A Dislodged Tectonic Block Wedged Beneath Tokyo, and Implications for the Enigmatic 1855 Ansei-Edo Earthquake

On the basis of microearthquake distributions, seismic tomography, and seismic stress inversion, we argue that a dislodged block of the Pacific plate is jammed between the Pacific (PAC), Philippine Sea (PHS) and Eurasian (EUR) plates beneath Tokyo at a depth of at depths of 40-100 km. We argue that the block controls much of Tokyos seismic behavior, including recent M~6 class earthquakes, and the 1855 M~7.2 Ansei-Edo shock as well. To illuminate the plate configuration and possible large earthquake sources, we examined 320,000 earthquakes recorded by NIED during 1979-2004 in a 3D GIS, from which we identify several new features of the Kanto triple junction. First, a 25-km-thick, 90 x 120-km-wide enclosed volume of seismicity with high seismic velocity lies between the PAC and the EUR beneath the Kanto Plain, which we interpret to be a lithospheric block dislodged from the descending PAC. Second, we find that the leading edge of the PHS lies at 35 km depth and abuts the southern margin of the block. Third, there is a pronounced bend to the double seismic zone defining the descending PAC slab, which closely parallels the sharp curvature of the volcanic front. Because of the bend, the PAC/EUR contact at shallow depth is nearly flat-lying, and undergoes episodic aseismic slip unaccompanied by large subduction earthquakes. We associate the corridor of active seismicity extending from upper Tokyo Bay for 100 km northward with the eastern edge of the dislodged block. Within this Kanto seismic corridor, eight M≥5.7 shocks have been recorded since 1985, and four M≥7 shocks have struck since 1603. We speculate the Ansei-Edo earthquake was an intermediate-depth (30-60 km) or deep (70-100 km) thrust interplate event on the upper or lower surface of the dislodged block, rather than being an intraplate or shallow crustal shock. The 1855 event, which devastated Edo (now Tokyo) and killed more than seven thousand people, is the only well-documented inland historical earthquake in the past 400 years. But its source and repeatability are still disputed due to the complicated plate structure.