We combine two complementary seismological methods to study the deep roots of arc volcanism in the Alaskan subduction zone: targeted seismic body wave tomography and imaging of the relative size distribution (b value) of earthquakes. For the tomography we apply a staggered inversion scheme, starting with the minimum 1-D velocity model, progressing to a 3-D velocity model. Inversions are based on traveltime data from about 5500 events (1971–2002) with approximately 100,000 P and 50,000 S high-quality arrivals, allowing us to resolve the 3-D velocity field on a 20 × 20 km grid. For the b value imaging we have introduced a new 3-D sampling approach to map the b values on surfaces in the Wadati-Benioff zone (WBZ). Our b value study is based on 12,474 relocated events of magnitude of completeness ≥2.4 and depth >40 km. We observe anomalously high b values at depths around 100 km, appearing locally concentrated beneath active subduction volcanoes. We believe that these zones are associated with dehydration of the slab and subsequent magma generation. The analysis of V_P/V_S ratios likewise shows indications of the presence of fluids: significantly higher V_P/V_S ratios rising in columns from the top of the WBZ at 100 km depth below active volcanic centers. On the basis of the joint interpretation, we propose that our images track fluids from their genesis at 100 km depth at the top of the subducting plate to the bottom of the crust below volcanoes.