High-resolution Rayleigh wave phase velocity maps from ambient noise tomography in North China

We presented high-resolution Rayleigh wave phase velocity maps at periods ranging from 5 s to 30 s in the northeast part of the North China Craton (NNCC). Continuous time-series of vertical component between October 2006 and December 2008, recorded by 187 broadband stations temporarily deployed in the NNCC region, have been cross-correlated to obtain estimated fundamental mode Rayleigh wave Green’s functions. Using the frequency and time analysis technique based on continuous wavelet transformation, we measured 3 667 Rayleigh wave phase velocity dispersion curves. High-resolution phase velocity maps at periods of 5, 10, 20 and 30 s were reconstructed with grid size 0.25°× 0.25°, which reveal lateral heterogeneity of shear wave structure in the crust and upper mantle of NNCC. For periods shorter than 10 s, the phase velocity variations are well correlated with the principal geological units in the NNCC, with low-speed anomalies corresponding to the major sedimentary basins and high-speed anomalies coinciding with the main mountain ranges. Within the period range from 20 s to 30 s, high phase velocity observed in eastern NCC is coincident with the thin crust, whereas low phase velocities imaged in central NCC is correlated to the thick crust. However, the low-velocity anomaly in the Beijing-Tianjin-Tangshan region displayed in the 20 s and 30 s phase maps may be associated with fluids.

Shear velocity structure of crust and uppermost mantle in China from surface wave tomography using ambient noise and earthquake data

We present a 3D model of shear velocity of crust and upper mantle in China and surrounding regions from surface wave tomography. We combine dispersion measurements from ambient noise correlation and traditional earthquake data. The stations include the China National Seismic Network, global networks, and all the available PASSCAL stations in the region over the years. The combined data sets provide excellent data coverage of the region for surface wave measurements from 8 to 120 s, which are used to invert for 3D shear wave velocity structure of the crust and upper mantle down to about 150 kin. We also derive new models of the study region for crustal thickness and averaged S velocities for upper, mid, and lower crust and the uppermost mantle. The models provide a fundamental data set for understanding continental dynamics and evolution. The tomography results reveal significant features of crust and upper mantle structure, including major basins, Moho depth variation, mantle velocity contrast between eastern and western North China Craton, widespread low-velocity zone in mid- crust in much of the Tibetan Plateau, and clear velocity contrasts of the mantle lithosphere between north and southern Tibet with significant E-W variations. The low velocity structure in the upper mantle under north and eastern TP correlates with surface geological boundaries. A patch of high velocity anomaly is found under the eastern part of the TP, which may indicate intact mantle lithosphere. Mantle lithosphere change from the western to The Tanlu Fault appears boundary. shows striking systematic eastern North China Craton. to be a major lithosphere

Surface wave tomography on a large-scale seismic array combining ambient noise and teleseismic earthquake data

We discuss two array-based tomography methods, ambient noise tomography （ANT） and two-plane- wave earthquake tomography （TPWT）, which are capable of taking advantage of emerging large-scale broadband seismic arrays to generate high resolution phase velocity maps, but in complementary period band： ANT at 8-40 s and TPWT at 25-100 s period. Combining these two methods generates surface wave dispersion maps from 8 to 100 s periods, which can be used to construct a 3D vs model from the surface to -200 km depth. As an illustration, we apply the two methods to the USArray/Transportable Array. We process seismic noise data from over 1 500 stations obtained from 2005 through 2009 to produce Rayleigh wave phase velocity maps from 8 to 40 s period, and also perform TPWT using -450 teleseismic earthquakes to obtain phase velocity maps between 25 and 100 s period. Combining dispersion maps from ANT and TPWT, we construct a 3D vs model from the surface to a depth of 160 km in the western and central USA. These surface wave tomography methods can also be applied to other rapidly growing seismic networks such as those in China.

Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography

We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green＇s functions obtained from seismic ambient noise cross-correlation. The data we use are from the China National Seismic Network, global and regional networks and PASSCAL stations in the region. We first acquire cross-correlation seismograms between all possible station pairs. We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s. After that, Rayleigh wave group and phase velocity dispersion maps on 1°by 1°spatial grids are obtained at different periods. Finally, we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node. The inversion results show large-scale structures that correlate well with surface geology. Near the surface, velocities in major basins are anomalously slow, consistent with the thick sediments. East-west contrasts are striking in Moho depth. There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau （TP） and Tianshan （Junggar, Tarim, Ordos, and Sichuan）. These strong blocks, therefore, appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape. In northwest TP in Qiangtang, slow anomalies extend from the crust to the mantle lithosphere. Meanwhile, widespread, a prominent low-velocity zone is observed in the middle crust beneath most of the central, eastern and southeastern Tibetan plateau, consistent with a weak （and perhaps mobile） middle crust.

Love wave tomography in Italy from seismic ambient noise

We estimate Love wave empirical Green＇s functions from cross-correlations of ambient seismic noise to study the crust and uppermost mantle structure in Italy. Transverse-component ambient noise data from October 2005 through March 2007 recorded at 114 seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia （INGV） national broadband network, the Mediterranean Very Broadband Seismographic Network （MedNet） and the Austrian Central Institute for Meteorology and Geodynamics （ZAMG） yield more than 2 000 Love wave group velocity measurements using the multiple-filter analysis technique. In the short period band （5-20 s）, the cross-correlations show clearly one-sided asymmetric feature due to non-tmiform noise distribution and high local activities, and in the long period band （〉20 s） this feature becomes weak owing to more diffusive noise distribution. Based on these measurements, Love wave group velocity dispersion maps in the 8-34 s period band are constructed, then the SH wave velocity structures from the Love wave dispersions are inverted. The final results obtained from Love wave data are overall in good agreement with those from Rayleigh waves. Both Love and Rayleigh wave inversions all reveal that the Po plain basin is resolved with low velocity at shallow depth, and the Tyrrhenian sea is characterized with higher velocity below 8 km due to its thin oceanic crust.

Surface Wave Group Velocity Tomography Imaging from Ambient Noise for Fujian Province and Its Adjacent Areas

Two-month continuous waveforms of 108 broadband seismic stations in Fujian Province and its adjacent areas are used to compute noise cross-correlation function(NCF). The signal quality of NCF is improved via the application of time-frequency phase weighted stacking. The Rayleigh and Love waves group velocities between 1 s-20 s are measured on the symmetrical component of the NCF with the multiple filter method. More than 5,000 Rayleigh wave dispersion curves and about 4,000 Love wave dispersion curves are obtained and used to invert for group velocity maps. This data set provides about 50 km resolution that is demonstrated with checkerboard tests. Considering the off great circle effect in inhomogeneous medium, the ray path is traced based on the travel time field computed with a finite difference method. The inverted group velocity maps show good correlation with the geological features in the upper and middle crust. The Fuzhou basin and Zhangzhou basin showed low velocity on the short period group velocity maps. On the long period group velocity maps, the low velocity anomaly in the high heat flow region near Zhangzhou and clear velocity contrast across the Zhenghe-Dapu faults, which suggests that the Zhenghe-Dapu fault might be a deep fault.

Ambient noise Love wave tomography of China

We first report on the Love wave tomography of China based on ambient noise cross-correlations. We used 3 years of continuous waveform data recorded by 206 broadband seismic stations on the Chinese Mainland and 36 neighboring global stations and obtained Love wave empirical Green's functions from cross-correlations of the horizontal components. The Love wave group velocity dispersion measurements were used to construct dispersion maps of 8-to 40-s periods, which were then inverted to obtain a threedimensional horizontally polarized S-wave(SH) velocity structure. The resolution was approximately 4°× 4° and 8°× 8° for eastern and western China, respectively, and extended to a depth of approximately 50 km. The SH model was generally consistent with a previously published vertically polarized S-wave(SV) model and showed large-scale features that were consistent with geological units, such as the major basins and changes in the crustal thickness across the north-south gravity lineament. The SH and SV models also showed substantial differences, which were used to examine the subsurface radial anisotropy. We define the radial anisotropy parameter asψ = 2(V_(SH)-V_(SV))/(V_(SH)+ V_(SV)). At a shallow depth, we observed significant radial anisotropy under major basins, which may be related to thick sedimentary layers. At the mid to lower crust, most of the Chinese continent showed strong positive radial anisotropy(SH > SV).Central and southern Tibet showed strong positive anisotropy, whereas the radial anisotropy was relatively weak at the northern and eastern margins, which suggests a change in deformation style from the plateau interior to its margins. The North China craton showed prominent positive radial anisotropy, which may be related to decratonization and strong extension since the Mesozoic Era. Love waves are less well retrieved than Rayleigh waves from ambient noise cross-correlations. Increasing the duration of the cross-correlation data beyond 4 to 8 years may not aid in retrieving Love waves of longer periods, for which improved methods need to be explored.

Ambient Noise Tomography of Jinan:The Migration of Groundwater and the Formation of Geothermal Water

Jinan is an important city in eastern China,with rich groundwater in the region.There are four famous springs in the urban area and an abundance of geothermal water in the northern part,which makes the migration of groundwater in this area a very important issue.To study the shallow shear wave velocity structure and groundwater migration in Jinan,we utilized almost a month of continuous waveform data from 175 short period seismometers deployed by the Chinese Academy of Geological Sciences,in order to calculate the cross-correlation function.We picked 7749 group dispersion curves and 6117 phase dispersion curves with a period range of 0.2–2 s.Through inversion,we obtained the fine threedimensional shear wave velocity and azimuthal anisotropy structure(0–2.4 km).Combining the results with local geological and hydrological data,the following conclusions were reached.(1)There are widespread high velocity anomalies in the region between the Qianfoshan and Wenhuaqiao faults,as well as to the east of the Wenhuaqiao Fault,which may be related to the intrusive gabbro known as the Jinan Intrusive Rock.(2)The two distinct high velocity anomalies in our model(referred to as west and east Jinan Intrusive Rock in this paper)may indicate that the Jinan Intrusive Rock was broken through crustal movement.(3)There is an obvious low velocity layer under the intrusive rock,which could be the channel of groundwater migration.The precipitation in the southern mountain region seeps down into the ground,then is blocked by the Jinan Intrusive Rock and can only progress downwards to a deeper part,where the groundwater is heated by the geothermal gradient.The heated water finally arrives at the northern part and forms geothermal water.(4)The depth of the low velocity layer beneath the Jinan Intrusive Rock varies laterally,which may indicate that the depth of the groundwater migration is different beneath the west and east Jinan Intrusive Rock.(5)There is strong azimuthal anisotropy in southern Jinan,with nearly E-W fast orientation,which may be related to the tilt limestone layering structure.

The influences of large earthquake signals on the recovery of surface waves from ambient noise cross-correlation functions

Ambient noise tomography(ANT)has been widely used to image crust and upmost mantle structures.ANT assumes that sources of ambient noise are diffuse and evenly distributed in space and the energy of different modes is equipartitioned.At present,the sources of the primary and the secondary microseisms are well studied,but there are only a few on the studies of long-period ambient noise sources.In this study,we study the effects of large earthquake signals on the recovery of surface waves from seismic ambient noise data recorded by seismic stations from the US permanent networks and Global Seismographic Network(GSN).Our results show that large earthquake signals play an important role on the recovery of long-period surface waves from ambient noise cross-correlation functions.Our results are consistent with previous studies that suggest the contribution of earthquake signals to the recovery of surface waves from cross-correlations of ambient noise is dominant at periods larger than 20–40 s.

Ambient Noise Tomography, Green’s Function and Earthquakes

Green’s function is well-known, among others, in the application of ambient noise tomography methodologies that may demonstrate the potential of hydrocarbon entrapment in the study area. Here it is also shown to be of key importance in identifying the fractal dimension in the unified scaling law for earthquakes as well as in studying an explicit relationship of a future strong earthquake epicenter to the average earthquake potential score. Such studies are now in progress.

