A szeizmikus anizotrópia jelentősége a szubdukciós zónák kutatásában

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1 A szeizmikus anizotrópia jelentősége a szubdukciós zónák kutatásában
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2 Szeizmikus anizotrópia: A rugalmas anyagok egy belső tulajdonsága, amely meghatározza a szeizmikus hullámok terjedési tulajdonságait. Leírása általános eset: rugalmassági tenzor (21 rugalmassági állandó), amely a feszültség és a deformáció kapcsolatát írja le Földköpeny magasabb szimmetriával közelíthető: 5 független állandó, szimmetriatengely Jelentősége: Deformáció és anizotrópia kapcsolata Ásványok kristálytani irányítottsága (CPO v. LPO) Alaki irányítottság (SPO) pl.: olvadékzsebek, törések Hatása a szeizmikus hullámok terjedésére: - Nyíró hullámok kettős törése (shear wave splitting) P hullámok sebessége irányfüggő P-S konvertálódás jellege A felszíni hullámok (Love és Rayleigh) terjedési sebessége eltérő Fajtái: Radiális anizotrópia: horizontálisan és vertikálisan polarizált hullámok terjedési sebessége különböző Azimutális anizotrópia: a hullámterjedési sebesség egy vízszintes síkon belük függ az azimuttól

3 Nem csak a földköpenyben, hanem a Föld egyéb rétegeiben is megfigyeltek szeizmikus anizotrópiát.
4%, VSH>SSV Olyan deformációs folyamat amely nem hoz létre CPO-t

4 Wenk 2004: Anisotropy was documented in layers of the transition zone (400–700 km) where olivine breaks down to high-pressure phases. Particularly, just above the 660 km discontinuity anisotropy is pronounced and may be related to intense deformation near those phase transitions (Trampert and Heijst 2002). But contrary to the regular pattern of anisotropy in the uppermost mantle, anisotropic regions in the transition zone are confined, vary in extent and depth, and are to some degree associated with subduction (Fouch and Fischer 1996). There is little evidence for anisotropy between 700 km and 2700 km (a few exceptions have been noted by Barruol and Hoffmann 1999), but this may be partially due to limited crossing ray coverage. However, the deep mantle, in the vicinity of the boundary with the core (D” layer), reveals itself as a fascinating and heterogeneous area of the Earth (e.g., Kellogg et al. 1999). Geodynamic modeling suggests very strongly deformed subducting slabs (McNamara et al. 2001) and seismologists have observed anisotropy that could be due to texturing (e.g., Fouch et al. 2001, Kendall and Silver 1996, Ritsema 2000, Thomas and Kendall 2002, Vinnik et al. 1998), periodic layering (Garnero 2000), or partial melt (Lay et al. 1998). The outer core is liquid and thus isotropic, but it is firmly established, both from body waves and free oscillation observations, that the solid inner core is again anisotropic.

5 A megfigyelt szeizmikus anizotrópiát a felső köpenyben általában az olivinhez kötik, amely nagy mennyiségben van jelen a felső köpenyben és nagy mértékben anizotróp (~18%). Long & Becker 2010 Mainprice 2007 Schematic diagram of the relationship between horizontal mantle flow and the resulting azimuthal anisotropy for different olivine fabric types. (A) In most regions of the asthenosphere, olivine fabric is expected to be A-, C-, or E-type, which would result in a fast axis of anisotropy that is parallel to the (horizontal) flow direction beneath a seismic station. (B) If B-type olivine fabric, rather than A-, C-, or E-type, dominates, then the same flow direction would result in an observed fast direction that is different by 90°. The classification of olivine CPO proposed by Jung and Karato (2001) as a function of stress and water content. The water content scale in ppm H/Si is that originally used by Jung and Karato. The water content scale in ppm wt is more recent calibration used by Bolfan-Casanova (2005). The numbers in brackets are the percentage of samples with the fabric types found in the database of Ben Ismaı¨l and Mainprice (1998).

6 A szubdukciós zónákban, az olivinen kívül, az olvadékok és egyéb víztartalmú ásványok is szerepet játszhatnak a megfigyelt anizotrópia kialakulásában. Mainprice and Ildefonse (2009): most hydrous minerals relevant for subduction zones have single-crystal anisotropies that are as strong as or stronger than that of olivine, including antigorite (shear wave anisotropy AVs =68%), talc (AVs = 68%), chlorite (AVs = 76%), hornblende (AVs = 31%), brucite (AVs = 31%), and phase A (AVs = 18%). Illustration of the possible effect of serpentinite LPO on subduction zone anisotropy. (a) Pole figures (equal-area lower-hemisphere projections) of crystallographic orientation of deformed antigorite for a sample deformed at 300_C to 200% strain. Horizontal line corresponds to the (dextral) shear direction. There is a strong concentration of [001] axes normal to the shear plane and a relatively weak concentration of [100] axes subparallel to the shear direction (the [001] axis of single-crystal antigorite is much slower than the [100] (fast) and [010] (intermediate) axes, leading to a maximum single-crystal shear wave anisotropy of 68% [Mainprice and Ildefonse, 2009]). (b) Predicted patterns of shear wave splitting for the experimentally determined LPO shown in Figure 10b. The predicted polarization of the fast shear wave for each possible wave propagation direction is shown. (c) Schematic cartoon of proposed contribution from antigorite anisotropy beneath the Ryukyu arc. Deformation of serpentine is concentrated in a relatively thin layer above the subducting plate, with the seismically slow axis (c axis) aligning normal to the plate interface. This results in a strong anisotropy with trench-parallel fast direction in the forearc wedge. [100] [010] [001] Katayama et al. [2009].

7 A szeizmikus anizotrópia felszíni és térhullámok segítségével mutatható ki. A leggyakrabban használt módszer a shear wave splitting, amely a teleszeizmikus S hullámok kettőstörésén alapszik. Sketch of seismic raypaths commonly used in shear wave splitting studies to probe different parts of subduction systems, including local S phases, SKS phases, and direct teleseismic S, after Long and Silver [2009]. (Long 2013) Schematic diagram of shear wave splitting due to upper mantle anisotropy, after Garnero ( and Crampin (1981). An incident, nearly vertical shear wave has an initial linear polarization before it hits the anisotropic volume (box). The shear wave splits into two components, one polarized parallel to the fast direction of the medium (blue) and one with orthogonal polarization (red). As the wave continues to propagate through the anisotropic medium, the components accumulate a time delay. The shear wave splitting parameters ϕ, which correspond to the azimuth of the fast component, and δt, the time delay between the fast and slow components, are measured directly from the seismogram. (Long & Becker 2010)

8 PRF (P Receiver Function) analízis az anizotróp réteg geometriájáról és/vagy dőlő réteghatárokról nyújthat információt. Szanyi Gyöngyvér engedélyével. Sketch of seismic raypaths commonly used in receiver function and P tomography studies to probe anisotropy in the mantle wedge and slab. (Long 2013) P hullám tomográfia elsősorban a köpenyék anizotrópiájának a meghatározására alkalmas.

9 A köpenyék S hullám anizotrópiája complex, jelentős területi változatosságot és széles tartományban szóró beérkezési idő különbséget figyeltek meg. Summary map of wedge splitting worldwide. Arrows indicate the first-order patterns in average fast direction; where multiple arrows are present, this indicates a spatial transition in observed φ. Arrows are color coded by fast direction observations; magenta arrows indicate dominantly trench-parallel φ, blue arrows indicate dominantly trench-perpendicular φ, yellow arrows indicate complex and variable φ, red arrows indicate a transition from trench-parallel φ close to the trench to trench-perpendicular φ farther away, and green arrows indicate the opposite transition (from trench-perpendicular φ close to the trench to trench-parallel f farther away). Beneath the name of each subduction zone, we indicate the range of observed delay times. (Long & Wirth 2013)

10 A szubdukálódó lemez anizotrópiájáról kevés, és egymásnak sokszor ellentmondó információval rendelkezünk. Tectonic background of the study area. The bold curves denote the plate boundaries. The dashed line shows the estimated boundary between the Amurian and Okhotsk plates. The solid triangles denote the active volcanoes. The bold arrow indicates the motion direction of the Pacific plate relative to the Eurasian plate. (Huang et al. 2011) Raypaths for the upper crust (blue lines), LF (red lines), upper plane (gray lines from black crosses), and lower plane earthquakes (gray lines from the open circles) in the subducting Pacific slab used in this study. Solid squares at the top denote the seismic stations, and the triangle indicates the location of the volcanic front (VF). The dashed lines denote the Conrad and the Moho discontinuities and the upper boundary of the subducting Pacific slab at the latitude of 40°N. The numbers of the measurements from the four types of earthquakes are shown in the inset.

11 A cartoon showing the anisotropic structures under NE Japan
A cartoon showing the anisotropic structures under NE Japan. Anisotropy is considered to exist in the gray shaded area. The dark and light colors indicate strong and weak anisotropy, respectively. The horizontal bars and circled crosses denote the trench‐normal and trench‐parallel FVDs, respectively. The curve with a downgoing arrow denotes the mantle flow in the mantle wedge driven by the subduction of the Pacific slab, while the upgoing arrow indicates the secondary mantle flow containing partial melts. The thick horizontal bar at the top denotes the land area. The triangles indicate the active volcanoes. The inverted triangle denotes the Japan Trench. Horizontal solid arrows denote the compressional regime in NE Japan, while double blank arrows denote the bending and unbending in the subducting Pacific slab. Dipping lines in the outer rise part of the Pacific plate denote the normal faults. The dashed lines denote the Conrad and the Moho discontinuities and the upper and lower boundaries of the subducting Pacific slab, respectively.

