Fig. 3

a Observed seismograms of SP-W5 to which a 1–15-Hz band-pass filter is applied. Red arrow highlights reflected waves from plate boundary (PB). Each trace is normalized by its maximum amplitude. b Synthetic seismograms for SP-W1 calculated from velocity model of (c) in which a thin low-velocity (3.0 km/s) layer atop the slab is assumed. c Ray diagrams of SP-W5 for the reflected waves from the plate boundary (black curves). Numerical values in the plot are P wave velocities estimated from first arrival times. For deeper parts (in parentheses), we assumed velocities using the surrounding experiments (Arai et al. 2009, 2013; Arai and Iwasaki, 2014). For reference, we plotted earthquakes that occurred within 5 km of the seismic profile in 2007 and 2008 (orange dots). For the plot, we used hypocentral data from the unified catalog of the Japan Meteorological Agency. Synthetic seismograms calculated from a velocity model in which 4.0 km/s (d), 6.0 km/s (e), and no low-velocity layer (f) is assumed for the PB. Note that a velocity model with a small (e) or no (f) velocity contrast at the PB cannot explain the reduced amplitudes of the first arrivals