Masayuki Obayashi

Stagnant slabs in the upper and lower mantle transition region

We made a region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660-km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre-Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

Subducting slabs stagnant in the mantle transition zone

The P wave velocity structure beneath the Western Pacific is found from the International Seismological Center first arrival data. Special attention was paid to the deep structure beneath the Wadati-Benioff zone. We discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks. This way of discretization minimizes a problem with tomographic studies of regional scale: difficulty in making corrections for the effects outside the region of interest. Our solution is iterative with the alternate step of the relocation of earthquakes, using the whole mantle model of Inoue et al. (1990) as a starting model. A first-order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model. The essential result depends little on whether the reference spherical model is smooth or discontinuous near 400- and 670-km depths. We examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere. The resolution is depth dependent but in general good enough to see the slab configuration beneath the Southern Kurile-Japan-Izu-Bonin arcs and the Java arc. It is relatively poor beneath the Northern Kurile and Mariana arcs. The seismic image of subducting slab beneath the Southern Kurile to Bonin arcs bends to subhorizontal near the leading edge of the Wadati-Benioff zone and extends continentward over a distance of more than 1000 km. The subhorizontal portion of the slab connects a high-velocity blob to the bottom that reaches a depth of at least 800 km across the 670-km discontinuity under the Japan arc. Although the image of the Java slab directly penetrates the 670-km discontinuity, it then bends to a shallow dip with a considerable spread, reaching a depth of about 1200 km. These results suggest that descending slabs of lithosphere in the Western Pacific tend to be stagnant in the transition zone under a subtle control of the 670-km discontinuity. Although stagnant slab materials eventually descend into the lower mantle, they no longer maintain their original configuration below the 670-km discontinuity.

Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity

A new P wave tomographic model of the mantle was constructed using more than 10 million travel times. The finite-frequency effect of seismic rays was taken into account by calculating banana-donut kernels at 2 Hz for all first arrival time data, and at 0.1 Hz for broadband differential travel time data. Based on this model, a systematic survey for subducted slab images was developed for the circum-Pacific; including the Kurile, Honshu, Izu-Bonin, Mariana, Java, Tonga-Kermadec, southern and northern South America, and Central America, arcs. This survey revealed a progressive lateral variation of the configuration of slabs along arc(s), which we interpret as an indication for successive stages of slab subduction through the Bullens transition region with the 660 km discontinuity at the middle. We identified the four distinct stages: I - slab stagnant above the 660 km discontinuity; II - slab penetrating the 660 km discontinuity; III - slab trapped in the uppermost lower mantle (at a depth of 660–1000 km); and IV - slab descending well into the deep lower mantle. The majority of slab images are found to be either at Stage I or III, suggesting that Stages I and III are relatively stable or neutral and II and IV are relatively unstable or transient. There is a remarkable distinction for the deepest hypocentral distribution between slabs at Stage I and slabs at Stages II or III.

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.

P and PcP travel time tomography for the core‐mantle boundary

PcP arrival times reported to the International Seismological Centre (ISC) are inverted for vertical travel time anomaly at the core-mantle boundary (CMB). We first invert about 2×106 ISC first P arrival data for the three-dimensional (3-D) structure of the mantle and hypocenter locations iteratively. The PcP data are corrected for the relocated hypocenters and the 3-D structure. This correction has reduced the observed PcP residuals. The corrected PcP data are then inverted for the vertical travel time anomalies at the CMB, which are due either to velocity anomaly or topographic anomaly. The spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the CMB (by ∼3% if the layer is 20 km thick) or a lowering of the CMB by 3 km. The heterogeneity pattern of the CMB is drastically different from that of the overlying D′′ layer. Negative vertical travel time anomalies appear in the northern high-latitude region, and positive anomalies appear under Southeast Asia and Middle to South America. The lateral dimensions of these anomalies are of the order of 40°. The anomalies can be interpreted as either a velocity perturbation in the boundary layer (±∼7% if the layer is 20 km thick) or an undulation of the CMB (±∼7 km). Various tests including the error and resolution estimations indicate that these three anomalies are the significant, resolvable features of the CMB. These anomalies are also corroborated by the result of the joint inversion of P and PcP for both the 3-D mantle and the 2-D CMB. Assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core, the PcP data are also inverted for the isostatic figure of the boundary layer. The velocity perturbation in this case is 5∼6% if the layer is 20 km thick, and the CMB undulation is only ±1∼2 km. Regardless whether the CMB heterogeneity is due to velocity and/or topographic perturbations, it is dominant in the zonal component of degree 2, while the degree 2 pattern in the lower mantle, including the D′′, is dominant in sectorial component.

Trans-Pacific temperature field in the mantle transition region derived from seismic and electromagnetic tomography

Abstract The trans-Pacific temperature field for the depth range 350–850 km was inferred from global seismic tomography and semi-global electromagnetic tomography. The seismic tomography incorporated millions of reported first arrival times and 7000 PP–P differential travel times measured on broadband seismograms. The electromagnetic tomography used voltage data from trans-Pacific submarine cables and magnetic field data from circum-Pacific geomagnetic observatories. The resultant P-wave velocity anomalies and electrical conductivity anomalies were converted to temperature anomalies using a proposed conversion formula and experimental results for mantle minerals, respectively. These conversions show consistently high-temperature anomalies of 200–300 K in the mantle transition region beneath the Hawaiian hotspot. At subduction zones, where slab-related cold anomalies and wedge mantle-related hot anomalies are likely to coexist in close proximity, the seismic and electromagnetic tomography did not always give consistent features, in part because of the preferred sensitivity of electromagnetic tomography to hot anomalies. Low-temperature anomalies of 200–300 K associated with subducted slabs are clearly resolved in the seismic tomography, but are less apparent in the electromagnetic tomography. The high-temperature anomaly in the intervening zone between the Mariana and Philippine slabs is very pronounced in the electromagnetic tomography but is marginal in the seismic tomography.

Upper mantle tomography in the northwestern Pacific region using triplicated P waves

We conducted delay time tomography of the upper mantle beneath the northwestern Pacific using P data from NorthEast China Extended SeiSmic Array, F-net, and nearby available stations. To improve resolution and accuracy in the vicinity of mantle discontinuities, we extracted traveltimes of both initial and secondary phases observed at triplication distances by using a waveform fitting technique. Compared with the model obtained by using only the initial phase, the resolution just above the 410 km discontinuity is especially improved, and low-velocity anomalies beneath the Changbai Volcano are clearly observed down to the 410 km discontinuity. Compared with previous models, low-velocity anomalies atop the 410 are more pronounced. The results of this study together with the previous receiver function analysis provide further support that we have hot material beneath the Changbai Volcano.

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.