Upper mantle low anisotropy channels below the Pacific Plate

A new 3D anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves. Though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field, a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals. New insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure. The model is best resolved in the Pacific Plate, the largest and the most active tectonic plate. Superimposed on the large-scale radial (ξ parameter) and azimuthal anisotropy (of V_SV velocity) within and below the lithosphere, correlated with present or past Pacific Plate motions, are smaller-scale (<1000 km) lateral variations of anisotropy not predicted by plate tectonics. Channels of low anisotropy down to a depth of 200 km (hereafter referred to as LAC) are observed and are the best resolved anomalies: one east-west channel between Easter Island and the Tonga-Kermadec subduction zones (observed on both radial and azimuthal anisotropies) and a second one (only observed on azimuthal anisotropy) extending from the south-west Pacific up to south-east Hawaii, and passing through the Polynesia hotspot group for plate older than about 40 Ma. These features provide strong constraints on the decoupling between the plate and asthenosphere. They are presumably related to cracking within the Pacific Plate and/or to secondary convection below the rigid lithosphere, predicted by numerical and analog experiments. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in the Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean.     

利用远震研究四川盆地及其周缘瑞利面波相速度和方位各向异性

基于四川盆地及周边的245个宽频带台站2010年9月-2014年9月期间的远震记录,提取双台路径瑞利面波相速度频散资料,反演得到四川盆地20~120 s的高分辨率瑞利面波相速度及各向异性空间分布.在丰富区域地球物理基础数据的同时,结合已有研究成果对地壳上地幔变形耦合进行探讨,结果表明短周期（20~30 s）的相速度分布与四川盆地的地质构造特征相吻合,作为川滇地块、松潘-甘孜地块和四川盆地之间的边界——龙门山断裂带和鲜水河断裂带对上述三个地块上地壳的速度结构具有明显的控制作用;松潘-甘孜地块,特别是川滇地块中下地壳普遍表现为明显的低速异常,表明中下地壳相对软弱;而四川盆地的中下地壳整体呈现相对高速,表明四川盆地具有相对坚硬的中下地壳.研究区域东南角接近北扬子地块与南扬子地块的缝合部位,呈现高速异常.四川盆地南部和东南邻区不同周期均具有较强的各向异性,且快波方向较为一致,反映这些地区不同深度变形耦合较好.四川盆地西部、北部及东北部邻区,不同周期的各向异性快波方向变化较大,不同深度变形耦合较差.这些特征与绕喜马拉雅东构造结的物质流动被扬子地块的高速地壳阻挡的宏观认识基本一致.

     

Calculated and observed seismic behaviour of exposed upper mantle

In order to determine the seismic properties of rocks representing the continental crust/upper mantle transition, a structural and petrographical study of peridotites of the Ivrea-Verbano zone (N. Italy) has been undertaken. The seismic behaviour (Vp, Vs, birefringence, orientation of the fast shear-wave polarisation plane) has been calculated from the modal composition of rock samples, together with the crystallographic preferred orientation and the single crystal elastic constants, using the methods of Mainprice (1990). It is shown that the intrinsic anisotropy of peridotites is mainly due to the lattice preferred orientation (LPO) of olivine and to the competing effects due to the LPO of orthopyroxene. If present, well-oriented hydrous phases such as phlogopite, leads to both P- and S-waves velocities becoming drastically reduced, but the anisotropy and the S-wave splitting (mainly on the foliation plane) is increased. An attempt has been made to apply this petrologic and microstructural approach to understand the seismic properties of ultramafic rocks with the aid of a mobile seismic network set up in the Ivrea-Verbano zone. Five recording units with 3-component geophones were installed on the most representative lithologies of the Ivrea-Verbano zone: kinzigites, gabbros, stronalites, and on metapelites of the adjacent outcrop of the Serie dei Laghi. During five months the seismic network recorded 49 events that were mainly regional earthquakes. The preliminary analysis of the first arrival revealed a high-velocity anomaly near the Finero ultramafic massif. Rough calculations indicated that this anomaly could be due to a shallow-depth, peridotitic body about six kilometres thick. S-wave polarisation analysis was carried out on the horizontal components of the seismograms of four regional earthquakes that showed a first arrival propagating almost vertically at the recording sites. Four earthquakes were chosen in order to reconstruct the orientation of the polarisation plane of the shear phases. The preliminary results of this analysis have shown a qualitatively good agreement between the orientation of the polarisation plane of the fast shear-wave and the strike of the foliation at the station near Finero, located on kinzigites of the Ivrea-Verbano zone, although the relationships between the local structural setting and the polarisation orientations at the other recording sites is not yet so clear.     

