High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity Masayuki Obayashi ⁎ , Hiroko Sugioka, Junko Yoshimitsu, Yoshio Fukao Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, 237-0061, Japan Received 2 June 2005; received in revised form 20 December 2005; accepted 27 December 2005 Available online 7 February 2006 Editor: R.D. van der Hilst Abstract Our P-wave whole mantle tomography revealed a low velocity region oceanward of the Northern Honshu slab of the Pacific plate at depths around the 410-km seismic discontinuity. Resolution tests and scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies. The existence of the slow anomalies is also supported by the analysis of the P-wave records from the J-array (a large- aperture seismic array in Japan) for a Bonin earthquake. The P arrivals to Northern Honshu (at epicentral distances of 13–20°) are strongly triplicated because of the 410-km discontinuity. The later arrivals along the retrograde branch, where ray paths pass through the low velocity region, are anomalously slow. Comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410-km discontinuity. This depression represents the direct evidence for the low velocity zone of primarily thermal origin. An excess temperature of 200 K and the associated fractional melt of less than 1% can explain both the results of the tomographic and waveform analyses. © 2006 Elsevier B.V. All rights reserved. Keywords: mantle; 410-km discontinuity; subducting lithosphere; seismic tomography 1. Introduction At subduction zones mantle lithosphere descends into the Earth's interior along with sediments and oceanic crust, which is collectively called “slab”. Because subducting slabs are colder than the ambient mantle, they are distinct as anomalously high seismic velocity regions (e.g. [1–3]) and they are also known as the places of distorting the seismic discontinuity plane at either 410 or 660 km depth. The 410-km discontinuity is generally attributed to the phase transition from α-olivine to β-wadsleyite [4–6]. Topographic high in the cold slab, which is expected from the positive Clapeyron slope of this transform, has been observed [7]. The mantle wedge, a part of the mantle overlying the subducting slab, is known in general as a zone of low seismic velocity and high attenuation [2,8]. Since sunduction-related activities are largely observed above the mantle wedge, attention has been mostly directed to this part (e.g. [9–12]). On the other hand, little attention has been given to the opposite side (trench side) of the mantle across the slab and no significant seismic anomaly has been reported in detail. In this study, we investigate the low velocity anomaly oceanward of the Northern Honshu Earth and Planetary Science Letters 243 (2006) 149 – 158 www.elsevier.com/locate/epsl ⁎ Corresponding author. Tel.: +81 46 867 9744; fax: +81 46 867 9745. E-mail address: obayashi@jamstec.go.jp (M. Obayashi). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.12.032