Dr David Rothery, Lecturer in the Department of Earth and Environmental Sciences at The Open University, said:
"Today’s magnitude 7 (variously quoted as M7.4 and M7.1) off the coast of Japan is the largest aftershock we’ve seen following the M9.0 quake on 11 March. It was caused by a continuation of the same process, namely the Pacific plate being forced (subducted) below Japan, as stress transfer occurs along the currently-displacing region of the subduction zone. Fortunately, at a source depth of nearly 50 km the ground (and sea-water) displacement is less than would have been caused by a shallower quake of the same magnitude. A 2 metre high tsunami was forecast by the Japan Meteorological Agency within 2 minutes of the quake’s occurrence, but only for immediately adjacent stretches of coastline.
"Except for any places where tsunami walls and especially tsunami gates have not been put back in repair after last month’s disaster, I doubt if this tsunami will cause much damage. It occurred at nearly midnight local time, so there will have been few if any people near the vulnerable sea-front."