This chapter discusses the deep structure of island arc magmatic regions as inferred from seismic observations. Recent seismic observations provide possible evidence for deep-seated magmatic activity in some of the subduction zones of the world. Tomographic inversions for seismic-wave velocity structures delineate low-velocity zones in the crust and in the mantle wedge beneath active volcanoes. Seismic attenuation tomography also delineates similar zones of low-Q value in the crust and mantle wedge beneath active volcanoes, although they are less clearly imaged because of lower spatial resolution. The most typical example is that of the area beneath the northeastern Japan arc, where inclined P-wave low-velocity zones have been clearly imaged using data acquired through microearthquake observations with dense networks. The low-velocity zones with 2–6% of velocity lows are continuously distributed from the upper crust right under active volcanoes to the depth of 100–150 km in the mantle wedge, their thicknesses being about 50 km. They are approximately parallel to the dip (∼30°) of the underlying subducted Pacific plate, and their lower edges are 30–60 km apart from the top of the subducted plate. These low-velocity zones probably reflect the pathway of magma ascent from a deeper part of the mantle to the earth's surface, along which additional evidence for deep magmatic activity has been found.
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