S-wave splitting from records of local micro-earthquakes in West Bohemia/Vogtland: An indicator of complex crustal anisotropy

Most S-wave particle motions of local micro-earthquakes in the West Bohemia/Vogtland region display S-wave splitting. The split S waves are usually well defined, being separated in time and polarized in roughly perpendicular directions in the horizontal projection. In most cases, the polarization of the fast S wave is aligned NW-SE (referred to as “normal splitting”), which is close to the direction of the maximum horizontal compression in the region. However, for some ray directions, the polarization of the fast S wave is aligned NE-SW (referred to as “reverse splitting”). The pattern of normal/reverse splitting on a focal sphere is station-dependent, indicating the presence of inhomogeneities in anisotropy. For some stations, the normal/reverse splitting pattern is asymmetric with respect to the vertical axis, indicating the symmetry axes of anisotropy are probably inclined. The presence of inclined anisotropy is confirmed by observations of directionally dependent delay times between split S waves. A complex and station-dependent anisotropy pattern is probably the result of a complicated anisotropic crust characterized by diverse geological structures. The spatial variation of anisotropy probably reflects the presence of a variety of different types of anisotropic rocks in the region.     

P-SH Conversions in Layered Media with Hexagonally Symmetric Anisotropy: A CookBook

—Reflectivity synthetic seismograms demonstrate that the type, layering and orientation of 1-D anisotropy influences strongly the coda of teleseismic P waves at periods T > 1 sec, particularly P-SH converted waves. We assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis ? of arbitrary orientation. The resulting phase velocities vary as cos 2ξ with respect to that axis. Using three families of simple crustal models, we compare the effects of an anisotropic surface layer with reverberations caused by both "thick" and "thin" layers of anisotropy at depth. If anisotropy in the surface layer is significant, the polarization of direct P can be distorted to generate a transverse component, followed by Ps and a prominent shear reverberation converted from direct P at the free surface. If the anisotropic layer is buried, the first, and often the most prominent, arrival on the transverse component is the P-to-SH conversion at its upper surface. If the anisotropic layer is sufficiently thin, P-to-SH conversions from its boundaries interfere to form a derivative pulse shape on the transverse component, which could be mistaken as the signature of shear-wave splitting. If ? is horizontal, compressional (P) and shear (S) anisotropy both produce similar waveform perturbations with four-lobed azimuthal patterns, suggesting that a weighted stack of P coda from different back-azimuths would improve signal-to-noise. For ? tilted between the horizontal and vertical, however, the effects of P- and S-anisotropy differ greatly. The influence of P-anisotropy on P-to-S conversion is greatest for a symmetry axis tilted at 45° to the vertical, where its azimuthal pattern has two lobes, rather than four. Combinations of P- and S-anisotropy typically lead to a composite azimuthal dependence in the P-coda reverberations.     

Seismic, Structural and Petrological Models of the Subcrustal Lithosphere in Southern Germany: A Quantitative Revaluation

—Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P _n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.¶Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.¶The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed. This ambiguity can be substantially reduced when shear waves (S-waves) are considered. The directional distribution of S-wave velocities and of the S-wave splitting exhibits characteristic patterns for the different olivine texture types. This could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle. A fundamental requirement for a more comprehensive interpretation is the availability of detailed S-wave observations. The maximum S-wave splitting in the peridotites investigated coincides with the maximum of the faster (leading) S-wave. This may be of importance to detect S-wave splitting in future seismic studies.     

Interpreting non-orthogonal split shear waves for seismic anisotropy in multicomponent VSPS

