A New Passive Tomography of the Aigion Area (Gulf of Corinth, Greece) from the 2002 Data Set

We present the results of a tomographic study performed in the framework of the 3F-Corinth project. The aim of this work is to better understand the rifting process by imaging the crustal structure of the western Gulf of Corinth. Forty-nine stations were deployed for a period of six months, allowing us to monitor the microseismicity. Delayed P and S first-arrival times have been simultaneously inverted for both hypocenter locations and 3-D velocity distributions. We use an improved linearized tomography method based on an accurate finite-difference travel-time computation to invert the data set. The obtained Vp and Vs models confirm the presence of a two-layer vertical structure characterized by a sharp velocity gradient lying at 5–7 km depth, which may be interpreted as a lithological contrast. The shallower part of the crust (down to 5 km depth) is controlled by the N-S extension and lacks seismicity. The deeper part (7–13 km depth) matches the seismogenic zone and is characterized by faster and more heterogeneous anomalies. In this zone, the background seismicity reveals a low-angle active surface dipping about 20° toward the north and striking WNW-ESE. The position of this active structure is consistent with both high Vp/Vs and low Vp.Vs anomalies identified at 8–12 km depth and suggesting a highly fracturated and fluid-saturated zone. Both the geometry of the active structure beneath the gulf and the presence of fluids at 8–12 km depth are in accordance with a low-angle detachment model for the western part of the Gulf of Corinth. S. Gautier and D. Latorre formerly at Géosciences Azur     

多震相菲涅耳体射线走时同时反演成像

高频假设下的地震射线理论以及相应的地震成像理论表明,在射线稀疏条件下,不可能得到较高分辨率的构造成像;而有限频射线理论更符合实际地震的传播规律,即地震波的走时不仅与中心射线(传统的几何射线)上的速度分布有关,而且与中心射线附近一定范围(称其为第一菲涅耳体)内的速度异常分布有关.鉴于此,本文提出了计算多震相地震波菲涅耳体有限频射线的方法,并定义了走时敏感核函数,同时给出了利用多震相菲涅耳体有限频射线进行速度模型和反射界面同时反演成像的公式.利用多震相走时资料,使用传统射线层析成像方法与有限频射线层析成像方法进行了速度和界面的同时反演成像.结果表明,当射线密度较小时,无论是对速度模型的重建还是对反射界面几何形状的更新,有限频射线层析成像方法均优于传统射线层析成像方法, 而变频有限频射线层析成像则是实际地震层析成像的首选反演算法.      

Simultaneous Inversion for Velocity and Reflector Geometry Using Multi-phase Fresnel Volume Rays

Traditional ray tomography methods based on the high frequency assumption are sometimes unable to obtain a high resolution tomographic picture due to a deficient coverage of ray paths in real applications, especially for low velocity anomalous regions. In contrast, finite-frequency ray theory is more suitable for handling real seismic propagation problems because the travel time depends not only on the velocity distribution along a central ray (or traditional geometric ray), but also on the velocity values within a region (referred to as the first Fresnel Volume) which incorporates the central ray. In this study, we develop an algorithm to calculate multi-phase Fresnel Volume finite-frequency rays, and then present an inversion method to simultaneous invert for both velocity and reflector geometry by using these multi-phase Fresnel Volume finite-frequency rays. Using synthetic data examples, we compare the reconstructions of the velocity field and the reflector orientation using the Fresnel Volume ray tomographic methods and the traditional ray tomography approach. Results show that the former is advantageous over the latter, especially when the ray density is relatively low. An additional benefit of the Fresnel Volume finite-frequency ray tomographic method is that it can start with a low frequency to capture the coarse velocity structure, thereby mitigating the local minimum trapping problem, and then be tuned to a high frequency for delineating the fine velocity structure.     