Processing Seismic Ambient Noise Data to Obtain Reliable Surface Wave Dispersion Measurements

Ambient noise tomography is a rapidly emerging field of seismological research. This paper presents the current status of ambient noise data processing and its development history over the past several years, with the intention to explain and justify this development through salient examples. The ambient noise data processing procedure can be divided into four principal phases:1 single station data preparation; 2 crosscorrelation and temporal stacking; 3 measurements of dispersion curves( p erformed with frequency-time analysis for both group and phase speeds); 4 quality control,i ncluding SNR analysis and selection of the acceptable measurements. In addition,we provide a specific solution for a better use of the seismic station data to ambient noise study.

Phase velocity maps of Rayleigh waves in the Ordos block and adjacent regions

We analyze continuous waveform data from 257 broadband stations of the portable seismic array deployed under the "China Seismic Array-northern part of NS seismic belt" project as well as data from a permanent seismic network from January 2014 to December 2015. The phase velocity dispersion curve of 7,185 Rayleigh waves is obtained with a method based on the image analysis of phase velocity extraction, and the inversion is obtained. The period of Rayleigh wave phase velocity distribution has a range of 5–40 s, and minimum resolution close to 20 km. The results show that the phase velocity structure image well reflects the geological structural characteristics of the crust and uppermost mantle, and that the phase velocity distribution has obvious lateral heterogeneity. The phase velocity of the 5–15 s period is closely linked to the surface layer and sedimentary layer, the low-velocity anomalies correspond to loose sedimentary cover, and the high-velocity anomalies correspond to orogenic belts and uplifts and the boundary between high and low velocity anomalies is consistent with the block boundary. The phase velocity of the 5–15 s period is strongly affected by the crust layer thickness, the northeastern Tibetan plateau has low-velocity anomalies in the middle to lower crust, the west side of the Ordos block is consistent with the northeastern Tibetan plateau, which may imply the material exchange and fusion in this area. The velocity variation is inversely related to the Moho depth in the 40 s period of Swave, and the lateral velocity heterogeneity represents the lateral variation of the Moho depth. The Ordos block and the northern margin of Sichuan basin are located in the uppermost mantle at this depth, and the depth in the transition zone is still located in the lower crust.

Rayleigh waves from correlation of seismic noise in Great Island of Tierra del Fuego,Argentina:Constraints on upper crustal structure

In this study, the ambient seismic noise cross-correlation technique is applied to estimate the upper structure of the crust beneath Great Island of Tierra del Fuego（TdF）, Argentina, by the analysis of shortperiod Rayleigh wave group velocities. The island, situated in the southernmost South America, is a key area of investigation among the interaction between the South American and Scotia plates and is considered as a very seismically active one. Through cross-correlating the vertical components of ambient seismic noise registered at four broadband stations in TdF, we were able to extract Rayleigh waves which were used to estimate group velocities in the period band of 2.5-16 s using a timefrequency analysis. Although ambient noise sources are distributed in homogeneously, robust empirical Green＇s functions could be recovered from the cross-correlation of 12 months of ambient noise, The observed group velocities were inverted considering a non-linear iterative damped least-squares inversion procedure and several 1-D shear wave velocity models of the upper crust were obtained.According to the inversion results, the S-wave velocity ranges between 1.75 and 3,7 km/s in the first10 km of crust, depending on the pair of stations considered. These results are in agreement to the major known surface and sub-surface geological and tectonic features known in the area. This study represents the first ambient seismic noise analysis in TdF in order to constraint the upper crust beneath this region.It can also be considered as a successful feasibility study for future analyses with a denser station deployment for a more detailed imaging of structure.

Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Plateau

To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods(5–12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16–26 s, that inverts to high-velocity anomalies at periods of 30–34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15–30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone.