There are two main sources of non-orthogonality in multicomponent shear-wave seismics: inherent non-orthogonal split shear waves arising from substantial ray deviation in off-symmetry planes due to strong anisotropy or complex overburden, and apparent non-orthogonal split shear waves in the horizontal plane due to variation of the angle of incidence even if the two shear waves along the raypath are orthogonal. Many techniques for processing shear-wave splitting in VSP data ignore these kinds of non-orthogonality of the split shear waves. Assuming inherent non-orthogonality in zero-offset VSPs, and apparent non-orthogonality in offset VSPs, we derive equations for the four-component data matrix. These can be solved by extending the linear-transform technique (LTT) to determine the shear-wave polarizations in zero-offset and offset VSPs. Both full-wave synthetic and field data are used to evaluate the technique and to examine the effects of non-orthogonal polarized split shear waves. If orthogonality is incorrectly assumed, errors in polarization measurements increase with the degree of non-orthogonality, which introduces a consistent decreasing trend in the polarization measurements. However, the effect of non-orthogonality on the estimation of geophone orientation and time delays of the two split shear waves is small and negligible in most realistic cases. Furthermore, for most cases of weak anisotropy (less than 5% shear-wave anisotropy) apparent non-orthogonality is more significant than inherent non-orthogonality. Nevertheless, for strong anisotropy (more than 10% shear-wave anisotropy) with complicated structure (tilted or inclined symmetry axis), inherent non-orthogonality may no longer be negligible. Applications to both synthetic and real data show that the extended linear-transform techniques permit accurate recovery of polarization measurements in the presence of both significant inherent and apparent non-orthogonality where orthogonal techniques often fail.     

Modelling S-wave radiation for SP-waves in isotropic and anisotropic media

Utilizing shear-wave (S-wave) data acquired with compressional waves (P-waves) is becoming more common as joint imaging and inversion techniques improve. Interest in S-waves radiated from vertical sources and buried explosives exploits conversion to P-waves as primary reflections (SP-waves) for reducing acquisition costs and for application to legacy data. However, recent investigations overstate the extent of SP-wave illumination and show isotropic processing results with narrow bandwidth frequency and wavenumber data. I demonstrate that illumination with SP-waves is limited in general to near vertical polar angles up to around 30° or 35° for V_P/V_S of 2 or 3, respectively. At greater angles, S-waves are typically in the P-wave evanescent range and cannot excite SP-wave reflections. Contrary to recent claims, these sources for P-wave do not radiate S_H-waves polarized in horizontal planes in all azimuths. I show these properties for isotropic media with radiation expressions for amplitude derived in vector slowness coordinates. Also, I extend these expressions to transversely isotropic media with a vertical symmetry axis to show agreement with synthetic seismic data that only quasi S_V-waves are radiated and become more narrowly focused towards 45°. Furthermore, in orthorhombic media, synthetic data show that fast S₁- and slow S₂-waves polarized parallel and perpendicular to fractures may appear as S_V- and S_H-waves. For the partially saturated fracture model studied here, S₁-wave radiation has broader azimuthal illumination than slow S₂-waves, which are more narrowly focused in azimuth. These produce SP-wave splitting signatures on vertical component reflection data that are nearly identical to PS-wave signatures on radial horizontal component data. Separating these fast and slow SP-waves is an additional processing challenge.     

蒙古中南部地区地壳各向异性及其动力学意义

利用蒙古中南部地区布设的69套宽频带数字地震仪2011年8月—2013年7月记录的远震事件,使用时间域反褶积方法提取接收函数,并挑选高质量Pms震相,通过改进的剪切波分裂方法对研究区地壳各向异性参数进行了研究,最终获取了1473对各向异性参数.经过统计分析,有48个台站可以归纳出两个方向的各向异性,11台站得到单个方向的各向异性,而剩余10个台站各向异性方向比较发散.结果显示,各向异性在蒙古中南部地壳中呈不均匀分布,有54个台站得到了NE-SW向各向异性,快波偏振方向平均值为N58°E±16°,与最大水平主应力σHmax方向和区域内主要断层走向一致,说明这部分地壳各向异性的主要成因存在于上地壳,可能与流体填充的微裂隙有关.而NW-SE向各向异性在53个台站被观测到,各向异性方向变化范围平均N132°E±16°,与研究区大部分SKS分裂快波方向具有较好的一致性,说明下地壳成岩矿物晶体定向排列是各向异性的主要成因.研究区地壳各向异性的分层特征总体上支持岩石圈受到NE-SW向挤压的动力学模型.     

