No abstract

[1] We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long-and short-period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments.

[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.

We conducted broadband dispersion survey by deploying two arrays of broadband ocean bottom seismometers in the northwestern Pacific Ocean at seafloor ages of 130 and 140 Ma. By combining ambient noise and teleseismic surface wave analyses, dispersion curves of Rayleigh waves were obtained at a period range of 5–100 s and then used to invert for one‐dimensional isotropic and azimuthally anisotropic βV (VSV) profiles beneath each array. The obtained profiles show ~2% difference in isotropic βV in the low‐velocity zone (LVZ) at a depth range of 80–150 km in spite of the small difference in seafloor ages and the horizontal distance of ~1,000 km. Forward dispersion‐curve calculation for thermal models indicates that simple cooling models cannot explain the observed difference and an additional mechanism, such as sublithospheric small‐scale convection, is required. In addition, the fastest azimuths of azimuthal anisotropy in the LVZ significantly deviate from the current plate motion direction. We infer that these observations are consistent with the presence of small‐scale convection beneath the study area. As for azimuthal anisotropy in the Lid, the peak‐to‐peak intensity is 3–4% at the depth from Moho to ~40 km. The fastest direction is almost perpendicular to magnetic lineation in area A at 130 Ma and oblique to magnetic lineations in area B at 140 Ma, suggesting complex mantle flow beneath the infant Pacific Plate surrounded by three ridge axes. The intensity of azimuthal anisotropy in the LVZ is ~2%, indicating that radial anisotropy is stronger than azimuthal anisotropy therein.

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.