Assessment of the stability of H/V spectral ratios from ambient noise and comparison with earthquake data in the Cologne area (Germany)

Situated in an active tectonic region the highly industrialised Cologne area (Germany) suffers from moderate-sized earthquakes. Our contribution to the mitigation of earthquake risk includes a microzonation study using ambient seismic noise and earthquake recordings from two field campaigns. An analysis of these data using the horizontal-to-vertical (H/V) as well as the classical spectral ratio (CSR) technique led to the following observations and conclusions: (1) The spatial variation in the thickness of the sedimentary cover is reliably retrieved using the fundamental resonance frequency estimated from the peak in the (H/V) ratio of ambient noise. (2) This fundamental resonance frequency is stable in time, but the amplitude of the peak (the amplification factor) is not. (3) The relative amplification variation of the H/V ratio in the area should therefore be checked systematically with repeated measurements before drawing conclusions about its significance. (4) The analysis of the H/V ratio of ambient noise provides the lower frequency bound, above which amplification may occur. (5) The shapes of the spectral ratios obtained by the different methods are generally in good agreement. However, the analysis of earthquake data shows that significant amplification of ground motion may also occur at frequencies higher than the fundamental one even when thick sediments are present.     

S-wave velocity images of the Dead Sea Basin provided by ambient seismic noise

Based on passive seismic interferometry applied to ambient seismic noise recordings between station pairs belonging to a small-scale array, we have obtained shear wave velocity images of the uppermost materials that make up the Dead Sea Basin. We extracted empirical Green’s functions from cross-correlations of long-term recordings of continuous data, and measured inter-station Rayleigh wave group velocities from the daily correlation functions for positive and negative correlation time lags in the 0.1–0.5 Hz bandwidth. A tomographic inversion of the travel times estimated for each frequency is performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved. Subsequently, the velocity-frequency curves are inverted to obtain S-wave velocity images of the study area as horizontal depth sections and longitude- and latitude-depth sections. The results, which are consistent with other previous ones, provide clear images of the local seismic velocity structure of the basin. Low shear velocities are dominant at shallow depths above 3.5 km, but even so a spit of land with a depth that does not exceed 4 km is identified as a salt diapir separating the low velocities associated with sedimentary infill on both sides of the Lisan Peninsula. The lack of low speeds at the sampling depth of 11.5 km implies that there are no sediments and therefore that the basement is near 10–11 km depth, but gradually decreasing from south to north. The results also highlight the bowl-shaped basin with poorly consolidated sedimentary materials accumulated in the central part of the basin. The structure of the western margin of the basin evidences a certain asymmetry both whether it is compared to the eastern margin and it is observed in north–south direction. Infill materials down to ∼8 km depth are observed in the hollow of the basin, unlike what happens in the north and south where they are spread beyond the western Dead Sea shore.     

Geological variation in S-wave velocity structures in Northern Taiwan and implications for seismic hazards based on ambient noise analysis

Ambient noise analysis in Northern Taiwan revealed obvious lateral variations related to major geological units. The empirical Green’s functions extracted from interstation ambient noise were regarded as Rayleigh waves, from which we analyzed the group velocities for period from 3 to 6 s. According to geological features, we divided Northern Taiwan into seven subregions, for which regionalized group velocities were derived by using the pure-path method. On average, the group velocities in mountain areas were higher than those in the plain areas. We subsequently inverted the S-wave velocity structure for each subregion down to 6 km in depth. Following the analysis, we proposed the first models of geology-dependent shallow S-wave structures in Northern Taiwan. Overall, the velocity increased substantially from west to east; specifically, the mountain areas, composed of metamorphic rocks, exhibited higher velocities than did the coastal plain and basin, which consist of soft sediment. At a shallow depth, the Western Coastal Plain, Taipei Basin, and Ilan Plain displayed a larger velocity gradient than did other regions. At the top 3 km of the model, the average velocity gradient was 0.39 km/s per km for the Western Coastal Plain and 0.15 km/s per km for the Central Range. These S-wave velocity models with large velocity gradients caused the seismic waves to become trapped easily in strata and, thus, the ground motion was amplified. The regionalized S-wave velocity models derived from ambient noises can provide useful information regarding seismic wave propagation and for assessing seismic hazards in Northern Taiwan.     

