How Are Vertical Shear Wave Splitting Measurements Affected by Variations in the Orientation of Azimuthal Anisotropy with Depth?
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Global Mantle Flow and the Development of Seismic Anisotropy: Differences Between the Oceanic and Continental Upper Mantle Clinton P
Global Mantle Flow and the Development of Seismic Anisotropy: Differences Between the Oceanic and Continental Upper Mantle Clinton P. Conrad Department of Earth and Planetary Sciences Johns Hopkins University Baltimore, MD 21218, USA Mark D. Behn Department of Geology and Geophysics Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA Paul G. Silver Department of Terrestrial Magnetism Carnegie Institution of Washington Washington, DC 20015, USA Submitted to Journal of Geophysical Research : July 3, 2006 Viscous shear in the asthenosphere accommodates relative motion between the Earth’s surface plates and underlying mantle, generating lattice-preferred orientation (LPO) in olivine aggregates and a seismically anisotropic fabric. Because this fabric develops with the evolving mantle flow field, observations of seismic anisotropy directly constrain flow patterns if the contribution of lithospheric fossil anisotropy is small. We use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and LPO, and constrain the asthenospheric contribution to anisotropy using a global compilation of observed shear-wave splitting measurements. For asthenosphere >500 km from plate boundaries, simple shear rotates the LPO toward the infinite strain axis (ISA, the LPO after infinite deformation) faster than the ISA changes along flow lines. Thus, we expect ISA to approximate LPO throughout most of the asthenosphere, greatly simplifying LPO predictions because strain integration along flow lines is unnecessary. Approximating LPO ~ISA and assuming A-type fabric (olivine a-axis parallel to ISA), we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy (average misfit = 12º). Continental anisotropy is less well fit (average misfit = 39º), but lateral variations in lithospheric thickness improve the fit in some continental areas. -
Seismic Anisotropy: a Review of Studies by Japanese Researchers
J. Phys. Earth, 43, 301-319, 1995 Seismic Anisotropy: A Review of Studies by Japanese Researchers Satoshi Kaneshima Departmentof Earth and Planetary Physics, Faculty of Science, The Universityof Tokyo, Bunkyo-ku,Tokyo 113,Japan 1. Preface Studies on seismic anisotropy in Japan began in the mid 60's, nearly at the same time its importance for geodynamicswas revealed by Hess (1964). Work in the 60's and 70's consisted mainly of studies of the theoretical and/or observational aspects of seismic surface waves. The latter half of the 70' and the 80's, on the other hand, may be characterized as being rich in body wave observations. Elastic wave propagation in anisotropic media is currently under extensiveresearch in exploration seismologyin the U.S. and Europe. Numerous laboratory experiments in the fields of mineral physics and rock mechanics have been performed throughout these decades. This article gives a review of research concerning seismic anisotropy by Japanese authors. Studies by foreign authors are also referred to whenever they are considered to be essential in the context of this article. The review is divided into five sections which deal with: (1) laboratory measurements of elastic anisotropy of rocks or minerals, (2) papers on rock deformation and the origin of seismic anisotropy relevant to tectonics and geodynarnics,(3) theoretical studies on the Earth's free oscillations, (4) theory and observations for surface waves, and (5) body wave studies. Added at the end of this article is a brief outlook on some current topics on seismic anisotropy as well as future problems to be solved. -
Shear Wave Splitting Time Variation by Stress-Induced Magma Uprising at Mount Etna Volcano
Shear wave splitting time variation by stress-induced magma uprising at Mount Etna volcano Francesca Bianco1, Luciano Scarfì2, Edoardo Del Pezzo1 & Domenico Patanè2 1Istituto Nazionale Geofisica e Vulcanologia - Osservatorio Vesuviano, Napoli (Italy) 2 Istituto Nazionale Geofisica e Vulcanologia Sez. Catania (Italy) Abstract Shear wave splitting exhibits clear time variations before the July 17th – August 9th, 2001 flank eruption at Mount Etna. The normalized time delays, Tn, detected through an orthogonal transformation of singular value decomposition, exhibit a clear increase starting 20 days before the occurrence of the eruption (July 17th); the qS1 polarization direction, obtained using a 3D covariance matrix decomposition, shows a 90°-flip several times during the analyzed period: the last flip 5 days before the occurrence of the eruption. Both splitting parameters also exhibit a relaxation phase shortly before the starting of the eruption. Our observations seem in agreement with Anisotropic Poro Elasticity (APE) modelling, suggesting a tool for the temporal monitoring of the build up of the stress leading to the occurrence of the 2001 eruption at Mt. Etna. Introduction Shear wave splitting is the elastic analogue of the well known optical birefringence phenomenon. A shear wave entering into an anisotropic volume, is split into two quasi shear waves, qS1 and qS2, propagating with different velocities and approximately orthogonal polarizations. The splitting parameters are the time delay between qS1 and qS2 wave (hereafter referred to as Td); and the polarization direction of the leading split (qS1) wave (hereafter referred to as Lspd, Linear split polarization direction).. The Extensive Dilatancy Anisotropy (EDA) model[Crampin et al, 1984], suggests that the principal source of seismic anisotropy in the crust is stress aligned fluid filled microcracks. -
