Depth of the 660-km discontinuity near the Mariana slab from an array of ocean bottom seismographs,

[1] High frequency records of deep Mariana earthquakes from a dense array of ocean bottom seismographs deployed in the Mariana arc and back-arc regions are stacked and searched for the phases P660p and S660p to constrain the depth of the 660-km discontinuity near the Mariana slab. Results of the high-resolution study suggest that around 18°N the 660-km discontinuity lies at about 710-730 km (±14 km) depth within or in the vicinity of the slab core. In the region seismicity ceases at $620 km depth. This implies that, although tomographic images show the Mariana slab penetrating into the lower mantle, deep seismicity in the region terminates$100 km above the base of the transition zone. These findings and similar observations in Tonga argue that factors other than the phase transition at the base of the upper mantle may control the maximum down-dip extent of the deep seismogenic region in the slab

Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

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 (BBOBS) 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 ray path coverage arguments

TIARES Project -Tomographic investigation by seafloor array experiment for the Society hotspot

http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr08-06_leg1/

Cruise Report of JAMSTEC R/V Natsushima NT0801

調査海域: 相模湾 / Area: Sagami Bay ; 期間: 2008年1月7日～2008年1月10日 / Operation Period: January 7, 2008～January 10, 2008http://www.godac.jamstec.go.jp/darwin/cruise/natsushima/nt08-01/

Shear wave speed structure beneath the South Pacific superswell using broadband data from ocean floor and islands

[1] We determined three-dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km by analyzing Rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the Pacific Ocean. The ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the French Polynesian region, which enabled us to study the upper mantle structure beneath the Superswell with unprecedentedly high resolution. We measured the dispersions of fundamental mode Rayleigh waves at periods between 40 and 140 seconds by the two-station method. We found pronounced slow anomalies near the hot spots and in the Lau Basin. The slow anomalies beneath the hot spots have deeper-rooted than those associated with the Lau basin. The slow anomalies near the Society, Macdonald, Marquesas, and Pitcairn hot spots continue down to at least 200 km depth