[1] Repeated GPS surveys in the Mariana Islands show that the Mariana block is moving apart from the Philippine Sea plate. The velocities of the islands relative to the Philippine Sea plate range from about 15 mm/yr in the northern islands to about 45 mm/yr near Guam. The data also suggest convergence rates for the Mariana forearc with respect to the Pacific plate of 35-45 mm/yr at 19°N increasing to 55-70 mm/yr at 13.5°N. In addition, the velocity vectors show a slight north-south expansion of the arc. The estimated location of the Euler pole of the Mariana forearc with respect to the Philippine Sea plate is well south of the geographical point where the back-arc basin narrows to zero width.

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After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata-Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 À15 (m 2 ). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N-S or NE-SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.