[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.

Abstract. Results from the Reykjanes-Iceland Seismic Experiment (RISE)show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62ø40•N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of •-130 øC, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63øN, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a •40øC change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9-11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seisrnogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (>_7.5 krn s -x), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age. Iceland and the Reykjanes RidgeIceland has been created where the slow spreading Mid-Atlantic Ridge passes directly over a mantle plume.McKenzie and Bickle [1988] showed that for decompression melting of the mantle at oceanic spreading centers the melt volume produced relies sensitively on the mantle temperature. Thus, assuming passive upwelling, the presence of the spreading center allows us to estimate the mantle temperature at a range of distances from the plume by measuring the igneous crustal thickness.•

An extensive seismic survey using ocean‐bottom seismographs (OBS) was performed in the area across the Jan Mayen Basin, North Atlantic, from the Jan Mayen Ridge to the Iceland Plateau. The Jan Mayen Ridge and surrounding area are considered to be a fragment of a continent which was separated from Greenland just prior to magnetic anomaly 6. This study presents the crustal structure of the Jan Mayen microcontinent and the ocean/continent transition to the west of the Jan Mayen Ridge. The crustal structures from the centre of the Jan Mayen Ridge to the Jan Mayen Basin are characterized by a deep sedimentary basin, a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the Iceland Plateau. The OBS data indicate that a continental upper crust (V p=5.8–6.1 km s−1) and lower crust (V p=6.7–6.8 km s−1) underlie the deep sedimentary basin. The thickness of the continental lower crust varies significantly from 12 km beneath the Jan Mayen Ridge to almost zero thickness beneath the northwestern part of the Jan Mayen Basin. An ocean/continent transition zone is found at the western edge of the Jan Mayen Basin. Within the 10 km wide transition zone, crustal velocities increase towards the Iceland Plateau, and approach the velocities of the oceanic crust obtained at the Iceland Plateau, that is 3.8–5.1 km s−1 (oceanic layer 2A), 5.9–6.5 km s−1 (oceanic layer 2B) and 6.8–7.3 km s−1 (oceanic layer 3). The crustal model indicates very thin oceanic crust (5 km) immediately oceanwards of the ocean/continent transition zone. Beneath the Iceland Plateau, the oceanic crust is thicker (9 km) than the typical thickness of normal oceanic crust. This might imply that the oceanic crust at the Iceland Plateau has been generated by asthenospheric material slightly hotter than normal. From the crustal structure obtained by the present study, it is proposed that the western part of the Jan Mayen Ridge may be referred to as a non‐volcanic continental margin, generated by a long duration of rifting. Even if the asthenospheric material upwelling along the margin were hotter than normal, only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting.