Collaborative Research: Anisotropy and Mantle Flow Beneath Japan from Seismological Observations and Geodynamical Modeling

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Proper characterization of elastic anisotropy can help understand the causes, mechanisms, and consequences of tectonic deformation in the upper mantle and in other regions of the Earth's deep interior and beneath regions of active tectonics. Unfortunately, our understanding of seismic anisotropy is still unsatisfactory and interpretation of seismological diagnostics of anisotropy (such as shear wave birefringence) is often ambiguous. Gaining full understanding of seismic anisotropy and flow beneath complex tectonic regions has, therefore, remained an outstanding challenge of modern seismology and geodynamics. In collaboration with colleagues in France, scientists from MIT and Colorado School of Mines propose to meet this challenge through the explicit integration of powerful data sets and forward and inverse modeling tools. In particular, they aim to improve the understanding of 3-D seismic anisotropy and tectonic deformation in the upper mantle near subduction zones, which are tectonic regions that are of particular societal interest because of their association with significant volcanic and seismic hazard. They will focus on Japan, where geophysical instrumentation is dense because of the need for the continuous monitoring of seismic and volcanic activity.Data from these dense networks can be obtained through the internet, thanks to scientists in Japan, and the MIT-CSM team has already assembled a large data set from 64 broadband seismic stations. Through examination of ~12,000 individual seismograms, they compiled a database of ~1330 high-quality shear wave splitting measurements for S, SKS/SKKS, and ScS, and more will be added later. They propose to improve the quantitative interpretation of the splitting maps by combining two complementary approaches. First, they wish to extend their numerical modeling of upper mantle flow from 2.5 to 3-D, incorporating (evolving) velocity boundary conditions and realistic geometries and rheologies. Second, they plan to develop a novel technique for tomographic inversion of the observed shear wave polarizations and split times. The flow models will be used to design the parameterization and regularization of the inversion, and, in turn, the seismic imaging results will be used to update the flow models. Such an explicitly cross-disciplinary approach will not only yield new insight into the 3-D anisotropy and flow field beneath Japan. Indeed, the powerful new tools and concepts for data analysis, modeling, and interpretation can also be applied to data from such new initiatives as USArray/Earthscope.