Azimuthal anisotropy of Rayleigh waves in the crust in southern Tohoku area, Japan

Abstract

To reveal seismic anisotropy within the crust in the southern Tohoku area, Japan, we analyze the azimuthal anisotropy of Rayleigh waves in the frequency range of 40-65 mHz. Rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014. The frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust: (1) north-south fast velocity direction around the upper crust and (2) trench-normal fast velocity direction in the deeper crust. We also conduct a time-lapse analysis of Rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 Tohoku-oki earthquake. We demonstrate that the azimuthal location of teleseismic events influences the accuracy of time-lapse analysis. Moreover, a decrease of azimuthal anisotropic ratio in time-lapse results might be related to changes in the velocity structure associated with release of crustal stress during the Tohoku-oki earthquake.

Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: effect of evolving melt texture

“…The interpretation of seismic and electrical anomalies in terms of melt fraction often results in conflicting estimations as to the extent of melting in the asthenosphere [Pommier and Garnero, 2014;Karato, 2014]. A conductivity model based on major element chemistry of melt attributed the apparent inconsistencies in conductivity measurements to possible chemical variations in the melt [Pommier and Garnero 2014]. Their model predicts that low degree melting of peridotite produces melt that is more conductive than basaltic compositions.…”

“…Both experimental and theoretical models acknowledge that the volume fraction and spatial distribution of melt play an integral part in modifying the seismic and electrical properties of partially molten rocks. However, large discrepancies still remain in the laboratory estimations (regardless of the technique) of the amount of melt volume fraction present in the asthenosphere [Pommier and Garnero, 2014;Karato, 2014]. Due to the large number of studies addressing the electrical properties of melt, the disagreement between laboratory-based electrical conductivity measurements is highly visible [Karato, 2014].…”

“…The influence of volatile contents could be one of the key parameters controlling the conductivity of the resulting melt [Yoshino et al, 2010;Ni et al, 2011;Sifré et al, 2014]. A model based on chemical variation in the melt has been proposed to explain the apparent disagreement on melt fraction estimations between electrical conductivity measurements and seismic models [Pommier and Garnero, 2014]. The discrepancy may primarily stem from the absence of systematic experimental investigation into the structural factors influencing the EC in partially molten systems.…”

Advanced surface-wave analysis for 3D ocean bottom cable data to detect localized heterogeneity in shallow geological formation of a CO2 storage site

“…If 3D seismic data were densely acquired in a wide variety of directions, the proposed 3D surface-wave analyses can be applied to estimate azimuthal-dependent surface waves. As demonstrated by Alvizuri and Tanimoto (2011) and Ikeda and Tsuji (2014) using teleseismic data, azimuthal anisotropic phase velocity can be effective in estimating underground stress conditions. In CCS projects, such anisotropic properties are important in evaluating fracture orientation and intensity, which directly controls the injected CO2 flow.…”

A Cosserat Breakage Mechanics model for brittle granular media

“…Particularly in a geological context, multi-physical couplings have a strong influence on the formation of localisation zones with temperature and pore fluid pressure interactions being well studied (e.g. Sulem, 2010;Sulem, Stefanou and Veveakis, 2011) and chemical changes such as decomposition and dehydration (Brantut and Sulem, 2012;Brantut, Sulem and Schubnel, 2011;Stefanou andSulem, 2013, 2014;Sulem and Famin, 2009;Veveakis, Alevizos and Vardoulakis, 2010;Veveakis, Stefanou and Sulem, 2013, among others) have also been shown to contribute towards forming but also potentially limiting the size of localisation zones. Recently, fully coupled systems that integrate the effects of temperature, pressurisation and chemical reactions have been explored (Platt, Rudnicki and Rice, 2014;Rattez, Stefanou and Sulem, 2018;Rattez et al, 2018a,b;Rice, Rudnicki and Platt, 2014;Sulem and Stefanou, 2016).…”