Credit: © Masahiro Hayata

Beneath Tokyo, both the Philippine Sea plate and the Pacific plate are subducted under the Eurasian plate, with the Philippine Sea plate squeezed into the wedge between the other two. This unique set-up may account for deep earthquakes that have damaged Tokyo in the past, and should be considered in future seismic hazard assessments, suggests a new study.

Francis Wu from Binghamton University, USA and colleagues1 examined this unique seismic setting — the Philippine Sea plate subducting in a north-westerly direction, with the westward-travelling Pacific plate diving directly beneath it. They compiled a series of three-dimensional models, which they superimposed onto tomographic cross-sections of the area to show the three-way interaction of the plates as they meet underneath Japan. As the Philippine plate subducts between the Eurasian and Pacific plates it is deformed, producing a zone of increased seismicity. This zone is likely to be the source of past and future magnitude 6 and 7 earthquakes, conclude the researchers.

The infamous 1923 magnitude 8 earthquake in the Kanto area in eastern Japan originated from the megathrust where the Philippine Sea plate meets the Eurasian plate. But the less intense and deeper-seated earthquakes that wreak havoc in the region more frequently are probably the result of double subduction.