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Progress in Earth and Planetary Science
Fig. 5 | Progress in Earth and Planetary Science

Fig. 5

From: The nature of the Pacific plate as subduction inputs to the northeastern Japan arc and its implication for subduction zone processes

Fig. 5

a Seismic reflection profiles along lines A4 and R2. The inverted triangles show OBS positions. On the northwestern part of the Pacific plate, sediment thickness is generally 300–400 s in two-way time, corresponding to 300–400 m assuming the average \(V_p\) within the sediments is 2.0 km/s, but we found areas of thin sediments such as the area from 260–380 km along this line. b Magnified seismic reflection section (left) and receiver function (RF) of controlled-source OBS data (right) in the area of thick sediments. One of characteristics of the sedimentary layer in this region is the existence of the chert unit at the bottom of the sediments, immediately above the basaltic crust (Moore et al. 2015; Shipboard Scientific Party 1980). The two P-to-S conversion interfaces imaged at about 2 s on the OBS RF section are interpreted as the top of the chert and the top of the basaltic crust, respectively. c The same figures as b but in the thin-sediment area. The characteristic appearance of the chert unit could not be recognized in the seismic reflection section, but multiple P-to-S conversion interfaces were observed in the OBS RF sections, suggesting that the strong reflection of the MCS reflection section is the apparent basement and multiple seismic structure boundaries exist immediately beneath the apparent basement (Fujie et al. 2020)

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