Seismic anisotropy of upper mantle in eastern China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation. Supported by Special Project for the Fundamental R & D of Institute of Geophysics, China Earthquake Administration (Grant No. DQJB06B06), Special Program of the Ministry of Science and Technology of China (Grant No. 2006FY110100), China Digital Earthquake Observation Network Project “North China Seismic Array”, and National Natural Science Foundation of China (Grant Nos. 40334041 and 40774037)     

华北地区地壳上地幔速度各向异性研究

本文介绍面波反演得到的华北地区地壳上地幔速度各向异性分布图像,并与S波分裂的结果作初步的定性比较.不同周期瑞利波群速度的方位各向异性图像呈现显著的横向变化,与华北地区地壳上地幔的构造分块和垂直分层结构有比较密切的联系.在鄂尔多斯和阿拉善等稳定地块中,岩石圈地幔到160 km深度都保持比较一致的显著各向异性;而在发生过岩石圈减薄的华北克拉通东部,大约80~150 km深度范围内基本没有探测到方位各向异性,可能说明岩石圈减薄过程抹去了原有的各向异性印迹而且没有显著的水平构造运动造成新的方位各向异性.地球介质的各向异性具有明显的分层特征,面波的反演结果是如此,而S波分裂测量所表现的离散性,也可能是由各向异性的分层差异和倾斜的各向异性对称轴等因素引起.采用多层的各向异性模型,在多数情况下可以定性地解释面波反演和S波分裂结果之间的差异.进一步的工作要求增大面波的探测深度和改善分辨能力,获取更多的S波分裂测量资料,从而建立定量或半定量的三维各向异性模型.     

利用地震面波频散重建川滇地区壳幔S波速度

利用适配滤波频时分析技术分析覆盖川滇地区的长周期面波记录,计算了周期10~100 s内的面波群速度频散,对研究区进行划分尺度大小1.5°×1.5°分格后,采用射线追踪方法求取各分段射线的长度和时间,得到各个格子的纯路径频散.继而采用阻尼最小二乘法求解,反演得到该研究区壳幔S波速度分布.研究结果表明,川滇地区表现出地壳增厚和缩短,在地壳和上地幔顶部,川滇菱形块体内部与其外部相比,虽然存在局部速度负异常,总体上呈相对高速,其周边的走滑断裂带呈现深至上地幔顶部的负速度异常,这有助于地壳块体沿断裂的侧向挤出;此外,云南西部和四川西部壳内和上地幔高导层的存在被认为是与部分熔融的物质或与滑脱构造相关联;从纬向剖面和经向剖面可以得到四川盆地莫霍面平均深度大约为45 km,云南地区莫霍面深度南北方向不一致,云南地区最北端深度达到49 km,南端莫霍面深度大约为36 km,这说明不同构造块体在构造运动过程中受到影响的程度不同.     

湖北地区上地幔各向异性及其动力学意义

SKS波分裂测量是研究大陆地幔的形变特征、探索大陆动力学和演化过程的重要工具.本文选用湖北数字地震台网(HB台网)和中国数字化宽频带地震台网(CB台网)提供的三分量宽频带数字化地震资料,使用旋转相关法和切向能量最小法,计算得到了湖北地区23个地震台站下方上地幔各向异性参数.结果表明,快波偏振方向较为复杂:大别造山带内,呈平行于大别造山带构造走向的NWW向,且与绝对板块运动方向基本一致,反映印支—燕山期扬子板块与华北板块南北向挤压碰撞造山的过程,而地幔流动则是造成这种南北挤压的力源.扬子板块内NWW向虽与绝对板块运动趋势一致,但比较离散,受地幔对流影响不明显,而NE向各向异性主要与燕山后期之后应力场的转变有关.分析认为,该地区的上地幔各向异性为地幔流动和固结在岩石圈中的"化石各向异性"共同作用的结果.根据上地幔各向异性方向与主要构造带的走向平行,可以作为地壳变形与上地幔连贯变形的证据.整个研究区域得到的分裂延迟时间在0.5~1.35 s之间,厚度在58~155 km之间.     

