[1] The anelastic structure of a subduction zone can place first-order constraints on variations in temperature and volatile content. We investigate seismic attenuation across the western Pacific Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment. This 11-month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc, island arc, and back-arc spreading center. We compute amplitude spectra for P and S arrivals from local earthquakes and invert for the path-averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake. Additionally, we investigate earthquake source parameter assumptions and frequencydependent exponents (a) ranging from 0 to 0.6. Tomographic inversion of nearly 3000 t* estimates (at a = 0.27) for 2-D Q P À1 and Q P /Q S structure shows a $75 km wide columnar-shaped high-attenuation anomaly with Q P $ 43-60 beneath the spreading center that extends from the uppermost mantle to $100 km depth. A weaker high-attenuation region (Q P $ 56-70) occurs at depths of 50-100 km beneath the volcanic arc, and the high-attenuation regions are connected at depths of 75-125 km. The subducting Pacific plate is characterized by low attenuation at depths greater than 100 km, but high attenuation is found in the plate between 50 and 100 km depth. The fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc. Q S structure is less well resolved than Q P because of a smaller data set, but Q P /Q S ratios are significantly less than 2 throughout the study region. As temperatures estimated from Q S À1 are unusually high, we interpret the arc and wedge core anomalies as regions of high temperature with enhanced Q À1 due to hydration and/or melt, the slab and fore-arc anomalies as indicative of slab-derived fluids and/or large-scale serpentinization, and the columnar-shaped high Q P À1 anomaly directly beneath the back-arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis.

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S U M M A R YShear wave splitting measurements provide significant information about subduction zone mantle flow, which is closely tied to plate motions, lithospheric deformation, arc volcanism, and backarc spreading processes. We analyse the shear wave splitting of local S waves recorded by a large 2003-2004 deployment consisting of 58 ocean-bottom seismographs (OBSs) and 20 land stations and by nine OBSs from a smaller 2001-2002 deployment. We employ several methods and data processing schemes, including spatial averaging methods, to obtain stable and consistent shear wave splitting patterns throughout the arc-backarc system. Observed fast orientation solutions are dependent on event location and depth, suggesting that anisotropic fabric in the mantle wedge is highly heterogeneous. Shear waves sampling beneath the northern island arc (latitudes 17.5 • -19 • N) and between the arc and backarc spreading centre show arcparallel fast orientations for events shallower than 250 km depth; whereas, fast orientations at the same stations are somewhat different for deeper events. Waves sampling beneath the central island arc stations (latitudes 15.5 • -17.5 • ) show fast orientations subparallel to both the arc and absolute plate motion (APM) for events <250 km depth and APM-parallel for deeper events. Ray paths sampling west of the spreading centre show fast orientations ranging from arc-perpendicular to APM-parallel. Arc-parallel fast orientations characterize the southern part of the arc with variable orientations surrounding Guam. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than ∼250 km and at large distances from the trench. We conclude that the arc-parallel fast orientations are likely the result of physical arc-parallel mantle flow and are not due to recently proposed alternative lattice preferred orientation mechanisms and fabrics. This flow pattern may result from along-strike pressure gradients in the mantle wedge, possibly due to changes in slab dip and/or convergence angles.

Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and back-arc spreading. We investigate the seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment, an 11 month deployment consisting of 20 broadband seismic stations installed on islands and 58 semibroadband ocean bottom seismographs. We determine the three-dimensional V P and V P /V S structure using over 25,000 local and over 2000 teleseismic arrival times. The mantle wedge is characterized by slow velocity and high V P /V S beneath the fore arc, an inclined zone of slow velocity underlying the volcanic front, and a strong region of slow velocity beneath the backarc spreading center. The slow velocities are strongest at depths of 20-30 km in the fore arc, 60-70 km beneath the volcanic arc, and 20-30 km beneath the spreading center. The fore-arc slow velocity anomalies occur beneath Big Blue seamount and are interpreted as resulting from mantle serpentinization. The depths of the maximum velocity anomalies beneath the arc and back arc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry, strongly indicating that the lowvelocity zones delineate regions of melt production in the mantle. The arc and back-arc melt production regions are well separated at shallow depths, but may be connected at depths greater than 80 km.