Seismic velocities, anisotropy, and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones

2013 ◽  
Vol 118 (3) ◽  
pp. 1015-1037 ◽  
Author(s):  
Shaocheng Ji ◽  
Awei Li ◽  
Qian Wang ◽  
Changxing Long ◽  
Hongcai Wang ◽  
...  
Keyword(s):  
Shear Wave ◽  
Subduction Zones ◽  
Shear Wave Splitting ◽  
Seismic Velocities ◽  
Tectonic Implications ◽  
Wave Splitting

scholarly journals Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> axis orientations

2018 ◽  
Vol 12 (5) ◽  
pp. 1715-1734 ◽  
Author(s):  
Johanna Kerch ◽  
Anja Diez ◽  
Ilka Weikusat ◽  
Olaf Eisen
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Ice Core ◽  
Shear Wave Splitting ◽  
P Wave ◽  
Seismic Velocities ◽  
Polar Ice ◽  
S Wave ◽  
Crystal Anisotropy ◽  
Wave Splitting

Abstract. One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135 m s−1 for P-wave and 200 m s−1 for S-wave velocity compared to the earlier framework (estimated 1 % difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45 m s−1 for the alpine ice core and 59 m s−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185 m s−1 for P-wave and 280 m s−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281 m s−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200 m s−1 for P wave and more than 200 m s−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50 m s−1 per 10 cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding.

Anisotropy-revealed change in hydration along the Alaska subduction zone

Geology ◽  
2021 ◽  
Author(s):  
Colton Lynner
Keyword(s):  
Subduction Zone ◽  
Shear Wave ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Pacific Plate ◽  
Seismic Gap ◽  
Megathrust Earthquakes ◽  
Wave Splitting ◽  
Subducting Slab

Megathrust earthquake behavior in subduction zones is controlled by a variety of factors including the hydration state of the subducting slab. Increased hydration reduces the occurrence of great, damaging earthquakes by diminishing the strength of the material along the interface between tectonic plates. Understanding variations in hydration in subductions zones is necessary for properly assessing the overall hazard posed by each region. Fortunately, seismic anisotropy is strongly dependent upon hydration of the subducting crust and lithosphere. I present shear-wave splitting measurements that illuminate changes in anisotropy, and therefore hydration, of the subducting Pacific plate beneath the Alaska subduction zone (northern Pacific Ocean). Variations in shear-wave splitting directly correlate to changes in the behavior of great, megathrust earthquakes. My measurements show that the Shumagin seismic gap is characterized by a hydrated subducting slab, explaining the long-term lack of great earthquakes. Observations in the immediately adjacent Semidi segment, which experiences great events regularly, indicate a far less hydrated slab. These results are driven by the preferential alignment of paleo-spreading fabrics of the Pacific plate. Where fabrics are more closely aligned with the orientation of the trench, outer-rise faulting and plate hydration is enhanced. These results highlight the importance of changes in preexisting slab structures and subsequent hydration in the production of great, damaging earthquakes.

scholarly journals Shear Wave Splitting and Mantle Flow in Mexico: What Have we Learned?

2017 ◽  
Vol 56 (2) ◽  
Author(s):  
Raúl W. Valenzuela ◽  
Gerardo León Soto
Keyword(s):  
Shear Wave ◽  
Slow Wave ◽  
Upper Mantle ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Special Focus ◽  
Mantle Flow ◽  
Wave Splitting

A review is presented of the shear wave splitting studies of the upper mantle carried out in Mexico during the last decade. When a seismic wave enters an anisotropic medium it splits, which means that a fast and a slow wave are produced. Two parameters are used to quantify anisotropy. These are the fast polarization direction and the delay time between the fast and the slow wave. An example of the measurement technique is presented using an SKS phase because most observations are based on teleseismic data. Results of two studies using local S waves from intraslab earthquakes are also discussed. Key aspects of the interpretation of splitting measurements are explained. These include the depth localization of anisotropy, the relation-ship between olivine fabrics and mantle flow, the role of absolute plate motion, and the role of relative plate motions with a special focus on subduction zones. An important motivation for studying seismic anisotropy is that it makes it possible to constrain the characteristics of upper mantle flow and its relationship to tectonic processes. Mexico has many diverse tectonic environments, some of which are currently active, or were formerly active, and have left their imprint on seismic anisotropy. This has resulted in a wide variety of mechanisms for driving mantle flow. Broadly speaking, the discussion is organized into the following regions: Baja California peninsula, Western Mexican Basin and Range, northern and northeastern Mexico, the Middle America Trench, the Yucatán peninsula, and lowermost mantle anisotropy. Depending on the unique characteristics encountered within each region, the relationship between anisotropy and mantle flow is explored.

scholarly journals Shear wave splitting in the crust in North China: stress, faults and tectonic implications

2011 ◽  
Vol 187 (2) ◽  
pp. 642-654 ◽  
Author(s):  
Yuan Gao ◽  
Jing Wu ◽  
Yoshio Fukao ◽  
Yutao Shi ◽  
Ailan Zhu
Keyword(s):  
Shear Wave ◽  
North China ◽  
Shear Wave Splitting ◽  
Tectonic Implications ◽  
Wave Splitting
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