Three-dimensional velocity model of the crust of the Bohemian Massif and its effects on seismic tomography of the upper mantle

We have compiled a representative three-dimensional P-velocity model of the crust of the Bohemian Massif (BM) to provide a basis for removing effects of the crustal structure in teleseismic tomography of the upper mantle. The model is primarily based on recently published 2D velocity models from findings of wide-angle refraction and near-vertical reflection seismic profiles of CELEBRATION 2000, ALP 2002, and SUDETES 2003 experiments. The best fitting 3D model of the BM crust (NearNeighbour model) is complemented by velocities according to the reference Earth model at sites where data are sparse, which precludes creating artificial heterogeneities that are products of interpolation method. To test the model, we have performed tomographic inversions of the P-wave travel times measured during the BOHEMA II experiment and compared the results obtained with and without crustal corrections. The tests showed that the presented crustal model decreases magnitudes of velocity perturbations leaking from the crust to the mantle in the western part of the BM. The tomographic images also indicated a highvelocity anomaly in the lower crust or just beneath the crust in the Brunovistullian unit. Such anomaly is not described by our model of the crust since no seismic profile intersects this part of the unit. The tests also indicated that crustal corrections are of the great importance especially for interpretations of the uppermost mantle down to depths of about 100 km.     

初至波菲涅尔体地震层析成像

根据地震波传播的有限频理论,对于某个特定震相的观测信息,不仅射线路径上的点对该信息具有影响,射线领域上的其他点对接收信息也具有影响,这种影响可以用核函数来表达.本文基于波动方程的Born近似与Rytov近似,给出了非均匀介质情况下初至地震波振幅与走时菲涅尔体层析成像单频、带限层析核函数的计算方法.通过对均匀介质情况下初至地震波菲涅尔体层析成像核函数解析表达式的理论模型实验与分析,给出了不同维度振幅、走时单频菲涅尔体的空间分布范围,进而给出了带限菲涅尔体边界的确定方法.将本文的走时菲涅尔体层析成像理论应用于表层速度结构反演中,理论模型试验与实际资料处理结果表明,初至波菲涅尔体地震层析成像方法比传统的初至波射线层析成像理论具有更高的反演精度.     

应用远震有限频率层析成像反演首都圈上地幔速度结构

本文选用首都圈数字地震台网2003年9月~2005年12月记录的300多个远震事件的波形资料,采用分频带多道互相关方法得到三个不同频段的P波相对走时数据共18499个,计算了每个频段的走时灵敏度核,应用有限频率层析成像反演得到首都圈地区的上地幔三维P波速度结构模型.利用检测板估计了反演结果的分辨率,并与射线层析成像方法的结果进行了比较,说明了反演结果的可靠性.研究结果表明,各构造单元具有明显不同的速度结构特征,其差异可到150 km深:燕山隆起区表现高速;太行山隆起区整体以低速为主并存在小范围高速块体;华北盆地、渤海湾下浅层上地幔中存在大范围的强低速异常,其顶面在50~70 km,可视为软流圈顶面的埋深,这一结果说明华北盆地、渤海湾下岩石圈明显减薄;张家口-蓬莱断裂带是上地幔浅部速度结构的变异带,也是岩石圈减薄的边界带,区内大部分强震都发生在该构造带上,由此看来该带上强震的发生不仅与地壳结构的不均匀性有关,还可能有较深的构造背景.     

Measurements of translation, rotation and strain: new approaches to seismic processing and inversion

We propose a novel approach to seismic tomography based on the joint processing of translation, strain and rotation measurements. Our concept is based on the apparent S and P velocities, defined as the ratios of displacement velocity and rotation amplitude, and displacement velocity and divergence amplitude, respectively. To assess the capability of these new observables to constrain various aspects of 3D Earth structure, we study their corresponding finite-frequency kernels, computed with a combination of spectral-element simulations and adjoint techniques. The principal conclusion is that both the apparent S and P velocities are generally sensitive only to small-scale near-receiver structure, irrespective of the type of seismic wave considered. It follows that knowledge of deeper Earth structure would not be required in tomographic inversions for local structure based on the new observables. In a synthetic finite-perturbation test, we confirm the ability of the apparent S and P velocities to directly detect both the location and the sign of shallow lateral velocity variations.     

Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia

We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data. We used 3D sensitivity kernels to combine the datasets, and mantle structure was parameterized with an irregular grid. In the best-sampled region our data resolve structure on scale lengths less than 150 km. The smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3D model. Our tomographic inversions reveal high-velocity roots beneath the Archean Ordos Plateau, the Sichuan Basin, and other continental blocks in SE Asia. Beneath the Himalayan Block we detect high seismic velocities, which we associate with subduction of Indian lithospheric mantle. This structure is visible above the 410 km discontinuity and may not connect to the remnant of the Neo-Tethys oceanic slab in the lower mantle. Our images suggest that only the southwestern part of the Tibetan plateau is underlain by Indian lithosphere and, thus, that the upper mantle beneath northeastern Tibet is primarily of Asian origin. Our imaging also reveals a large-scale high-velocity structure in the transition zone beneath the Yangtze Craton, which could have been produced in multiple subduction episodes. The low P-wave velocities beneath the Hainan Island are most prominent in the upper mantle and transition zone; they may represent counter flow from the surrounding subduction zones, and may not be unrelated to processes beneath eastern Tibet.     