A strategy for automated analysis of passive microseismic data to image seismic anisotropy and fracture characteristics

Monitoring of induced seismicity is gaining importance in a broad range of industrial operations from hydrocarbon reservoirs to mining to geothermal fields. Such passive seismic monitoring mainly aims at identifying fractures, which is of special interest for safety and productivity reasons. By analysing shear‐wave splitting it is possible to determine the anisotropy of the rock, which may be caused by sedimentary layering and/or aligned fractures, which in turn offers insight into the state of stress in the reservoir. We present a workflow strategy for automatic and effective processing of passive microseismic data sets, which are ever increasing in size. The automation provides an objective quality control of the shear‐wave splitting measurements and is based on characteristic differences between the two independent eigenvalue and cross‐correlation splitting techniques. These differences are summarized in a quality index for each measurement, allowing identification of an appropriate quality threshold. Measurements above this threshold are considered to be of good quality and are used in further interpretation. We suggest an automated inversion scheme using rock physics theory to test for best correlation of the data with various combinations of fracture density, its strike and the background anisotropy. This fully automatic workflow is then tested on a synthetic and a real microseismic data set.     

川滇地区壳幔地震各向异性研究进展

根据川滇地区已有的地震各向异性研究结果,利用体波、面波资料的结果,分析川滇地区不同构造尺度、不同深度的地震各向异性特征。对比不同方法研究川滇地区介质各向异性的特点,探讨了该地区的介质连续性及壳幔耦合状态。分析认为,地壳上地幔各向异性的差异表明,川滇地区具有复杂的地壳及上地幔形变机制。因此,对于川滇地区壳幔地震各向异性的深入理解,需在理论上和高密度数据资料基础上加强量化分析和综合研究。     

青藏高原东南缘基于背景噪声的Rayleigh面波方位各向异性研究

青藏高原东南缘地区是现今地壳形变和地震活动最强烈的地区之一,也是研究青藏高原现今变形机制和构造演化规律的重要区域.本研究使用云南区域地震台网的55个宽频带地震台站连续地震背景噪声数据,采用双台站互相关方法获得Rayleigh(瑞利)面波经验格林函数,提取相速度频散曲线,反演得到云南地区周期5~34s范围内方位各向异性分布图像.反演结果揭示:短周期(5~12s)Rayleigh面波快波优势方向与区域断裂走向有很好的一致性,快波方向随着断裂走向的变化而变化.周期16~26s快波优势方向与反映上地壳特性的5~12s图像总体图像相似,但细节略有不同.其中,滇中块体内易门断裂和滇中块体内东侧的普渡河断裂附近,各向异性快波方向从NS向NW方向旋转;易门断裂以西呈NW向.这反映了青藏高原物质东流和川滇块体受到青藏块体的南东向挤压作用.周期30~34s范围的各向异性,滇缅泰块体和印支块体,快波优势方向为NS和NNW向;而在滇中块体内部,各向异性快波方向呈顺时针旋转变化,可能与青藏高原物质向东逃逸有关.本文还开展了与体波各向异性的对比分析,通过与近震S波分裂、Pms转换波分裂和远震SKS、PKS和SKKS(以后简称为XKS)分裂的对比研究,发现随着周期的增大,得到的快波优势方向与XKS剪切波快波偏振方向趋向一致,与地壳快剪切波偏振方向呈一定夹角.本研究认为,青藏高原东南缘地区壳幔各向异性具有不同的特征和形成机制.     

A Tertiary asthenospheric flow beneath the southern French Massif Central indicated by upper mantle seismic anisotropy and related to the west Mediterranean extension

Upper mantle flow beneath the French Massif Central is investigated using teleseismic shear wave splitting induced by seismic anisotropy. About 25 three-component stations (short period, intermediate and broadband) were installed during the period 1998-1999 in the southern Massif Central, from the Clermont Ferrand volcanic area to the Mediterranean Sea. Teleseismic shear waves (SKS, SKKS and PKS) were used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 0.7 and 1.5 s have been observed at most of the sites. The azimuths of the fast split shear waves trend homogeneously NW-SE in the southern Massif Central suggesting a homogeneous mantle flow beneath this area. The observed NW-SE direction differs from the N100°E Pyrenean anisotropy further south. It does not appear to be correlated to Hercynian structures nor to the present-day motion of the plate but is well correlated to the Tertiary extension direction. We propose that the opening of the western Mediterranean induced by the rotation of the Corsica-Sardinia lithospheric block and the roll-back to the SE of the Tethys slab may have generated a large asthenospheric mantle flow beneath the southern Massif Central and a deflection of the up going plume centered beneath the northern Massif Central toward the SE.     

