过0.8的地磁测深响应函数.并将其转换为大地电磁测深的响应函数,获取了105~107 s周期范围内的视电阻率和相位.应用ρ+理论对数据进行了一致性检验和反演,结果表明417km,850km深度附近可能存在电性间断面.同时采用基于一维最光滑模型的Occam反演方法得到了300~1000km范围的地幔电性结构,并与前人在其他地区的研究结果进行了对比.发现华北地区地幔过渡带的电导率在大兴安岭—太行山重力梯度带东西两侧表现不同,重力梯度带附近及西侧台站下方过渡带深度的电导率和北美的Tucson地区相当,而华北地区东部的电导率在地幔过渡带范围高出西侧约2~5倍,这很可能和太平洋板块的俯冲有关.     

死谷亮点反映加利福尼亚州大盆地南部的中地壳岩浆体吗?

在大盆地(Great Basin)南部的死谷(Death Valley)地区,已经可以用COCORP(大陆反射剖面联合会)的深部地震反射剖面探测出以前未认识的中地壳岩浆体。在6s处(深15km)的强信号和较宽带的反射,是由于部分熔融物质近水平的侵入引起的。这个“亮点”(bright spot)在死谷中部下方横向延伸至少15km。一个中度倾斜的正断层,可能从这个推测的岩浆房,一直向上延伸到一个690000年前的玄武岩质火山渣锥。推测该断层带在这个火山渣锥形成期间,已成了岩浆的通道。根据地壳反射特性的垂直变化假定,死谷中的岩浆体可能已经沿一条解耦带侵入,这个解耦带把断裂的脆性上地壳与比较可塑的和/或可侵入的下地壳分隔开。死谷亮点类似于1977年在Rio Grande裂谷、由COCORP记录到的反射,后一地区的地球物理和大地测量资料支持这样一种推论:在20km深处存在着一个板状岩浆房。     

滇西三江构造带电性结构特征——以福贡—巧家剖面为例

为查明滇西三江构造带及邻区复杂的构造特征,并揭示该区深部电性结构,沿福贡—巧家布设了一条长约410 km的大地电磁剖面.共观测到61个物理点,其中宽频大地电磁测点41个,长周期大地电磁测点20个.通过对采集到的数据进行一系列的处理、反演,得到了沿剖面的壳幔电性结构模型.并结合研究区内区域地质资料及其他地球物理资料,对剖面所经过的各个主要地质构造单元及主要断裂带进行了综合解释.电性结构模型揭示沿剖面地壳电性层次复杂,深部电性结构由西往东呈分块展布,横向变化大,壳内广泛发育低阻异常.在中甸构造带(香格里拉地块)和盐源—永胜构造带深部壳幔存在大规模低阻异常,这可能与地下局部熔融体和地热流有关;康滇构造带壳幔存在大规模高阻异常,表明地壳中曾经有地幔物质侵入;在大凉山构造带地下10~50 km深处存在一呈横向“半月形”展布的低阻体,电阻率值不满10Ωm,结合地质资料与前人的研究成果,推测该低阻体成因应与青藏高原东南缘“地壳管道流”有一定关联.     

MAGNETOTELLURIC IMAGING OF MAGMA DISTRIBUTION BENEATH MA'ANLING AND LEIHULING VOLCANOES OF NORTHEASTERN HAINAN,CHINA

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.     

DEEP “ARCH-BRIDGE” MAGMATIC SYSTEM OF THE AERSHAN VOLCANIC GROUP AND ITS STABILITY ANALYSIS

The Aershan volcanic group has been active since the Pleistocene to the modern and has potential to erupt, so it is of great significance to strengthen the study on the Aershan volcano group and evaluate its activity. The magmatism is characterized by low resistivity in electrical properties. The electrical structure obtained by magnetotelluric sounding can be used to study the magmatic occurrence and volcanic activity. It is an effective method to detect the deep structure of volcanic area. Based on the magnetotelluric sounding data of the Aershan volcanic group, the two-dimensional nonlinear conjugate gradient inversion is obtained after the normalization of the data and the two-dimensional electrical structure of the Aershan volcanic group is obtained. It is found that there is a large-scale "arch-bridge" low-resistivity anomaly (resistivity less than 320Ω·m) and there are obvious high-conductivity anomalies (resistivity less than 40Ω·m) respectively on the west of the town of Ershi and the east of Chaihe town, the former is relatively small in sizes, buried at the depth of 40~60km, and the size of the latter is larger, buried at the depth of 60~90km, or even deeper. Combined with geological and geochemical data, it is inferred that the "arch bridge" anomaly is the channel of the basaltic magma transport from the epithermal basins on its both sides. The two high-conductivity anomalies it contains are probably the uncondensed or gathering magma chamber, so the Aershan magma system consists of "arch bridge" channel and asthenosphere-derived basaltic magma, the volcanic group has a unified magmatic system. Further analysis shows that the melting percentage of the "arch bridge" channel material is not less than 0.5%, and the lithosphere structure tends to be stable. The melting degree of the two magma chambers it contains is 2.5%~11.5%, and the grain boundary may all be wetted by the melt, rock flow intensity is relatively low, lithosphere structural stability is poor. In addition, the regional seismic distribution and the formation of hot springs also have a certain correlation to the Aershan volcanic group magma system. There are indications that the Aershan volcano group is in dormancy, rather than an extinct volcano, there is the possibility of eruption, so it should be closely monitored.     

