[S-IT16] Structure and Dynamics of Earth and Planetary Mantles

[SIT16-08] Slow anomalies atop the 410-km discontinuity oceanward of the Kurile slab

*Masayuki Obayashi¹, Junko Yoshimitsu¹, Yoshio Fukao¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:410-km discontiuity, triplication, tomography

Previous studies have shown slow anomalies atop the 410-km discontinuity below the subducting Honshu slab, which have been attributed to thermal anomalies (e.g. Obayashi et al., 2006; Bagley & Revenaugh, 2008). The slow anomaly area is, however, located toonly oceanward of the Honshu slab and northern Izu-Bonin slab and its spatial extent is still unknown. At the JpGU-AGU joint meeting 2020, we showed the retrograde-branch waves from the 410-km discontinuity on the Hi-net records from Kamchatka earthquakes arriving later by 2 or 3 seconds than the predictions form IASP91. We attributed this phenomenon to the 2 or 3 % slow anomaly atop the 410-km discontinuity along the Kurile and northern Honshu slabs. And at the 2020 SSJ fall meeting, we suggested the model M200M (Obayashi et al., 2006) that contains slow anomalies and depression of the 410-km discontinuity due to 200℃ excess temperature and less than 1 % melts, by comparing the observed waveforms with the synthetics by the model.

In this study, we inverted differential traveltimes between the first arrivals and later arrivals along the retrograde branch in addition to the first arrival time data of International Seismological Centre catalogue to investigate where these slow anomalies occur. The differential traveltimes are obtained by picking the peak of each phase on the squared envelope waveforms (Figure 1). The obtained tomography model shows strong slow anomalies oceanward Kurile slab near the 410-km discontinuities (Figure 2).