Comparison of ground truth location of earthquake from InSAR and from ambient seismic noise: A case study of the 1998 Zhangbei earthquake

Because ambient seismic noise provides estimated Green’s function （EGF） between two sites with high accuracy, Rayleigh wave propagation along the path connecting the two sites is well resolved. Therefore, earthquakes which are close to one seismic station can be well located with calibration extracting from EGF. We test two algorithms in locating the 1998 Zhangbei earthquake, one algorithm is waveform-based, and the other is traveltime-based. We first compute EGF between station ZHB （a station about 40 km away from the epicenter） and five IC/IRIS stations. With the waveform-based approach, we calculate 1D synthetic single-force Green’s functions between ZHB and other four stations, and obtain traveltime corrections by correlating synthetic Green’s functions with EGFs in period band of 10–30 s. Then we locate the earthquake by minimizing the differential travel times between observed earthquake waveform and the 1D synthetic earthquake waveforms computed with focal mechanism provided by Global CMT after traveltime correction from EGFs. This waveform-based approach yields a location which error is about 13 km away from the location observed with InSAR. With the traveltime-based approach, we begin with measuring group velocity from EGFs as well as group arrival time on observed earthquake waveforms, and then locate the earthquake by minimizing the difference between observed group arrival time and arrival time measured on EGFs. This traveltime-based approach yields accuracy of 3 km, Therefore it is feasible to achieve GT5 （ground truth location with accuracy 5 km） with ambient seismic noises. The less accuracy of the waveform-based approach was mainly caused by uncertainty of focal mechanism.

利用背景噪声资料研究海原断裂带及邻区Rayleigh波相速度和方位各向异性

海原断裂带是位于青藏高原东北缘的一条NWW走向大型边界断裂带,为了研究该区域地壳速度结构与变形特征,使用中国地震局地震预测研究所布设的跨海原断裂带流动地震台阵(SACHY-Array)和研究区域内固定地震台站共61个台站(40个流动台站和21个固定台站)的垂向连续波形数据.采用背景噪声互相关方法,提取了面波相速度频散曲线,反演得到周期范围为5~30 s、分辨率1°×1°的Rayleigh波相速度和方位各向异性图像.结果表明,短周期5~10 s内,河西走廊过渡带东部、鄂尔多斯地块西北部以及银川地堑南部均呈现低速异常体,祁连造山褶皱带东段表现为相对高速体;海原断裂带西南侧快波偏振方向为NWW、NW向,鄂尔多斯西缘及邻区快波偏振方向主要为近NS向,各向异性方向与区域断裂走向基本一致.15~30 s周期内,河西走廊过渡带东部及银川地堑南部的低速异常逐渐减弱且范围不断减小,15 s周期左右,河西走廊过渡带东部的低速体在烟筒山断裂带下方有错断的趋向,陇中盆地中央、鄂尔多斯西南缘均存在高速异常体;方位各向异性方向与短周期基本一致,不过各向异性强度较弱.本文认为,海原断裂带是高低速过渡带,位于断裂带北侧的河西走廊过渡带表现为低速异常,而位于南侧的祁连褶皱造山带表现为高速异常,海原断裂带及邻区具有复杂的地壳结构,并可能存在局部的地壳破坏变形;地壳方位各向异性作为地壳变形的重要约束指标,说明了海原断裂带及邻区各向异性可能主要由青藏高原东北缘地壳的剪切变形所产生,青藏高原的NE向推挤作用是这一地区的主要动力来源.

