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In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10-13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

This paper describes a new testing method to examine the mechanical behavior of railroad ballast subjected to repeated train passages on ballasted track. Two types of cyclic loading tests, namely a single-point loading test and a movingwheel loading test, were performed with small-scale models of ballasted track. Next, a 'multi-ring shear apparatus' was developed as a type of torsional simple shear apparatus, and the applicability of a newly proposed multi-ring shear test to an element test of railroad ballast subjected to moving-wheel loads was examined by comparing the results of multi-ring shear tests with those of small-scale model tests. As a result, it was recognized that cumulative strain obtained from multi-ring shear tests is almost equivalent to the one derived from small-scale model tests. Moreover, it was revealed that the difference between loading methods has a considerable influence on the cyclic plastic deformation of railroad ballast because settlement in a moving-wheel loading test was much larger than the one in a single-point loading test. These results lead to the conclusion that a multi-ring shear test has an excellent applicability to the estimation of deformation behavior of granular materials subjected to moving- wheel loads. Key words: base course, cyclic load, gravel, railroad, special shear test. Cet article decrit une nouvelle methode d'essai pour etudier le comportement mecanique d'un ballast de voie ferree soumis a des passages repetitifs des trains sur le ballast de la voie. Deux types d'essais de chargement cyclique, l'essai a chargement sur un point et l'essai a chargement par une roue en deplacement, ont ete realises avec des modeles de ballast de voie a petite echelle. Ensuite, un << appareil de cisaillement multi-anneaux >> a ete developpe en tant que type d'appareil de cisaillement simple en torsion. L'applicabilite de l'essai propose en cisaillement multi-anneaux a ete evaluee sur un element de ballast de voie ferree soumis a un chargement par une roue en deplacement en comparant les resultats des essais de cisaillement multi-anneaux avec ceux des essais obtenus avec le modele a petite echelle. Il a ete observe que la deformation cumulative obtenue par les essais de cisaillement multi-anneaux est a peu pres equivalente a celle derivee des essais avec le modele a petite echelle. De plus, il a ete demontre que la difference entre les methodes de chargement a une influence considerable sur la deformation plastique cyclique d' un ballast de voie ferree, puisque le tassement subit lors de l' essai a chargement par une roue en deplacement est beaucoup plus grand que celui subit lors d' un essai a chargement sur un point unique. Ces resultats permettent de conclure que l'essai de cisaillement multi-anneaux est bien applicable a l'estimation du comportement en deformation de materiaux granulaires soumis a des chargements par une roue en deplacement. Mots-cles : assise de base, chargement cyclique, gravier, voie ferree, essai de cisaillement special. [Traduit par la Redaction], Introduction The study of 'track deterioration' is one of the principal assignments in railway engineering because track deterioration has serious consequences on the safety of train operations (Selig and Waters [...]

The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000-10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (~50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental crust. Keywords: intra-oceanic arc, continental crust, crustal evolution, seismic imaging, Izu-Bonin arc. doi: 10.1130/G23901A.1