Shear-wave velocity model of the Chukuo fault zone,Southwest Taiwan,from cross correlation of seismic ambient noise

The Chia-Nan (Chiayi-Tainan) area is in the southwestern Taiwan, and is located at the active deformation front of the collision of the Eurasian continental plate and the Philippine Sea plate, which causes complex folds as well as thrust fault systems in the area. The Chukuo fault zone is a boundary between the Western Foothill and the Western Coastal Plain in the Chia-Nan area. The nature of the crustal structure beneath the fault zone, especially the eastern part of the fault zone with mountain topography, has not been well known in detailed due to lack of drilling data as well as its limitation in using other geophysical methods, such as active source survey. In this study, we deployed an array with 11 broadband seismic stations to monitor the seismicity of the Chukuo fault zone. The array has recorded more than 1000 microearthquakes around this area. It provides an opportunity to use P- and S-wave travel time data to investigate the both the crustal P- and S-velocity in the fault zone, however due to the nature of the earthquake distribution, the ray density is relatively low at depth between 0 and 7 km. In addition, the uncertainty of S-wave reading for small earthquake also a limit in building precise S-velocity profile, Thus, we take the advantages of using cross-correlation of seismic ambient noise to investigate crustal S-velocity profile in the Chukuo fault area, especially in the mountain area where crustal faulting is a dominated phenomenon. The results indicate that S-wave velocity in the uppermost crust in the Chukuo fault zone is shown to be slower than previous studies. A low velocity layer exists at depth between 1 and 2 km in the east of the Chukuo Fault. The low S-velocity is related to a highly fractured upper crust due to intensive deformation caused by the orogenic process.     

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120 s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15 km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1 km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2 km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150 km) beneath the Taiwan Strait is about 0.1–0.3 km/s lower than the AK135 model. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves.     

On the amplitudes of correlations and the inference of attenuations,specific intensities and site factors from ambient noise

While techniques for retrieval of seismic velocities from wavelet arrival times in ambient noise correlations are now well established, interpretation of wavelet amplitudes remains unsatisfactory. It is clear that such amplitudes contain information on seismic attenuation, but they are also affected by ambient noise intensity, site amplification, and any nonlinear preprocessing that may have been applied to the noise signals. Disentangling these many factors in order to reliably recover seismic attenuation is challenging. It is argued here that noise intensity, while rarely isotropic or homogeneous, may nevertheless be modeled by a radiative transfer equation. It is then shown that this recognition sufficiently constrains the noise intensity that we may hope to fit measured correlation amplitudes to models for spatially varying attenuation and site amplification factors. One-bit preprocessing, it is shown, is not compatible with such fits except in the special case of spatially constant noise intensity. An alternative procedure for accelerating convergence is suggested. Numerical simulations for a case of homogeneous attenuation and homogeneous seismic velocity are presented in support of the assertions. Attenuation, site factors, and noise intensity are successfully retrieved from correlations of numerically simulated imperfectly diffuse waves measured on a linear array of sensors.     

Structure of the Mt Isa region from seismic ambient noise tomography

We use seismic tomography, exploiting group velocities derived from ambient noise, to delineate the crustal structure beneath Mt Isa and the surrounding blocks and basins. The depth extent of the blocks can be traced into the mid-crust and the spatial extent of the associated velocity anomalies mapped over an area of approximately 500 km by 500 km. The Proterozoic Mt Isa block is imaged as a region of elevated seismic velocities comparable to the Yilgarn craton in Western Australia, while the surrounding basins have relatively low velocities. Seismic velocity anomalies display correlations with the regional Bouguer gravity data and with high crustal temperatures in the region. There are a number of isolated low-velocity anomalies under the Millungera basin that suggest either previously unknown thermal anomalies or zones with high permeability, which can also produce lowered velocities.     

