Junko Yoshimitsu

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

Received 23 July 2013; revised 14 October 2013; accepted 17 October 2013. [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. Citation: Obayashi, M., J. Yoshimitsu, G. Nolet, Y. Fukao, H. Shiobara, H. Sugioka, H. Miyamachi, and Y. Gao (2013), Finite frequency whole mantle P wave tomography: Improvement of subducted slab images, Geophys. Res. Lett., 40, doi:10.1002/ 2013GL057401.

Tearing of Stagnant Slab

Tearing the Plate Recent seismic data show a widely variable geometry of subduction: Some plates penetrate deep into the mantle; others bend and become horizontal at 670 kilometers near a prominent phase boundary. Obayashi et al. (p. 1173; see the Perspective by Nolet) provide a detailed view of a situation where subduction of juxtaposed plates in the western Pacific, in somewhat different directions, seems to have ripped a gash in the plates starting at a depth of about 300 kilometers. The geometry of the gash provides information on the past evolution of this plate boundary. A tear in the plate in the mantle provides information on the subduction history of the western Pacific. Subducted slabs of oceanic lithosphere below the western Pacific tend to be stagnant in the transition zone with poorly known mechanical properties. Typical examples are the Izu-Bonin and Japan slabs that meet each other to form a cusplike junction beneath southwest Japan. Here, we show that these two slabs are torn apart at their junction when they bend to flatten over the 660-kilometer discontinuity, as is expected from a simple geometric argument. We present three lines of evidence for this ongoing slab tear.

Mantle plumes beneath the South Pacific superswell revealed by finite frequency P tomography using regional seafloor and island data

We present a new tomographic image beneath the South Pacific superswell, using finite frequency P wave travel time tomography with global and regional data. The regional stations include broadband ocean-bottom seismograph stations. The tomographic image shows slow anomalies of 200-300 km in diameter beneath most hot spots in the studied region, extending continuously from the shallow upper mantle to 400 km depth. Narrow and weak slow anomalies are detected at depths of 500–1000 km, connecting the upper mantle slow anomalies with large-scale slow anomalies with lateral dimension of 1000–2000 km prevailing below 1000 km depth down to the core-mantle boundary. There are two slow anomalies around the Society hot spot at depths shallower than 400 km, which both emerge from the same slow anomaly at 500 km depth. One of them is located beneath the Society hot spot and the other underlies 500 km east of the Society hot spot, where no volcanism is observed.