Preliminary seismic anisotropy in the upper crust of the south segment of Xiaojiang faults and its tectonic implications

The Xiaojiang faults,striking north-to-south(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults.     

基于背景噪声研究云南地区面波速度非均匀性和方位各向异性

基于中国地震科学探测台阵项目一期在南北地震带南段架设的300多个地震台站,利用2011年10月至2012年9月的连续观测记录,采用基于背景噪声互相关函数的面波层析成像技术,研究了青藏高原东南缘的云南地区面波群速度和方位各向异性分布.结果显示,地壳的面波快波方向呈现近南北向,整体表现出围绕东喜马拉雅构造结顺时针旋转的趋势,和地表GPS速度场以及S波分裂的快波方位较为一致.小江断裂东西两侧的快波方位有一定差异.对反映深度大概在下地壳和上地幔顶部的长周期面波,快波方向从近南北向逐渐向北西向过渡,在菱形块体附近,26°N以南,快波方向和红河断裂的走向趋于一致;其他区域相比上地壳的快波方向也有较大变化,这种结果较为支持青藏高原东南缘的云南地区壳幔变形的解耦.在滇西南,澜沧江向东弧形展布区域,中下地壳快波方位呈现局部的圆周旋转趋势,结合该区地震分布规律和应力主轴方向,推测这种现象和块体挤压及旋转具有一定相关性.     

四川长宁页岩气开采区上地壳各向异性及其变化特征

四川长宁页岩气开采区2018年12月16日和2019年1月3日相继发生了M_L5.7和M_L5.3地震, 两次中强地震给附近建筑物带来了严重的破坏.本文收集了长宁地区2018年11月至2019年2月的地震波形数据资料, 利用剪切波分裂方法获得了研究区内14个台站的剪切波分裂参数快波偏振方向和慢波延迟时间, 分析了长宁地区地壳各向异性特征, 以及地壳应力场的变化, 初步探讨了两次M_L5.0地震孕育的应力环境.结果显示, 多数台站快波偏振方向有两个优势方向, 北东向和北西向.研究区位于四川盆地南缘盆山结合地带, 构造环境复杂, 剪切波分裂参数较为离散.北西向的快波偏振方向与区域主压应力方向、震源机制解结果一致, 北东向快波偏振方向与之正交.整个研究区域内慢波延迟时间普遍较高, 均值为6.16 ms·km^-1.慢波延迟时间最大值来源于位于两次M_L5.0地震震中之间的XIC台站.离散的剪切波分裂参数以及较高的各向异性强度, 可能与页岩气开采压裂作业有关.水力压裂施工, 页岩地层中注入高压流体, 从而改变了岩体和断层的应力状态, 引起应力环境的改变.慢波延迟时间在M_L5.3地震发生之前表现出明显增加的趋势, 表明地震发生之前地壳应力的积累.M_L5.3地震发生之后观测到慢波延迟时间的降低, 反映了震后地壳应力的突然释放.

     

中国东西剖面的地壳 Q 结构研究

本文利用地震面波和体波的资料推断了一条横跨中国东西部剖面的地壳Q结构,并具体分析了中国大陆地台、山前皱褶带、高原和内陆盆地等几个典型构造单元内介质速度与吸收的特点。指出该剖面地壳的Q结构在深度上基本可以划分成上中下三层,其Q_β值分别为(100—300)、(200—550)和(30—190),它可能反映着地壳由弱固结层到低温低压的脆性层和高温高压的韧性层的变化,在横向构造上,以南北构造带为界分成东西两大部分。青藏高原的Q结构具有一定的特殊性。该东西剖面内中下层地壳Q值系统性偏低,这可能是中国地壳的一个普遍现象,在一定程度上反映着上地慢热活动的直接影响。该区震源深度的分布同地壳Q结构表现有良好的相关性,在东部地区绝大多数的构造地震发生在Q值较大的地壳中部层位内。作者由此推断,地壳中大多数的断层是自地表延续到中地壳附近,并认为温度较低、岩石较硬和岩体内有一定的断裂切割是地震易于发生的深部条件。     