Upper Crustal Velocity Structure along the Yangtze River from Ma'anshan to Anqing

We applied the 3D first arrival travel time tomography method to the Anhui active seismic source experiment data,and obtained the imaging of the upper crust velocity structure beneath the Yangtze River from Ma'anshan,Tongling to Anqing. Data fitting reveals the tomographic model fits the data with uncertainties, without overfitting, and with a minimum of complexity. The tomographic result shows an obvious heterogeneous upper crust which consists of a series of uplifts and depression basins. The velocity model and region imply that this region has experienced crustal uplift and extensional tectonism with concomitant magmatism since the Cenozoic.     

应用远震有限频率层析成像反演中国东北地区上地幔P波三维速度结构

选取黑龙江、吉林、辽宁、内蒙古区域地震台网,以及NECESSArray流动台阵记录的223个远震事件的波形资料,采用多道互相关方法得到了22569个P波相对走时数据,并计算了相应的走时灵敏度核,应用有限频率层析成像反演得到中国东北地区上地幔600 km以上的P波三维速度结构模型,利用检测板评估了反演结果的分辨率。结果表明,松辽盆地下方80～200 km的深度上呈主体的低速异常,与这一地区上地幔浅部的高地温值和低密度的特征相互对应,可能暗示了部分熔融的地幔。南北重力梯度带两侧的速度结构明显不同,这一差异可以延伸到200 km以下,表明在中国东北地区南北重力梯度带有可能是一条上地幔内部结构的变化带,或是深部结构的分界线。长白山火山区下呈大范围的低速异常,并可从上地幔浅部延伸到地幔转换带中,推测此低速异常可能反映了地幔转换带内上涌的热物质,上涌的原因则主要是受到太平洋板块俯冲运动的作用。     

二阶Born近似有限频率走时敏感核

有限频率层析成像考虑了非均匀介质中波的散射、衍射、波前愈合等物理性质,使得其对速度异常体的分辨能力远大于射线层析成像.推导和计算有限频率敏感核是进行有限频率层析成像的关键,当前推导有限频率敏感核多借助一阶Born近似,但这只适用于弱散射介质的情况.本文基于二阶Born近似并利用傅里叶变换推导了三维均匀介质情况下有限频率敏感核的解析表达式,并将其推广到非均匀介质中得到了三维非均匀介质中有限频率敏感核.研究表明:当介质中速度扰动小于2%时,基于二阶Born近似的有限频率敏感核与基于一阶Born近似的有限频率敏感核差别很小,可近似认为相同;当介质中速度扰动大于5%时,基于二阶Born近似的有限频率敏感核与基于一阶Born近似的有限频率敏感核有较大不同,表明此时已不能忽略二次散射.     

远震P波层析成像研究阿尔金断裂带东端及其邻区深部结构

本文利用"中国地震科学台阵探测——南北地震带北段"项目在内蒙古阿拉善西部及甘肃西北部地区布设的80个流动宽频带地震仪及16个固定台站,于2013年10月—2015年6月所记录的787个远震事件,采用波形相关方法拾取了共49052个高质量的P波走时残差数据,并利用Fast-Marching远震走时层析成像方法,反演获取了研究区下方的三维P波速度结构.结果显示:阿尔金断裂带东段、祁连山、北山地区下方地壳结构表现为低速异常特征,具有明显的造山带构造特征;阿拉善地体下方地壳结构表现为高速异常特征,为典型的大陆地壳结构特征;阿拉善地块沿着青藏高原北边界逆冲断裂(NBT)南向俯冲,其在祁连山造山带下与北向俯冲的柴达木岩石圈形成了面对面的碰撞接触关系;阿尔金断裂带的末端并没有北东向延伸到阿拉善块体,而是受到刚性的阿拉善岩石圈阻挡沿着其南缘断裂带继续向东发展.     

Wave equation tomographic velocity inversion method based on the Born/Rytov approximation

This paper discusses Born/Rytov approximation tomographic velocity inversion methods constrained by the Fresnel zone. Calculations of the sensitivity kernel function and traveltime residuals are critical in tomographic velocity inversion. Based on the Born/Rytov approximation of the frequency-domain wave equation, we derive the traveltime sensitivity kernels of the wave equation on the band-limited wave field and simultaneously obtain the traveltime residuals based on the Rytov approximation. In contrast to single-ray tomography, the modified velocity inversion method improves the inversion stability. Tests of the near-surface velocity model and field data prove that the proposed method has higher accuracy and Computational efficiency than ray theory tomography and full waveform inversion methods.     

基于邻域算法的瑞利面波垂直-水平振幅比及频散曲线联合反演及应用

瑞利面波垂直-水平振幅比(或ZH振幅比)是一个随频率变化的函数,对于台站下方浅层地壳结构非常敏感,且具有和频散资料不同的深度敏感核,是传统频散反演方法的一个很好的补充,从而可以将基阶瑞利面波的ZH振幅比和面波频散数据联合起来更好地反演获得观测台站下方的速度结构.本文提出了基于邻域算法的面波频散曲线与ZH振幅比联合反演方法,我们进行了基于理论模型的模拟测试,证明了联合反演是一种更为可靠的反演方法,且能更好地约束浅层地壳结构.相比于频散曲线单独反演,联合反演不仅可以精确反演获得地壳的Vs结构,对分层地壳的Vp/Vs也能很好地约束.然后我们将联合反演算法应用于实际测量数据,获得了中国西南昆明台(KMI)下方更为准确的地壳横波速度结构及Vp/Vs模型.     

利用背景噪声成像研究合肥-金华剖面地壳速度结构及径向各向异性的东西差异

背景噪声成像能够为地壳内部结构提供地震波速的观测证据,能够增进对地壳结构与物性的认识.本文利用背景噪声成像方法对合肥-金华流动地震台阵55个台站数据进行了分析反演,获得了华北克拉通与华南块体东部构造边界附近区域的地壳S波速度与径向各向异性结构.成像结果表明,以东经118°-118.5°附近为界,西北部表现为地壳低速异常和中下地壳正的径向各向异性,东南部表现为地壳高速异常,下部地壳存在负的径向各向异性.由此,本文推测地震波速结构差异反映了地壳内部物质与温度的差异,这种差异与两个地区自中生代晚期以来经历了截然不同的岩浆活动相关.     

Estimation of velocity in the uppermost crust in a part of the western Gulf of Corinth,Greece, from the inversion of <Emphasis Type="Italic">P</Emphasis> and <Emphasis Type="Italic">S</Emphasis> arrival times using the neighbourhood algorithm

We determine the velocities in an upper crustal model, composed of three homogeneous layers, for one subregion of the western part of the Gulf of Corinth, NE of the town of Aigion, Greece. We have used local events that occurred there in the year 2001 and were recorded by the Corinth Rift Laboratory Network. Weighted P and S arrival time residuals are minimized using the Neighbourhood Algorithm of Sambridge (1999), combined with the grid search for source locations. The resolution of the inversion is tested by delete-one jackknifing. The model obtained is compared with some other models derived or applied to the subregion. A fast velocity increase between depths of 5 and 7 km is confirmed as the major structural element.     

Tomographic Imaging of the Seismic Structure Beneath the East Anatolian Plateau, Eastern Turkey

The high level of seismic activity in eastern Turkey is thought to be mainly associated with the continuing collision of the Arabian and Eurasian tectonic plates. The determination of a detailed three-dimensional (3D) structure is crucial for a better understanding of this on-going collision or subduction process; therefore, a body wave tomographic inversion technique was performed on the region. The tomographic inversion used high quality arrival times from earthquakes occurring in the region from 1999 to 2001 recorded by a temporary 29 station broadband IRIS-PASSCAL array operated by research groups from the Universities of Bo?azi?i (Turkey) and Cornell (USA). The data was inverted and consisted of 3,114 P- and 2,298 S-wave arrival times from 252 local events with magnitudes (M _D) ranging from 2.5 to 4.8. The stability and resolution of the results were qualitatively assessed by two synthetic tests: a spike test and checkerboard resolution test and it was found that the models were well resolved for most parts of the imaged domain. The tomographic inversion results reveal significant lateral heterogeneities in the study area to a depth of ~20?km. The P- and S-wave velocity models are consistent with each other and provide evidence for marked heterogeneities in the upper crustal structure beneath eastern Turkey. One of the most important features in the acquired tomographic images is the high velocity anomalies, which are generally parallel to the main tectonic units in the region, existing at shallow depths. This may relate to the existence of ophiolitic units at shallow depths. The other feature is that low velocities are widely dispersed through the 3D structure beneath the region at deeper crustal depths. This feature can be an indicator of the mantle upwelling or support the hypothesis that the Anatolian Plateau is underlain by a partially molten uppermost mantle.     

