Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

Abstract

Abstract.The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one-dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper "660" km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab-related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying fiat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.

Upper mantle velocity structure in the western Pacific rim estimated from short-period recordings at Matsushiro Seismic Array System

“…Nevertheless, our findings of high velocity anomaly in two paths qualitatively agree with high velocity anomalies that are found in other regional studies (e.g., Suetsugu and Nakanishi, 1987;van der Hilst et al, 1991;Fukao et al, 1992). High velocity anomaly in the transition zone beneath the Philippine Sea has been linked to the accumulation of subducted slab which do not penetrate into the lower mantle effectively (Okino et al, 1989;van der Hilst and Seno, 1993;Ohtaki and Kaneshima, 1994;Tajima and Grand, 1998). The origin of the high velocity anomalies could be both thermal and chemical considering the compositional difference between the oceanic plate and the ambient mantle (e.g., Gaherty et al, 1999).…”

“…Lateral heterogeneity in the transition zone velocities has been reported for the Philippine Sea region (Brudzinski et al, 1997;Tajima and Grand, 1998;Nowack et al, 1999) so that it might be crude to discuss the structure in detail with our results from a one-dimensional approach. Nevertheless, our findings of high velocity anomaly in two paths qualitatively agree with high velocity anomalies that are found in other regional studies (e.g., Suetsugu and Nakanishi, 1987;van der Hilst et al, 1991;Fukao et al, 1992).…”

Subduction-transition zone interaction: A review

“…Such time scales are much longer than the time scales of stagnation estimated from the seismically imaged volume of material in the transition zone, which range from ~20-60 m.y., even if some amount of buckling is accounted for (Section 2.1) (Tajima and Grand, 1998;Schmid et al, 2002). This would indicate that penetration is commonly triggered before the stagnant slab becomes negatively buoyant (Agrusta et al, 2017).…”

Spatial variations of the 660-km discontinuity in the western Pacific subduction zones observed from CEArray triplication data

“…The multiple arrivals recorded at a station share very similar ray paths near the source and the receiver sides, thus the differential travel time and relative amplitude among these arrivals are not sensitive to shallow structures. This method has been widely used to study the 660-km discontinuity beneath various subduction zones (e.g., Tajima and Grand, 1998;Brudzinski and Chen, 2003;Wang et al, 2006;Wang and Chen, 2009). …”