Figure 4.

Seismic imaging of Philippine Sea Plate subduction thrust hosting slow slip and tremor beneath Tokai District in central Japan. (a) Map showing slip contours of long-term SSE (blue),low frequency earthquakes (green circles), and northwest-southeast profile line (black). (b) Top: Depth section of S-wave velocity perturbation (dVs/Vs) from seismic tomography. Centre: Ratio of P and S-wave velocities (Vp/Vs). Bottom: Receiver function results (RF amplitude) highlighting sharp changes in seismic velocities that illuminate the plate interface and the continental and oceanic Moho. (c) Top: Schematic interpretation of seismic structures. Blue shades in subducting oceanic crust reflect fluid pressure variations. Green arrows denote potential fluid pathways in the subduction zone. Bottom: Profile showing variations of dVs/Vs and Vp/Vs within the subducting oceanic crust. Relocated hypocentres are plotted for events within 10 km of the cross-section with grey circles indicating regular earthquakes and red stars indicating LFEs. The deep ETS zone hosts LFEs and M6 slow slip events recurring every few months [26]. An intermediate-depth section of the plate boundary hosts long-term M7 slow slip events (LTSS) that last for years and shorten the deeper ETS recurrences. The updip locked zone fails in M8 megathrust ruptures every approximately $150\hspace{0.17em}\text{years}$. (All images provided by A. Kato based on [25].) (Online version in colour.)