Recovering Shear-wave Anisotropy of the Lower Crust: The Influence of Systematic Errors on Travel-time Inversion

—Studies of seismic anisotropy in situ can help to discriminate between different rock types for the lower crust. In this context we investigate the sensitivity of an iterative linearized 3-D travel-time inversion scheme for transversely isotropic media with respect to two types of systematic errors wrong velocities and interface topography of the hanging wall of the lower crust. The computations simulate realistic field conditions such as found for the Variscan crust at the Urach geothermal anomaly. The study focusses on the possible information content of split S _M ?S arrivals observed along two orthogonal expanding spread profiles. It ensues that an imperfect knowledge of the layer geometry is of minor importance compared to errors in the velocities of the hanging wall. In particular, upper crust anisotropy has to be considered carefully. Generally, the anisotropy of transversely polarized shear waves (SH waves) was recovered with higher accuracy than the anisotropy of vertically polarized shear waves (SV waves).     

四川宜宾地区S波分裂特征

本文采用纵横比与偏振分析相结合的方法测定了2013年4月25日~2015年12月31日四川宜宾地区10个台站S波分裂参数,即快波偏振方向和慢波延迟时间。结果表明,华蓥山断裂两侧台站呈现不同的快波偏振优势方向,断裂带以西的台站偏振优势方向为NW向,与区域应力场方向一致;位于断裂带以东的台站优势偏振方向为NE向,与断裂走向一致。在地震密集分布区域内的CNI台的优势偏振方向为NE向,与台站附近的断裂带走向基本一致。研究区域南段的3个台站(JLI、YAJ、XWE)优势偏振方向近NS向。各个台站平均慢波延迟时间在3.07~11.95ms/km范围内,慢波延迟时间最大的台站是CNI台,距离2013年4月25日06时10分M_L5.2地震震中位置最近,这反映出震源区地震波各向异性程度较强。CNI台站的慢波延迟时间显示,在2015年2月7日M_L4.8地震前观测到慢波延迟时间有明显的上升趋势。     

Time variations in parameters of split S waves from weak local earthquakes beneath eastern Hokkaido

Parameters of seismic waves from clusters of local weak earthquakes that occurred at the upper boundary of the seismofocal zone in depth intervals of 40–60 and 70–90 km along eastern Hokkaido are investigated for the period 1998–2003, including the strong (M = 8.0) Tokachi-Oki earthquake of September 26, 2003. Analysis of data indicates that the distribution of anisotropic properties along Hokkaido is inhomogeneous and parameters of split waves (the azimuth of the fast S wave and the time delay between split S waves) are sensitive to variations in the stress-strain state of the medium. Unstable behavior of split wave parameters and increased values of the ratio V _P /V _S for clusters of events in the areas of the Hidaka Mountains and Nemuro Peninsula imply that the medium is in a mechanically weakened state (in the regime of intense dilatant deformation). On the contrary, the regions beneath the Tokachi and Kushiro plains are more rigid and are characterized by lower V _P /V _S values and comparatively stable behavior of wave parameters. Anomalous parameters of split waves from events of clusters in areas of different stations correlate with each other and are related to occurrence times of large earthquakes around Hokkaido, which may point to a redistribution of stresses and strains and fluid migration in the subduction zone.     

Anisotropy and temporal variations in the fast azimuth of the fast shear wave beneath southern Kamchatka during 1993–2002

Parameters of split shear waves from local earthquakes in the area of the PET IRIS station (town of Petropavlovsk-Kamchatski) were measured over the period 1993–2002 for the study of anisotropic properties of rocks in the subduction zone and variations in the fast azimuth of the fast shear wave (?). The dominating fast shear wave polarization directions were oriented in 1993–2002 along N90°E ± 20° in agreement with the direction of the Pacific plate motion. The normalized shear wave delay times δt _SS increase to a depth of 150 km. The values of δt _SS are largest (up to 20 ms/km) for earthquakes at depths of 50–60 and 90–150 km and smallest (up to 6 ms/km) for earthquakes at depths greater than 200 km. The fast azimuths for events with H < 80 km are described in terms of a horizontal transversely isotropic (HTI) model of the medium, with the axis oriented northward. Temporal variations in the fast azimuths with an amplitude of up to 90° and a predominant period of about 400–600 days are observed for events at depths of 80–120 km. The anisotropy of rocks is described by effective models of the orthorhombic and HTI symmetries. The predominant fast shear wave fast azimuths from events at depths of 120–310 km vary with time: the polarization axis was oriented to the north in 1993–1995, to the north and east in 1996–1998, to the east in 1999–2000, and to the northeast and southeast in 2001–2002. The anisotropy of rocks can be described in terms of the HTI model with the symmetry axis subparallel to the focal zone dip.     

Complex deep seismic anisotropy below the Scandinavian Mountains

Several seismological projects focused on the deep structure of the Scandinavian Mountains, in Norway and neighbouring Sweden. We use these recordings to study seismic anisotropy by analysing the birefringence of SKS and SKKS phases. These phases, which should be polarised radially, are split into an additional transverse component if they propagate through an anisotropic medium. Our results are directions Φ of the apparent fast shear wave polarisation and delay times δt between the split phases. For station KONO in Southern Norway, we find frequency-dependent Φ and δt values, indicating a depth-dependent anisotropy. Additionally, Φ and δt values vary with epicentre backazimuths in Norway, indicating a complex anisotropic structure in the crust and upper mantle. Stacking of the SKS/SKKS waveforms improves the signal-to-noise ratio along one station line and allows us to better determine the splitting parameters. A unique and complete model of the complex anisotropy cannot be obtained due to the limited observed backazimuth range. Near-surface tectonic structures correlate with the splitting pattern and thus the crust is one anisotropic layer in the region. Partly preferred orientations in the rock fabric at the surface can be correlated with Φ. Below one or more anisotropic layers must exist to explain the backazimuth- and frequency-dependent observations, as well as the long δt values (>2 s) which cannot be explained with crustal anisotropy alone. The spatial distribution of the splitting results indicates that different tectonics units, e.g. the Sveconorwegian, the Central and Northern Svecofennian and the Caledonian nappes, are each characterised by specific anisotropic signatures.     

Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran

IntroductionWhenpropagatingthroughananisotropicmedium,ashearwavesplitsintotwo(quasi)shearwaveswithdifferentpropagationspeedsandpolarizedorthogonally.Owingtotherecentdevel-opmentofseismicobservationsystem,detectionofshearwavessplittingwithverysmalldelaytimesbetweenfasterandslowershearwavesbecameavailableandprovidedpowerfulapproachfordetectionofcrustalanisotropy.Crampin(1978)emphasizedtheroleofalignedmicrocracksasacauseofcrustalanisotropyandpointedoutthatforverticallyalignedmicrocracksthedirecti…     

Backazimuthal Variations of Splitting Parameters of Teleseismic SKS Phases Observed at the Broadband Stations in Germany

—SKS phases observed at broadband stations in Germany show significant shear-wave splitting. We have analyzed SKS and SKKS phases for shear-wave splitting from 13 stations of the German Regional Seismic Network (GRSN), from 3 three-component stations of the Gräfenberg array (GRF) and from one Austrian station (SQTA). The data reveal strong differences in the splitting parameters (fast direction φ and delay time δt from a single event at various stations as well as variations at the individual stations for events with different backazimuths. The backazimuthal variations of the splitting parameters at some stations can be explained by two-layer anisotropy models with horizontal symmetry axes. The best resolved two-layer model is the GRA1 model (upper layer φ = 40°, δt = 1.15s; lower layer φ = 115°, δt = 1.95s). The upper layer can be attributed to the lithosphere. Because of the magnitude of the delay time of the upper layer, the lower layer must lie within the asthenosphere. At other stations splitting parameters are consistent with an anisotropic one-layer model for the upper mantle. Stations near the Bohemian Massif show fast directions near EW. Throughout NE Germany the directions are oriented NW/SE. The reason for this direction is probably the nearby Tornquist-Teisseyre line. The observed fast axes are subparallel to this prominent Transeuropean suture zone. At stations in southern Germany near the Alps we observed ENE/WSW directions. Below some stations we also found indications of inclined anisotropic layers.     

Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

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鄂尔多斯块体及其周缘上地幔各向异性分析研究

对布设在鄂尔多斯块体及其周缘的固定宽频带地震台网90个台站记录作远震SKS波形资料偏振分析,采用最小切向能量法求得每个台站的SKS快波偏振方向和快慢波时差,获得块体及其周缘上地幔各向异性图像,并结合1999～2007年相对鄂尔多斯块体的GPS水平地壳运动速度场,分析块体及其周缘的各向异性特征和壳幔耦合特征并重新认识块体...     

Seismological Evidence of Active Faulting in the Tendaho Rift (Afar Triangle, Ethiopia)

—A temporary network has been used to study the seismic activity in the central-northern part of the Tendaho Rift (Afar Triangle, Ethiopia). Small size earthquakes (M < 2.5) characterize the seismicity during the time interval 1993–1994 when the seismic array was operating. Shocks are mainly located in the central part of the Tendaho rift and their epicentral distribution highlights a remarkable coincidence with NW-SE trending tectonically active structures. Focal depths are confined in the range of 3–8?km. ? The few acceptable fault plane solutions suggest the occurrence of both strike-slip and dip-slip rupture mechanisms, which are often characterized by a nodal plane oriented approximately NW-SE. ?A detailed study of earthquake waveforms using polarization analysis showed the occurrence of splitting phenomena for the S waves. Despite the differences in focal mechanisms, the similarity of the faster S-wave directions (NW-SE to NNW-SSE) suggests that the direction of geological structures and/or the systems of fractures adjacent to the faults could bias shear-wave propagation. The splitting time between fast and slow shear waves shows no systematic relationship with either hypocentral distance or focal depth, suggesting that the anisotropic volume is constrained to depths shallower than 3?km.     

Dispersion splitting of Rayleigh waves in layered azimuthally anisotropic media

Rayleigh wave dispersion can be induced in an anisotropic medium or a layered isotropic medium. For a layered azimuthally anisotropic structure, traditional wave equation of layered structure can be modified to describe the dispersion behavior of Rayleigh waves. Numerical stimulation results show that for layered azimuthal anisotropy both the dispersion velocities and anisotropic parameters depend principally on anisotropic S-wave velocities. The splitting S-wave velocities may produce dispersion splitting of Rayleigh waves. Such dispersion splitting appears noticeable at azimuthal angle 45°. This feature was confirmed by the measured results of a field test. The fundamental mode splits into two branches at azimuthal angle 45° to the symmetry axis for some frequencies, and along the same direction the difference of splitting-phase velocities of the fundamental model reaches the maximum. Dispersion splitting of Rayleigh waves was firstly displayed for anisotropy study in dispersion image by means of multichannel analysis of surface waves, the image of which provides a new window for studying the anisotropic property of media.     

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...     

Seismic Anisotropy and Velocity Variations in the Mantle beneath the Saxothuringicum-Moldanubicum Contact in Central Europe

—We report on results of a passive seismic experiment undertaken to study the 3-D velocity structure and anisotropy of the upper mantle around the contact zone of the Saxothuringicum and Moldanubicum in the western margin of the Bohemian Massif in central Europe. Spatial variations of P-wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere. A joint interpretation of P-residual spheres and shear-wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units. Lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the Moldanubicum.     

肃南5.7级地震过程剪切波分裂特征

剪切波分裂应用于地震预报中的３个主要困难是：（１）如何自动定量识别剪切波快慢波到时；（２）用以认识其基本特征的震例太少；（３）因各次地震的复杂性，对所给剪切波分裂结果难以给出恰当的解释，本文研究了１９８８年１１月２２日甘肃肃南５．７级地震过程剪切波分裂的特征。所用资料为１９８８年８月到１９９０年８月中法合作张掖数字台网的记录。通过对张掖～肃南地区地壳结构模型的研究，确认了所用各次地震的真实出射角基