The Upper Mantle and Transition Zone
The Upper Mantle and Transition Zone Daniel J. Frost* DOI: 10.2113/GSELEMENTS.4.3.171 he upper mantle is the source of almost all magmas. It contains major body wave velocity structure, such as PREM (preliminary reference transitions in rheological and thermal behaviour that control the character Earth model) (e.g. Dziewonski and Tof plate tectonics and the style of mantle dynamics. Essential parameters Anderson 1981). in any model to describe these phenomena are the mantle’s compositional The transition zone, between 410 and thermal structure. Most samples of the mantle come from the lithosphere. and 660 km, is an excellent region Although the composition of the underlying asthenospheric mantle can be to perform such a comparison estimated, this is made difficult by the fact that this part of the mantle partially because it is free of the complex thermal and chemical structure melts and differentiates before samples ever reach the surface. The composition imparted on the shallow mantle by and conditions in the mantle at depths significantly below the lithosphere must the lithosphere and melting be interpreted from geophysical observations combined with experimental processes. It contains a number of seismic discontinuities—sharp jumps data on mineral and rock properties. Fortunately, the transition zone, which in seismic velocity, that are gener- extends from approximately 410 to 660 km, has a number of characteristic ally accepted to arise from mineral globally observed seismic properties that should ultimately place essential phase transformations (Agee 1998). These discontinuities have certain constraints on the compositional and thermal state of the mantle. features that correlate directly with characteristics of the mineral trans- KEYWORDS: seismic discontinuity, phase transformation, pyrolite, wadsleyite, ringwoodite formations, such as the proportions of the transforming minerals and the temperature at the discontinu- INTRODUCTION ity. -
Seismic Anisotropy and Shear- Wave Splitting
Seismic Anisotropy and shear- wave splitting Steve Gao, Missouri S&T http://www.mst.edu/~sgao 1 Anisotropy and Seismic Anisotropy • Anisotropy – property of a material is directionally dependent • Seismic anisotropy – seismic property (i.e., traveling velocity of waves) is directionally dependent 2 What is seismic anisotropy? • For P-waves and same-type surface waves: dependence of seismic wave speed on the direction of propagation of the waves • For S-waves and different type surface waves: dependence of seismic wave speed on the polarization (or vibration) direction of the waves 3 Seismic Phases Inside the earth, the velocity is not a smooth function of depth. To make things more complicated, there are several major interfaces including the core-mantle boundary and the outer core-inner core boundary, as well as the crust- mantle boundary (Moho). At the boundaries (and also at everywhere), the raypath is controlled by the Snell’s law Q: Why the raypath curves toward the center in the outer core? What kind of wave is it? 4 Raypaths for selected seismic waves P: direct P wave traveled (refracted) through the mantle PP: P wave refracted from the mantle, reflected at Earth’s surface,then refracted again PPP: P wave reflected twice at the Earth’s surface PKP: P wave refracted from the mantle through the outer core, then back through the mantle PKIKP: P wave refracted from the mantle, through the outer core, inner core and back out PKiKP: P wave bounced back from the inner core/outer core boundary PcP: P wave reflected off the outer core 5 -
Faults (Shear Zones) in the Earth's Mantle
Tectonophysics 558-559 (2012) 1–27 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Review Article Faults (shear zones) in the Earth's mantle Alain Vauchez ⁎, Andréa Tommasi, David Mainprice Geosciences Montpellier, CNRS & Univ. Montpellier 2, Univ. Montpellier 2, cc. 60, Pl. E. Bataillon, F-34095 Montpellier cedex5, France article info abstract Article history: Geodetic data support a short-term continental deformation localized in faults bounding lithospheric blocks. Received 23 April 2011 Whether major “faults” observed at the surface affect the lithospheric mantle and, if so, how strain is distrib- Received in revised form 3 May 2012 uted are major issues for understanding the mechanical behavior of lithospheric plates. A variety of evidence, Accepted 3 June 2012 from direct observations of deformed peridotites in orogenic massifs, ophiolites, and mantle xenoliths to seis- Available online 15 June 2012 mic reflectors and seismic anisotropy beneath major fault zones, consistently supports prolongation of major faults into the lithospheric mantle. This review highlights that many aspects of the lithospheric mantle defor- Keywords: Faults/shear-zones mation remain however poorly understood. Coupling between deformation in frictional faults in the upper- Lithospheric mantle most crust and localized shearing in the ductile crust and mantle is required to explain the post-seismic Field observations deformation, but mantle viscosities deduced from geodetic data and extrapolated from laboratory experi- Seismic reflection and anisotropy ments are only reconciled if temperatures in the shallow lithospheric mantle are high (>800 °C at the Rheology Moho). Seismic anisotropy, especially shear wave splitting, provides strong evidence for coherent deforma- Strain localization tion over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults. -
