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Earth, Planets and Space
Fig. 4 | Earth, Planets and Space

Fig. 4

From: Upper boundaries of the Pacific and Philippine Sea plates near the triple junction off the Boso Peninsula deduced from ocean-bottom seismic observations

Fig. 4

Distribution of focal mechanisms for the shallow earthquake group with magnitudes greater than 2.3. Red, yellow, and blue beach balls correspond to normal, strike-slip, and reverse-faulting earthquakes, respectively. Focal mechanisms are in lower-hemisphere projections. Line A–A′ defines the line projected onto the cross sections, parallel to the direction of PAC subduction. The blue and brown lines indicate the upper boundaries of the PHS (Uchida et al. 2010) and PAC (Nakajima and Hasegawa 2006; Nakajima et al. 2009), respectively. Light and dark gray dots show hypocenters based on the three-dimensional velocity model (Fig. 3b), with light gray dots corresponding to the period for which we could use ERI OBS seismic data, and the dark gray dots corresponding to the period for which we could not. Vertical cross sections have no vertical exaggeration. a, c Normal faults. b, d Strike-slip and reverse faults

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