Fig. 3: Dependence of splitting parameters on the depths of interplate earthquake sources.

(a) Geometry of subduction interface along a margin-normal profile at 40°N [Modified from Uchida et al., 2010⁵⁰]. Crosses schematically show earthquakes, and black squares schematically show seismic stations that record shear waves from these events. (b) Azimuth of fast shear-wave polarization beneath each station measured clockwise from the strike direction of the Pacific slab. Note that an azimuth −130° is the same as 50°. (c) Delay time between the fast and slow polarized waves. The depth axis is scaled to distance using the interface geometry shown in (a). In (b) and (c), red, green, and black circles show station averaged values based on interplate events, upper-plate events, and intraslab events, respectively. The intraslab data are from Nakajima et al. [2006]¹⁷. Filled red circles indicate the number of waveforms is larger than 10. Blue circles with error bars (one standard deviation) represent average values for events within every 20 km depth interval, except for the shallowest one which is an average from the 7 km trench depth to 20 km. For each depth average, we show the mean azimuth and circular standard deviation that utilize the azimuth and length of the summed unit vectors⁵³. Blue and red vertical bars mark the maximum depth of slab-mantle decoupling³³ and the volcanic front, respectively.