sPdepth phase at small epicentral distances and estimated subducting plate boundary

A remarkable later phase has been detected in seismograms between P- and S-wave arrivals at epicentral distances of about 150 km or more for events occurring in the north-eastern Japan arc. The characteristics of this phase support an interpretation as an sP phase at small epicentral distances, an upgoing S wave from the focus which is then reflected and converted to a P wave at the Earth's surface, diving into the Earth again, and finally reaching the observation stations. The arrival-time difference between this phase and the first P wave is very sensitive to the focal depths of events, and is useful for accurate estimation of the focal depths of those events, in a similar way to using teleseismic depth phases of pP and sP. The utilization of this phase has the advantage of yielding estimates of focal depths for relatively small magnitude events that do not have enough energy to generate teleseismic waves. By using the observed sP phases at small epicentral distances, shallow events beneath the Pacific Ocean in the north-eastern Japan arc, whose focal depths are poorly constrained by the land seismograph network of Tohoku University, have been relocated. The estimated hypocentre distribution, together with the focal mechanism solutions of some of the events, shows that the Pacific plate beneath this region subducts at an extremely low dip angle of less than 10° for the first descent to about 20 km depth and then gradually becomes a steeper dip angle of about 30° at depths deeper than 30 km.