琼东南盆地天然气水合物及其成藏模式的海洋可控源电磁研究

南海琼东南盆地是天然气水合物重要远景区之一.由于盆地大部分地区海底地形平缓、地层近于水平,增加了利用地震反射剖面识别似海底反射（BSR,bottom simulating reflector）的难度,从而影响了对水合物的评价.为了进一步开展琼东南盆地水合物调查研究,本文在研究海域进行了海洋可控源电磁探测试验,将自主研发的10台接收机以500 m的间距,投放至水深约为1360 m的海底,完成了一条4.5 km剖面的电磁数据采集.通过对采集的数据进行处理与二维（2D）反演,获得了研究剖面海底的电阻率断面图.反演结果显示,研究区海底60~330 mbsf（meter bottom of seafloor）的地层中,存在多个横向不连续分布的高阻异常体,电阻率介于2~10 Ωm之间;在海底330 mbsf之下,横向上发育了电阻率为2~4 Ωm的3个高阻体.根据研究区热力学条件,本文估算了生物成因气与热成因天然气的水合物稳定带（GHSZ,gas hydrate stability zone）厚度,结合高阻体的分布特征推断了地震剖面上BSR的位置.在此基础上,对反演的电阻率断面进行解释,推断了研究区水合物的分布及游离气运移通道.研究表明,勘探区具有形成天然气水合物矿藏的地质与地球物理条件,其成藏模式可能属于"断层、裂隙输导的下生上储型",水合物的气源为生物成因气.     

Upper crustal electrical resistivity structures in the vicinity of the Çatalca Fault, Istanbul, Turkey by magnetotelluric data

A magnetotelluric survey was performed at the Çatalca Region, west of Istanbul, Turkey with the aim of investigating geoelectrical properties of the upper crust near the Çatalca Fault and its vicinity. Broadband magnetotelluric data were collected at nine sites along a single southwest-northeast profile to image the electrical resistivity structure from surface to the 5 km depth. The dimensionality of the data was examined through tensor decompositions and highly two-dimensional behavior of the data is shown. Following the tensor decompositions, two-dimensional inversions were carried out where E-polarization, B-polarization and tipper data were utilized to construct electrical resistivity models. The results of the inversions suggest: a) the Çatalca Fault extends from surface to 5 km depth as a conductive zone dipping to southwest; b) the thickness of the sedimentary cover is increasing from SW to NE to 700 m with low resistivity values between 1–100 Ωm; c) the crystalline basement below the sedimentary unit is very resistive and varies between 2000–100000 Ωm; d) a SW-dipping resistivity boundary in the northeastern part of our profile may represent the West Black Sea Fault.     

阿尔山火山区地壳上地幔电性结构初探

在阿尔山活火山区一条北北西向测线上进行了7个大地电磁测深点的观测。2-D解释结果表明,研究区内新、老两条火山带可能存在通往深部的岩浆通道。其中,新发现的活火山带地下在10~12km还保持着较高的热状态,很可能富含流体,在30~50km处可能是地幔向上的供热通道;而老火山带30km以上可能存在一个正在冷却的岩浆通道,两条火山条带的深部可能是同源的。     

滇西三江地区深部电性结构特征及其意义

沿盈江一姚安布置了一条长约350 km的长周期大地电磁剖面,共观测宽频大地电磁测深点53个,长周期大地电磁测深点26个;利用多种数据处理方法对观测资料进行了预处理、定性分析和二维反演,得到沿剖面的地壳和上地幔电性结构模型.对主要地块和断裂带的电性结构进行了分析,电性结构模型揭示滇西三江地区广泛存在壳内低阻层,但沿剖面方向即东西向埋深和厚度不一,在保山地块和滇中地块地下存在较大规模的壳幔低阻带;腾冲地块地下埋深13~20 km,厚约10~17 km的壳内低阻层可能是岩浆囊的反映;初步将保山壳幔韧性剪切带作为印支地块挤出的深部东边界.     

海底地震仪远震记录接收函数反演——以南海西南次海盆为例

由于海底环境和海底地震仪(OBS)结构的特殊性,用OBS远震记录进行接收函数岩石圈反演研究因为存在一定的困难,所以还很少见.在深入分析问题的基础上,以国产I-4C型宽频带OBS在南海西南次海盆记录的天然地震为实例,我们将傅里叶变换和小波变换相结合以压制海底地震仪记录中的非平稳干扰,获得了信噪比较高、震相清晰的地震记录,进而成功开展了远震记录的岩石圈结构接收函数反演.主要结论是:(1)OBS接收函数的求取是可行的,关键是压制非平稳干扰.(2)西南次海盆的Moho面埋深为海底下10~12km(地壳厚6~8km),沉积物厚度为1~2km,浅部地壳存在低速区,与沉积物和海底扩张停止后的岩浆喷发产生的岩石碎屑和裂隙有关.(3)在扩张脊中央Moho面上方6~12km存在S波低速区,推测扩张中心可能存在下地壳熔融或岩浆房,在17~30km区间S波速度呈负梯度,我们认为扩张中心更深的地方存在热物质的供给.