Silent fault slip following an interplate thrust earthquake at the Japan Trench

Recent global space geodetic measurements have revealed that the velocities of tectonic plates over timescales as short as a decade(1) are consistent with models of velocities averaged over the past few million years. The slip inferred from interplate thrust earthquakes at deep sea trenches and and number of earthquakes, however, often falls short of that predicted from these observed plate convergence rates(2,3). Here we report transient crustal movements recorded by a permanent Global Positioning System (GPS) network in northeastern Tapan following a typical interplate earthquake that occurred in December 1994 at the Japan Trench. Cumulative fault slip was estimated from the postseismic displacements at the GPS points over the first year after the event, and the inferred amount of seismic moment released by the afterslip was comparable to that released in the high-speed rupture. Such seismically 'invisible' slip map therefore account for the shortage of seismic slip relative to that required by time-averaged plate velocities.