Seismic images and rock properties of the very shallow structure of Campi Flegrei caldera (southern Italy)

In September 2001, an extensive active-seismic investigation (Serapis experiment) was carried out in the Gulfs of Naples and Pozzuoli, with the aim of investigating and reconstructing the shallow crustal structure of the Campi Flegrei caldera, and possibly identifying its feeding system at depth. The present study provides a joint analysis of the very shallow seismic reflection data and tomographic images based on the Serapis dataset. This is achieved by reflection seismic sections obtained by the 3D data gathering and through refined P-velocity images of the shallowest layer of Pozzuoli Gulf (z < 1,000 m). From the refined Vp model, the overall picture of the velocity distribution confirms the presence of a complex arc-shaped anomaly that borders the bay offshore. The deeper part of the anomaly (beneath 700 m, with Vp > 3,500 m/s) correlates with units made up of agglomerate tuffs and interbedded lava, which form the southern edge of the caldera, which was probably formed following the two large ignimbritic eruptions that marked the evolutionary history of the area under study. The upper part of the anomaly that tends to split into two parallel arcs is correlated with dikes, volcanic mounds and hydrothermal alteration zones noted in previous shallow reflection seismic analyses. The depth of the transition between the upper and lower parts of the anomaly is characterized by an abrupt Vp increase on the one-dimensional (1D) profiles extracted from the 3D tomographic model and by the presence of a strong reflector located at about 0.6/0.7 s Two Way Time (TWT) on Common Mid Point gathers. The move-out velocity analysis and stack of the P–P and P–S reflections at the layer bottom allowed to estimate relatively high Vp/Vs values (3.7 ± 0.9). This hypothesis has been tested by a theoretical rock physical modeling of the Vp/Vs ratio as a function of porosity suggesting that the shallow layer is likely formed by incoherent, water saturated, volcanic and marine sediments that filled Pozzuoli Bay during the post-caldera activity.     

有限频层析成像方法研究进展

本文详细介绍了地震学中最近发展起来的一种非常重要的层析成像方法-有限频层析成像方法,对有限频理论进行了重点阐述,并对有限频层析成像方法的有效性进行了探讨,接下来总结了有限频地震层析成像方法的研究进展,并对有限频层析成像方法在地震学中的应用进行了展望.由于在非均匀介质中考虑了地震波遇到速度异常时的散射现象和波前复原效应,有限频层析成像方法大大提高了对速度异常体的分辨能力.     

Seismic ray path variations in a 3D global velocity model

A three-dimensional (3D) ray tracing technique is used to investigate ray path variations of P, PcP, pP and PP phases in a global tomographic model with P wave velocity changing in three dimensions and with lateral depth variations of the Moho, 410 and 660 km discontinuities. The results show that ray paths in the 3D velocity model deviate considerably from those in the average 1D model. For a PcP wave in Western Pacific to East Asia where the high-velocity (1-2%) Pacific slab is subducting beneath the Eurasian continent, the ray path change amounts to 27 km. For a PcP ray in South Pacific where very slow (−2%) velocity anomalies (the Pacific superplume) exist in the whole mantle, the maximum ray path deviation amounts to 77 km. Ray paths of other phases (P, pP, PP) are also displaced by tens of kilometers. Changes in travel time are as large as 3.9 s. These results suggest that although the maximal velocity anomalies of the global tomographic model are only 1-2%, rays passing through regions with strong lateral heterogeneity (in velocity and/or discontinuity topography) can have significant deviations from those in a 1D model because rays have very long trajectories in the global case. If the blocks or grid nodes adopted for inversion are relatively large (3-5°) and only a low-resolution 3D model is estimated, 1D ray tracing may be feasible. But if fine blocks or grid nodes are used to determine a high-resolution model, 3D ray tracing becomes necessary and important for the global tomography.