基于地震环境噪声的油页岩勘探

作为重要的非常规油气资源,油页岩的勘探开发日益受到重视。利用地震环境噪声(微动)对地下介质的横波速度成像是一种无源、高效、低成本的地震勘探方法,是更符合“环保要求的潜在油页岩勘探新方法。本文首次利用共中心面元空间自相关微动勘探方法,在松辽盆地开展了含油页岩地层识别研究。研究结果表明,共中心面元空间自相关微动勘探方法的观测系统可根据实际情况灵活多变,横波速度剖面与测线位置钻孔编录结果对应较好,能很好地划分主要地层,主要含油页岩的嫩江组二段呈现低横波速度特征。     

Seismic attenuation of coda waves in the eastern region of Cuba

Cuba's seismic attenuation had never been studied in detail. In this paper we present the results of the research on the seismic attenuation of Cuba's eastern zone based upon the information collected by the seismological Cuban network from 1998 to 2003. 581 earthquakes were selected from the Cuban catalogue to make this study. All of them, recorded by at least three seismic stations, had their epicenters located in the eastern Cuban region (19.3–22 N, 79–73 W), epicentral distances between 15 km and 213 km, their coda duration magnitudes ranging from 2 and 4.1 and their focal depths reaching up to 30 km. The seismic wave attenuation was studied using coda waves. The single scattering method proposed by Sato in 1977 was applied, the attenuation and frequency dependency for different paths and the correlation of the results with the geotectonics of the region are presented in this paper.The mean Q_c value calculated was Q_c = (64 ± 2)f^{0.84 ± 0.01}. The relatively low Q₀ and the high frequency dependency agree with the values of a region characterized by a high tectonic activity. The Q_c values of seven subregions of eastern Cuba were calculated and correlated with the geology and tectonics of the area.     

Simulation of seismic-prospecting random noise in the desert by a Brownian-motion-based parametric modeling algorithm

Random noise has a negative impact on seismic-prospecting record processing. An important step to improve the methods aimed at the attenuation of random noise is to scientifically characterize the properties of the noise. Numerical modeling is useful to understand the nature of the random noise. In this study, we present a Brownian-motion-based parametric modeling algorithm for the simulation of seismic-prospecting random noise in the desert. The optimal Hurst exponent required to implement the method can be determined by comparing the spectral properties related to the noise data and the simulated results. The data used to analyze the properties of the noise were acquired in the Tarim Basin (Northwest of China). We verify the performance of the modeling algorithm by comparing the results obtained after the simulation with the real noise data in both the time domain and the spatio-temporal domain. The experimental results thus obtained prove the accuracy and efficiency of the proposed modeling algorithm. This study can be used as a basis to investigate the seismic-prospecting random noise characteristics and thus contribute to its mitigation.     

基于背景噪声的自相关与HVSR联合成像在广东惠州城市浅层结构探测中的应用

随着我国城市化进程快速推进,大中小城市呈现地下空间全面开发的态势。然而城市地下空间开发利用存在地下情况不明、结构不清的问题,这在很大程度上影响了地下空间的开发利用和城市战略布局规划。广东惠州是典型的东部中型城市,为了避免老城区建成后地下水土质量不清楚、地下空间资源量不明确、城市地下空间资源开发无科学数据支持等问题,特在研究区开展绿色无损的背景噪声成像研究,以查明惠州地区的第四系及断层的发育情况,为新区成功建成提供数据支持。笔者等以台间距10 m、测线总长度1 km的密集地震台阵为基础,利用两条测线共200台的背景噪声数据,使用噪声自相关与噪声HVSR两种方法分别对研究区两条线性测线分别进行计算,通过数据预处理,单台数据处理,多台数据整合,最后得到了两条测线的自相关剖面和HVSR剖面。研究显示,研究区第四系基岩顶面时间域范围为20~100 ms,深度域范围为5~20 m,浅层P波速度范围为400~500 m/s,这与收集到的钻孔等资料相一致。研究表明基于密集台阵的噪声自相关与HVSR联合成像方法是一种高效、绿色、低成本的方法,可用于调查第四系分界面（强波阻抗分界面）及浅层沉积物中的活动断层。     

基于背景噪声的自相关与HVSR联合成像在广东惠州城市浅层结构探测中的应用

随着我国城市化进程快速推进,大中小城市呈现地下空间全面开发的态势。然而城市地下空间开发利用存在地下情况不明、结构不清的问题,这在很大程度上影响了地下空间的开发利用和城市战略布局规划。广东惠州是典型的东部中型城市,为了避免老城区建成后地下水土质量不清楚、地下空间资源量不明确、城市地下空间资源开发无科学数据支持等问题,特在研究区开展绿色无损的背景噪声成像研究,以查明惠州地区的第四系及断层的发育情况,为新区成功建成提供数据支持。笔者等以台间距10 m、测线总长度1 km的密集地震台阵为基础,利用两条测线共200台的背景噪声数据,使用噪声自相关与噪声HVSR两种方法分别对研究区两条线性测线分别进行计算,通过数据预处理,单台数据处理,多台数据整合,最后得到了两条测线的自相关剖面和HVSR剖面。研究显示,研究区第四系基岩顶面时间域范围为20~100 ms,深度域范围为5~20 m,浅层P波速度范围为400~500 m/s,这与收集到的钻孔等资料相一致。研究表明基于密集台阵的噪声自相关与HVSR联合成像方法是一种高效、绿色、低成本的方法,可用于调查第四系分界面（强波阻抗分界面）及浅层沉积物中的活动断层。     