12 A szubdukálodó lemez alatti köpeny (sub-slab) S hullám anizotrópiája kevésbé változatos, mint a köpenyék anizotrópiája. A gyorsabb S hullám orientációja többnyire párhuzamos, ritkábban merőleges a szubdukciós zónára, azonban széles tartományban szóró beérkezési idő különbséget figyeltek meg. Summary map of sub-slab anisotropy worldwide, updated from the compilations of Long and Silver [2009] and Paczkowski [2012]. Arrows indicate the dominant sub-slab fast direction (either trench-parallel or trench-perpendicular). The range of observed delay times is indicated beneath the name of each subduction zone.

13 Számos numerikus és laboratoriumi modellezési eljárást fejlesztettek ki, hogy megfejtség a szubdukciós zónákban lejátszódó geodinamikai folyamatokat. Faccenda and Capitanio 2012 Long et al. 2007 (a) Side view of the model showing the development of seismic anisotropy (colored bars). Purple surface is the contour of the density field around the subducting plate. Bars represent the orientation of the symmetry axis of anisotropy (based on a hexagonal symmetry class); length and color are proportional to the amount of anisotropy. White arrows indicate a layer of trench-perpendicular fast axes directly beneath the subducting slab (due to entrained flow) and a deep zone of trench-parallel fast axes beneath this layer (due to toroidal flow). (b) Oblique view of the velocity field at 300 km depth associated with this model. (a) Velocity field for the basic model setup (model A). The portion of the model corresponding to depths between 50 and 660 km is shown. Arrows correspond to velocities at individual nodes. Axis labels correspond to the depths (z) and horizontal coordinates (x) in km. (b) Streamlines traced through the velocity field displayed in (a). Alternating colours are for clarity. (c) Finite strain ellipses calculated for model A along the streamlines shown in (b). Finite strain ellipses are shown as coloured bars, with the orientation and length of the bar corresponding to the orientation and strength of the maximum finite strain direction.

14 Eddig még nem sikerült egy általános modelt létrehozni, amely valamennyi szubdukciós zónára alkalmazható. Long & Silver 2008 Sub-slab mantle: trench-parallel flow model B-típusú olivin CPO A szubdukálódó lemez magával vonszolja az asztenoszférát is Óceáni lemez: “frozen lithospheric anisotropy” Hidratáció és törések Schematic diagram of model, showing the dominance of 3D flow beneath the slab and the competing influence of 2D and 3D flow fields in the mantle wedge. Köpenyék: 2D corner flow + B-típusú olivine CPO Trench-parallel flow model Lokális köpenyáramlások a köpenyékben +/- olvadék

15 Hivatkozott irodalom (a vastagon szedett két cikkel érdemes kezdeni):
Faccenda, M., & Capitanio, F. A. (2012). Development of mantle seismic anisotropy during subduction-induced 3-D flow. Geophysical Research Letters, 39(11), L doi: /2012GL051988 Huang, Z., Zhao, D., & Wang, L. (2011). Frequency-dependent shear-wave splitting and multilayer anisotropy in northeast Japan. Geophysical Research Letters, 38(8), L doi: /2011GL046804 Katayama, I., Hirauchi, K.-i., Michibayashi, K., & Ando, J.-i. (2009). Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge. Nature, 461(7267), doi: Long, M. D. (2013). CONSTRAINTS ON SUBDUCTION GEODYNAMICS FROM SEISMIC ANISOTROPY. Reviews of Geophysics, 51(1), doi: /rog.20008 Long, M. D., & Becker, T. W. (2010). Mantle dynamics and seismic anisotropy. Earth and Planetary Science Letters, 297(3-4), doi: DOI /j.epsl Long, M. D., Hager, B. H., De Hoop, M. V., & Van Der Hilst, R. D. (2007). Two-dimensional modelling of subduction zone anisotropy with application to southwestern Japan. Geophysical Journal International, 170(2), doi: /j X x Long, M. D., & Silver, P. G. (2008). The Subduction Zone Flow Field from Seismic Anisotropy: A Global View. Science, 319(5861), doi: /science Long, M. D., & Silver, P. G. (2009). Mantle flow in subduction systems: The subslab flow field and implications for mantle dynamics. Journal of Geophysical Research: Solid Earth, 114(B10), B doi: /2008JB006200 Long, M. D., & Wirth, E. A. (2013). Mantle flow in subduction systems: The mantle wedge flow field and implications for wedge processes. Journal of Geophysical Research: Solid Earth, 118(2), doi: /jgrb.50063 Mainprice, D. (2007) Seismic Anisotropy of the Deep Earth from a Mineral and Rock Physics Perspective. In G. Schubert (Ed.), Treatise on Geophysics (pp ). Amsterdam: Elsevier. Paczkowski, K. (2012). Dynamic analysis of modifications to simple plate tectonics theory. (Ph.D. Thesis), Yale University. Wenk, H. R. (2004). The Texture of Rocks in the Earth's Deep Interior: Part II. Application of Texturing to the Deep Earth. In K. H. J. B. Editors-in-Chief:  , W. C. Robert, C. F. Merton, I. Bernard, J. K. Edward, M. Subhash & V. Patrick (Eds.), Encyclopedia of Materials: Science and Technology (Second Edition) (pp. 1-11). Oxford: Elsevier. Videók, amiket érdemes megnézni: