Title: On convective memory

Abstract:

Plate tectonics is the surface expression of mantle convection, but many aspects of the present-day tectonic setting depend on the history of how this system has evolved over time. I review recent work across spatio-temporal scales addressing how such convective memory can be used to validate tectonic scenarios and to better understand and predict how plate boundaries evolve consequently. Seismic anisotropy in the upper mantle is one recorder of convective deformation and the duration over which textures are reworked controls the lifespan of memory. This means that the asthenospheric depths show us present-day to recent convection, while shallower regions may allow distinguishing between different plate tectonic scenarios over the last ~50 Ma. Uncertainties about those scenarios and slab rheology mean that our understanding of the important issue of the path that slabs take into the deep mantle is still incomplete, leading to a range of estimates of mantle memory of past subduction. One particular question is how slabs are deformed upon bending in the trench, and I show results from convection models with rheological memory that can reproduce large offset faulting at the trench. Those models also show emergent slab segmentation, which may be relevant for the interpretation of deep mantle anomalies such as under Japan. The same sort of lithospheric rheologies with damage produce interesting effects in global mantle convection models which generate plate-like convection, and I close with an exploration of the typical timescales of fluctuations in convective heat transport as a function of plate boundary dynamics.

題目：損傷制御型コンクリート系構造の実現を目指して

要旨：

これまで，地震災害時の人命保護はもとより，建築物の骨組，非構造材，継続使用性などを保護できる損傷制御型コンクリート系構造の実現を目指して研究を実施してきた。制振ダンパーをコンクリート系構造物に合理的に活用して，地震時の応答低減，靭性能向上，損傷抑制を実現するため，コンクリート系部材との接合部，制振ダンパーを取り付けたコンクリート系構造の部材・部分架構・全体架構の構造実験と数値解析を行い，その力学挙動把握と構造性能評価について研究を行ってきた。近年では，鉄筋とコンクリートの付着を除去することで部材の損傷を制御する技術や，既存建築物の耐震補強工法の開発，柱梁接合部の新たな損傷制御技術の開発なども実施している。また，ドローンを用いた建築物の損傷評価，災害調査に関する先端的な研究も行っている。さらに，国内外で起こった大規模地震の際には現地調査を実施し，調査結果の分析や，被災した建築物の再現解析なども行っている。本セミナーでは，これまでに行ってきた研究活動と，今後予定している研究活動について紹介したい。

Title: An Ocean-Bottom View of Mantle Convection Beneath the Pacific Basin

Abstract:

The Pacific basin provides an outstanding natural laboratory for studying a wide range of tectonic and upper-mantle dynamic processes: seafloor spreading, hotspots and other midplate melting and volcanism, multiscale thermal convection, and subduction dynamics, to name just a few.  Historically, direct geophysical constraints on these processes were limited to low-resolution, long-wavelength imaging due to the restriction of most geophysical instrumentation on the surrounding continents and isolated islands.  Over the last decade, advances in seafloor instrumentation (both seismic and electromagnetic) has enabled innovative high-resolution, localized investigation of many of these processes under the umbrella of the grassroots international collaboration PacificArray.  Here I introduce new emerging results from the US ORCA (OBS Research on Convecting Asthenosphere) project, consisting of two year-long seismic arrays in the central and south Pacific.  ORCA was designed to image upper-mantle seismic wavespeeds in two regions where satellite-derived gravity variations display long, linear structures suggestive of small-scale convection in the upper mantle.  At the first site, we find linear blobs of fast and slow material in the mantle beneath the oceanic plate, parallel to the gravity features. These represent cold sinking and warmer rising material, revealing a highly dynamic convective system underneath the plate.  By combining these results with constraints on absolute shear velocity and seismic anisotropy in the two ORCA regions and previous analyses from across the Pacific, we are gaining an improved understanding of the processes controlling the formation, modification, and evolution of the ocean plates and the underlying convection system.

Title: Mapping subduction-induced mantle flows

Abstract:
Laboratory experiments and geodynamic flow simulations demonstrate that poloidal- and toroidal-mode mantle flows develop around subduction zones, driven by the downdip motion and rollback of subducting slabs. To date, inferring deep mantle flow circulation patterns in actual subduction environments using shear wave splitting or surface wave tomography remains elusive, due to limited depth or lateral resolution of these techniques. In this study, we use a new 3-D azimuthal anisotropy model constrained by full waveform inversion to infer deep subduction-induced mantle flows underneath the Middle American and Cascadian subduction zones. At depths shallower than 150~km, poloidal-mode mantle flow is observed perpendicular to the trajectory of the Middle American and Cascadian Trenches. From 300 to 450~km depth, return flows surround the lateral edges of the descending Rivera, Atlantic and Gorda slabs. Furthermore, at 700~km, the study region is dominated by the Farallon anomaly, with fast axis directions perpendicular to its strike, suggesting the development of latticepreferred orientations by substantial strains. These observations provide depthdependent constraints on seismic anisotropy for future mantle flow simulations, and call for further investigations about the deformation mechanisms and elasticity of minerals in the transition zone and uppermost lower mantle.