Characterization of site effects in Montreal,Canada

Recent destructive earthquakes have clearly shown that near-surface geological conditions play a major role in the level of ground shaking in urban areas. In Canada, Montreal is ranked second for seismic risk after Vancouver considering its population and regional seismic hazard. The city is largely built on recent unconsolidated marine and river deposits and most of its infrastructure is old and deteriorated. A seismic risk project that includes a combined methodology for site effects zoning in large cities, using microtremor measurements (H/V method) coupled with 1D numerical modelling (SHAKE91), has been initiated. The experimental approach gives good estimates of the fundamental frequency of soft deposits, while the numerical approach provides good estimates of the soil response in terms of amplification factor related to frequency. Main mechanical properties of soft soils were compiled from various data available, and a sample of input rock motions from real and synthetic earthquakes was used to compute soil response. The influence of marine clays on soil response is significant and is well correlated with thickness of these deposits. PGA amplification factors range from 2 to 4 at frequencies from 2 to 7 Hz, with some occasional larger values. The results demonstrate that the methodology used for our study is both fast and efficient to determine the influence of soft soils in urban environments. Such studies are essential for the effective deployment of seismic instrumentation, land-use planning and seismic mitigation.     

青藏高原东部基于噪声的面波群速度分布特征

通过收集青海、甘肃、四川三省的76个地震台记录的2008年1—12月三分量的连续噪声数据,利用噪声面波层析成像的方法获得了青藏高原东部的面波群速度分布特征。首先采用多重滤波方法提取了1 000多条台站对5～50 s的三分量面波群速度频散曲线,然后将研究区域划分为0.2°×0.2°的网格,利用O ccam方法反演了瑞利波(R-R)和勒夫波(T-T)的群速度分布。反演得到的群速度分布特征与地表地质和构造特征表现出较好的相关性,清晰地揭示了地壳内部的横向速度变化。层析成像的结果显示在短周期(8～20 s)内,拥有较厚的沉积层的四川盆地表现为明显的低速特征,而青藏高原东部则表现为较高的群速度分布特征;随着周期的增加(>20 s),群速度的分布特征呈现出与短周期相反的特性,青藏高原东部下方的速度远远低于四川盆地,这可能与青藏高原东部中、下地壳低速层相关联,同时也意味着研究区域的地壳结构具有明显的横向不均匀性。在群速度分布图上,龙门山不仅是四川盆地与青藏高原的地形和构造分界带,同时也对应着高群速度与低群速度的过渡带。     

利用背景噪声层析成像研究济南浅层横波速度结构

城市地球物理探测面临众多人文挑战,像城市交通、密布电网、参差楼群等,对应用不同方法进行数据处理分析提出了技术挑战,需要用创新的方式和技术进行城市探测。为探索和发展适用于城市地下空间资源开发利用的地球物理探测新技术,中国地质科学院地球深部探测中心第一次启动了城市地下空间勘查评价试点工程,并选择济南市作为试点城市,试验短周期密集台阵噪声层析技术,并取得了较好成效。获得的主要认识如下：(1)在非弥散场或非均匀噪声场源的人文干扰较为严重的城市区域,通过长时间的噪声信号的采集,可获得高信噪比的面波信号,因此密集台阵噪声成像技术适应于城市地下空间的背景岩层结构探测工作。(2)本次研究提取出了周期范围为0.2～1.5 s的高信噪比面波信号,并取得与实际地质特征相一致的横波速度结构信息。(3)从横波速度结构特征来看,研究区300 m以浅,主要岩性为灰岩且主要分布于研究区西侧,而侵入岩体主要分布于东侧,中间存在一个明显的分界面,指示存在一个近南北向的隐伏断裂。随着深度的增加,大面积的侵入岩体展布于深层,灰岩只在北侧局部有显现。总体来说,本项研究证明了密集台阵噪声层析方法能够适用于人文干扰严重的城市地下空间背景岩层结构探测,这一成果对认识研究区地下结构、地震防灾、工程应用具有重要指导意义。     