基于背景噪声研究青藏高原东北缘瑞利波相速度和方位各向异性

本研究收集了甘肃、青海、宁夏等118个宽频带数字地震台站的连续波形资料,利用噪声互相关,经过计算和筛选,在5~38 s范围内,共得到5773条瑞利波相速度频散曲线.然后采用1°×1°的网格划分,反演获得青藏高原东北缘相速度和方位各向异性分布.结果表明：短周期8~12 s内,鄂尔多斯从低速异常变为高速异常;该周期范围内各向异性结果与区域断裂走向有很好的一致性.18~25 s周期内,祁连地块、松潘-甘孜地块、羌塘地块低速异常范围逐渐变大,随周期增加地壳低速异常与人工探测结果相符;鄂尔多斯表现为速度随周期增加逐渐变大,说明其中下地壳速度相对偏高,不存在低速异常;该周期范围内的各向异性特征表现为,祁连地块和松潘甘孜地块大致呈NW-SE方向,而青藏高原内部快波方向显示了顺时针旋转的形态.在30~35 s范围内面波速度主要受莫霍面深度和莫霍面附近介质速度的影响,与地壳厚度分布有非常好的吻合.综合不同方法获得的各向异性研究结果,支持印度-欧亚板块的碰撞使青藏高原东北缘地壳发生缩短和逐渐隆升的观点,认为整个岩石圈的垂直缩短变形是青藏高原东北缘的主要形成机制.

     

青藏高原东南缘地质构造基本形态与地震各向异性基本特征

青藏高原东南缘受印度板块NE向推挤和高原物质SE向挤出及四川盆地、华南块体阻挡的共同作用,成为高原物质SE向逃逸的关键通道。本文综述了青藏高原东南缘由不同震相和不同方法得到的不同深度的地震各向异性结果,结合区域内断裂分布、地表运动、构造应力以及深部结构等方面,全面分析了青藏高原东南缘上地壳至中下地壳及上地幔的介质各向异性与变形耦合特征。青藏高原东南缘壳幔地震各向异性的差异反映了区域内复杂的深部构造和壳幔变形。由于青藏高原形成机制、壳幔耦合状态和软弱层分布形态等科学问题尚处于学术探讨之中,有效结合不同数据和综合多种方法,有益于获得更加准确、精细的地壳—上地幔地震各向异性图像,对深部物质运动与动力模式进行更有效的约束。     

滇西地区地壳上地幔电性结构与地壳构造活动的关系

本文根据滇西地区18个大地电磁测深点资料的数据处理和分析结果,对测区深部导电率在纵、横向上的变化特征进行了研究。结果表明:滇西地区深部电性为多层结构,大致可分四至五个电性结构层;深部电性结构横向变化大,明显受区域构造控制;该区上部地壳内普遍存在低阻层;上地幔高导层明显存在两个隆起区,一个以剑川—鹤庆为中心呈北北西向展布的隆起区,另一个以腾冲—潞西为轴呈南北向展布的隆起区。 本文还讨论了地壳上地幔电性结构与大地构造的关系,滇西北裂陷区盆地的形成,以及该区地震活动与深部构造的关系     

天山构造带地震各向异性及动力学机制研究进展

天山构造带及邻区的深部动力学机制是地球动力学研究的热点,而地震各向异性是区域构造深部动力学机制的一个重要性质。研究表明,天山构造带上地壳各向异性结果呈现区域性分区,受到构造带与断裂走向和区域应力影响;上地幔各向异性的结果认为快波偏振方向和构造带走向基本平行,但在伊塞克湖附近、塔里木盆地和准噶尔盆地挤压区域各向异性快波方向变化复杂,垂直方向上的变化可能由区域性双层各向异性引起,但局部复杂性原因有待进一步探讨。诸多研究支持天山构造带的地壳与上地幔垂直连贯变形机制。此外,地幔柱、软流圈变形、小尺度地幔对流等概念均被用来解释天山构造带的动力学背景,表明该地区的深部动力学机制非常复杂,需要更深入的探讨。