基于短周期密集台阵的广州—佛山地区地壳浅部三维S波速度结构研究

广州—佛山地区位于珠三角经济区的核心地带,三条区域性断裂从中交错穿过.广州—佛山地区精细的地壳浅部三维S波速度结构,对于研究区内的城市规划建设、地震灾害评估、强地面运动模拟等具有重要意义.本文利用布设于广州—佛山地区短周期密集地震台阵中的1104个台站记录到的连续波形数据,采用背景噪声互相关方法计算了台站对间的互相关函数并进一步提取到周期范围在0.1~5 s内的39488条高质量面波群速度频散曲线,然后通过基于射线追踪的面波直接反演方法计算得到了研究区内0.3~3 km深度的高分辨率三维S波速度结构.结果显示:研究区内地壳浅部S波速度横向差异显著,其中南部的珠江三角洲断陷盆地S波速度结构相对较为复杂,而北部的广花盆地内部S波速度结构则表现相对简单.研究区内的低速异常主要位于断裂附近,且大都位于断裂的下降盘.珠江三角洲内三水盆地东缘表现为马鞍状低速异常,向下最大延伸至2 km,低速体南北两侧边界与控盆断裂位置基本一致.广花盆地内部存在两个平行排列的高速异常,中间夹持一平行构造走向的低速异常,总体速度结构特征符合广花盆地为一复式向斜的地质构造背景.低速异常向下延伸至1.4 km深度处,可能指示了卷入向斜地层的最大埋深.沿广州—从化断裂发育条带状低速异常,与沿断裂形成的沉积地层有关.条带状异常在浅部分为两段,表明广州—从化断裂在历史上可能具有分段活动性.玉带公园周围被高速异常环绕,异常向上延伸至距地表约1.3 km处,结合研究区已有资料推测该高速异常可能为早期岩浆侵入在地壳浅部的反映.

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.

东北地区背景噪声的Rayleigh和Love波相速度层析成像

本文利用中国数字地震台网位于东北地区的122个宽频地震台站的18个月记录的三分量连续地震噪声数据,采用互相关方法提取了Rayleigh和Love波经验格林函数,并利用时频自动分析技术获取了相应的相速度频散曲线.通过反演频散曲线,获得了Rayleigh和Love波周期为8-35s的二维相速度分布.结果表明,东北地区相速度的分布存在横向和垂向的不均匀性.短周期的相速度分布同地表地质构造密切相关,松辽盆地及山间沉积盆地呈现低速异常,而大兴安岭、小兴安岭及东部的一些山岭显示高速异常.随着周期的增加,位于中间的松辽盆地变为高低速相间,两侧的造山带呈现低速异常.这种异常的转变,可能是受构造活动或者莫霍面深度的影响.另外,在周期为20-35s频段内,Rayleigh和Love波同一周期的相速度在松辽盆地和位于吉林地区的郯庐断裂带表现不一致,表明可能存在径向各向异性.

郯庐断裂带中南段及邻区Rayleigh波相速度与方位各向异性

与方位各向异性郯庐断裂带中南段及邻区处于华北克拉通和扬子克拉通的结合带,是研究陆内碰撞、大陆聚合及生长、地球动力学和岩石圈形变等问题的天然实验场。为获取研究区内更为精细的壳幔S波速度和方位各向异性的分布,文中利用郯庐断裂带中南段及邻区范围内共261个固定宽频带地震台站2a记录的连续波形数据,利用时频分析法提取了5~50s周期的Rayleigh面波相速度频散曲线。之后,将研究区域划分为0.25°×0.25°等间隔的网格点,并采用Tarantola方法反演获得了研究目标区内的二维Rayleigh波相速度与方位各向异性的分布图像。此外,还分析研究了6个具有代表性周期的相速度及方位各向异性分布图像,这些图像揭示出郯庐断裂带中南段及邻区壳幔速度结构的横向非均匀性及方位各向异性的空间差异特征。研究结果表明,相速度的分布特征与地质构造单元有着较好的对应关系。在地壳浅部(6~10s),拥有较厚松散沉积层覆盖的盆地和基岩出露的造山带分别呈现为低速异常和高速异常。随着周期递增(15~20s),地壳介质受浅部沉积层的影响有所减弱,河淮盆地、苏北盆地等平原地区局部出现了高速异常。下地壳至上地幔(25~30s)的相速度分布受地壳厚度的影响,总体上与莫霍面埋深呈负相关,如莫霍面厚40km的大别造山带由短周期高速转为低速分布。由于研究区内各个地质构造单元所处的构造环境存在差异,Rayleigh波相速度的方位各向异性也存在较为明显的空间差异性。总体上,各向异性的强度随着周期(深度)的递增而有所增强,快波方向更具规律性。综合苏北盆地自地壳浅部至岩石圈地幔的连贯低速分布及方位各向异性的一致性,我们认为其下方可能存在着壳-幔强耦合的现象。