Radial Seismic Anisotropy As a Constraint for Upper Mantle Rheology ⁎ Thorsten W
Available online at www.sciencedirect.com Earth and Planetary Science Letters 267 (2008) 213–227 www.elsevier.com/locate/epsl Radial seismic anisotropy as a constraint for upper mantle rheology ⁎ Thorsten W. Becker a, , Bogdan Kustowski b, Göran Ekström c a Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA b Department of Earth and Planetary Sciences, Harvard University, Cambridge MA, USA c Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Palisades, NY, USA Received 26 July 2007; received in revised form 9 November 2007; accepted 22 November 2007 Available online 8 December 2007 Editor: R.D. van der Hilst Abstract Seismic shear waves that are polarized horizontally (SH) generally travel faster in the upper mantle than those that are polarized vertically (SV), and deformation of rocks under dislocation creep has been invoked to explain such radial anisotropy. Convective flow of the upper mantle may thus be constrained by modeling the textures that progressively form by lattice-preferred orientation (LPO) of intrinsically anisotropic grains. While azimuthal anisotropy has been studied in detail, the radial kind has previously only been considered in semi-quantitative models. Here, we show that radial anisotropy averages as well as radial and azimuthal anomaly-patterns can be explained to a large extent by mantle flow, if lateral viscosity variations are taken into account. We construct a geodynamic reference model which includes LPO formation based on mineral physics and flow computed using laboratory-derived olivine rheology. Previously identified anomalous vSV regions beneath the East Pacific Rise and relatively fast vSH regions within the Pacific basin at ~150 km depth can be linked to mantle upwellings and shearing in the asthenosphere, respectively. -
No.294 "Flow and Seismic Anisotropy in the Mantle Transition Zone: Shear
第294回ジオダイナミクスセミナー Deformation of olivine and implicationsfor the dynamics of Earth’s upper mantle Speaker:Tomohiro Ohuchi (Postdoctral Fellow, GRC) 主催:愛媛大学地球深部ダイナミクス研究センター 日時:4/22(金)午後 4時30分~ Abstract 場所:総合研究棟4F 共通会議室 Crystallographic preferred orientation (CPO) of olivine, which is developed by dislocation creep, controls the seismic anisotropy in the upper mantle. One of the remarkable observations on the upper mantle near subduction zones is a striking rotation of fast direction of shear-wave splitting across an arc. Trench-normal fast directions are observed in the back-arc side, but trench-parallel ones are observed in the fore-arc side (e.g, Smith et al., 2001; Nakajima and Hasegawa, 2004). The rotation of fast direction of shear-wave splitting has been attributed to the transition of mantle-flow direction from trench-normal flow (in the back-arc side) to trench-parallel flow (in the fore-arc side) under the assumption that the A-type olivine fabric (developed by the (010)[100] slip system), which has a seismic fast-axis orientation subparallel to the shear direction, is assumed to be the unique cause of seismic anisotropy (e.g., Russo and Silver, 1994). However, this model is not fully supported by other observations such as geodetic observations. Recent laboratory results have shown that the flow-parallel shear wave splitting is caused not only by A- type olivine fabric but also by C- (developed by the (100)[001] slip system) and E-type (by the (001)[100] slip system) olivine fabrics (Jung and Karato, 2001; Katayama et al., 2004). Moreover, flow-perpendicular shear wave splitting is also found to be caused by the B-type olivine fabric (developed by the (010)[001] slip system) (Jung and Karato, 2001). -
Chapter 20. Fabric of the Mantle
Chapter 20 Fabric of the mantle A nisotropy this edition. There was a very large section on anisotropy since it was a relatively new concept And perpendicular now and now to seismologists. There was also a large section devoted to the then-novel thesis that seismic transverse, Pierce the dark soil and as velocities were not independent of frequency, they pierce and pass Make bare the and that anelasticity had to be allowed for in esti secrets of the Earth's deep heart. mates of mantle temperatures. Earth scientists no longer need to be convinced that anisotropy Shelley, Prometheus Unbound and anelasticity are essential elements in Earth physics, but there may still be artifacts in tomo Anisotropy is responsible for large variations in graphic models or in estimates of errors that seismic velocities; changes in the orientation of are caused by