华南大陆东部地壳上地幔S波速度结构研究

正华南大陆东部地区地处欧亚板块和太平洋板块的交汇区域,长期受到印度洋板块、太平洋板块和菲律宾海板块的俯冲影响,是欧亚大陆东部边缘构造框架极其重要的一部分,也是研究板块相互作用的天然实验室。已有的大尺度全国地震体波和面波成像结果可以很好的分辨出主要的盆地和块体分布,但体波成像的射线在地壳和上地幔顶部覆盖     

3-D crustal structure of the extensional Granada Basin in the convergent boundary between the Eurasian and African plates

Three-dimensional P and S wave velocity models of the crust under the Granada Basin in Southern Spain are obtained with a spatial resolution of 5 km in the horizontal direction and 2 to 4 km in depth. We used a total of 15407 P and 13704 S wave high-quality arrival times from 2889 local earthquakes recorded by both permanent seismic networks and portable stations deployed in the area. The computed P and S wave velocities were used to obtain three-dimensional distributions of Poisson's ratio (σ) and the porosity parameter (V_p×V_s). The 3-D velocity images show strong lateral heterogeneities in the region. Significant velocity variations up to ±7% in P and S velocities are revealed in the crust below the Granada Basin. At shallow depth, high-velocity anomalies are generally associated with Mesozoic basement, while the low-velocity anomalies are related to the neogene sedimentary rocks. The south–southeastern part of the Granada Basin exhibits high σ values in the shallowest layers, which may be associated with saturated and unconsolidated sediments. In the same area, V_p×V_s is high outside the basin, indicating low porosity of the mesozoic basement. A low-velocity zone at 18-km depth is found and interpreted as a weak–ductile crust transition that is related to the cut-off depth of the seismic activity. In the lower crust, at 34-km depth, a clear slow V_p and V_s anomalous zone may indicate variations in lithology and/or with the rigidity of the lower crust rocks.     

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between “deeper” deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and “shallower” deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard.     

Scattering and absorption imaging of a highly fractured fluid-filled seismogenetic volume in a region of slow deformation

Regions of slow strain often produce swarm-like sequences, characterized by the lack of a clear mainshock-aftershock pattern. The comprehension of their underlying physical mechanisms is challenging and still debated. We used seismic recordings from the last Pollino swarm (2010–2014) and nearby to separate and map seismic scattering (from P peak-delays) and absorption (from late-time coda-wave attenuation) at different frequencies in the Pollino range and surroundings. High-scattering and high-absorption anomalies are markers of a fluid-filled fracture volume extending from SE to NW (1.5–6 ​Hz) across the range. With increasing frequency, these anomalies approximately cover the area where the strongest earthquakes occurred from the sixteenth century until 1998. In our interpretation, the NW fracture propagation ends where carbonates of the Lucanian Apennines begin, as marked by a high-scattering and low-absorption area. At the highest frequency (12 ​Hz) the anomalies widen southward in the middle of the range, consistently marking the faults active during the recent Pollino swarm. Our results suggest that fracture healing has closed small-scale fractures across the SE faults that were active in the past centuries, and that the propagation of fluids may have played a crucial role in triggering the 2010–2014 Pollino swarm. Assuming that the fluid propagation ended at the carbonates barrier in the NW direction, fractures opened new paths to the South, favoring the nucleation of the last Pollino swarm. Indeed, the recently active faults in the middle of the seismogenic volume are marked by a high-scattering and high-absorption footprints. Our work provides evidence that attenuation parameters may track shape and dynamics of fluid-filled fracture networks in fault areas.     

浅谈城市浅层背景噪声体波提取研究

正随着我国城市化进程的加快,城市建设由地表向地下空间拓宽,城市地质环境受到的影响和面临的压力与日俱增。诸如城市活断层、地裂隙、地下空洞等,都是市政建设中急需解决的问题。在城市规划中,由于人口密集和安全问题,主动源实验并不能很好的在城区研究浅部地壳结构。因此寻求绿色且无破坏性的研究方法至关重要。