anisotropy. At the time of the mantle minerals, or in the direction of seismic first edition of this book - 1989 - the Earth waves, cause larger changes in velocity than can was usually assumed to be perfectly elastic and be accounted for by changes in temperature, isotropic to the propagation of seismic waves. composition or mineralogy. Plate-tectonic pro These assumptions were made for mathemati cesses, and gravity, create a fabric in the mantle. cal and operational convenience. The fact that Anisotropy can be microscopic - orientation of a large body of seismic data can be satisfacto crystals - or macroscopic - large-scale lamina rily modeled with these assumptions does not tions or oriented slabs and dikes. Discussions of prove that the Earth is isotropic or perfectly velocity gradients, both radial and lateral, and elastic. -
Discovery of Post-Perovskite and New Views on the Core–Mantle Boundary Region
Discovery of Post-Perovskite and New Views on the Core–Mantle Boundary Region Kei Hirose1 and Thorne Lay2 DOI: 10.2113/GSELEMENTS.4.3.183 phase transition of MgSiO3 perovskite, the most abundant component the core and by mantle convection should exist in the boundary layer. of the lower mantle, to a higher-pressure form called post-perovskite Chemical heterogeneity is likely to Awas recently discovered for pressure and temperature conditions in the exist in the boundary layer as a vicinity of the Earth’s core–mantle boundary. This discovery has profound result of ancient residues of mantle implications for the chemical, thermal, and dynamical structure of the lowermost differentiation and/or subsequent contributions from deep subduction mantle called the D” region. Several major seismological characteristics of the of oceanic lithosphere, partial melt- D” region can now be explained by the presence of post-perovskite, and the ing in the ultralow-velocity zone specific properties of the phase transition provide the first direct constraints on (ULVZ) just above the core–mantle boundary (CMB), and core–mantle absolute temperature and temperature gradients in the lowermost mantle. chemical reactions. The current Here we discuss the current understanding of the core–mantle boundary region. understanding of D” resulting from recent progress in characterizing KEYWORDS: post-perovskite, phase transition, D” layer, seismic discontinuity this region is reviewed below. INTRODUCTION DISCOVERY OF THE POST-PEROVSKITE Large anomalies in seismic wave velocities have long been PHASE TRANSITION observed in the deepest several hundred kilometers of the For several decades, MgSiO3 perovskite has been recognized mantle (the D” region) (see Lay and Garnero 2007) (FIG. -
The Hawaii-2 Observatory: Observation of Nanoearthquakes, Seismol
Butler, R. The Hawaii-2 Observatory: Observation of Nanoearthquakes, Seismol. Res. Lett., 74(10), 290-297, (2003). 1 The Hawaii-2 Observatory: Observation of Nanoearthquakes Rhett Butler The IRIS Consortium, Washington DC Introduction The Hawaii-2 Observatory (H2O, http://www.iris.edu/GSN/) was established on the seafloor between Hawaii and California (Figure 1) in 1998 using the retired AT&T Hawaii-2 undersea telephone cable for power and telemetry (Butler et al., 2000; Duennebier et al., 2002, Pettit et al., 2002; Harris and Duennebier, 2002). This is the first undersea seismic station in the Global Seismographic Network. The on-site instrumentation includes a broadband Guralp CMG-3 and 4.5 Hz Geospace HS1 geophones, buried 0.5 m into the seafloor mud, and an OAS E2PD hydrophone 0.5 m above the seafloor. All of the data are natively sampled at 160 sps continuously and sent via the Hawaii-2 cable to Oahu where the data are distributed by the University of Hawaii to IRIS. Data are available in real-time via a Live Internet Seismic Server (LISS) server, and are being used by the Pacific Tsunami Warning Center and the National Seismic Network. In early 2002, the Ocean Drilling Program drilled a borehole through 29±1 m of sediment into basalt about 1.5 km northeast of the current H2O instrumentation as a site for a future borehole sensor (Stephen et al., 2003). H2O was primarily sited to fill a large gap in global coverage between Hawaii and California, and not to monitor any particular local or regional seismicity. -
Isotropic and Anisotropic P and S Velocities of the Baltic Shield Mantle
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 653 Isotropic and Anisotropic P and S Velocities of the Baltic Shield Mantle Results from Analyses of Teleseismic Body Waves TUNA EKEN ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 UPPSALA ISBN 978-91-554-7548-2 urn:nbn:se:uu:diva-102501 2009 Dissertation presented at Uppsala University to be publicly examined in Hambergsalen, Geocentrum, Villavägen 16, SE-752 36 Uppsala, Sweden, Uppsala, Friday, June 12, 2009 at 13:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Abstract Eken, T. 2009. Isotropic and Anisotropic P and S Velocities of the Baltic Shield Mantle. Results from Analyses of Teleseismic Body Waves. Acta Universitatis Upsaliensis. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 653. 110 pp. Uppsala. ISBN 978-91-554-7548-2. The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P- wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy.