Short term afterslip in the 1994 Sanriku-Haruka-Oki earthquake

The Sanriku-Haruka-Oki earthquake, that occurred on December 28, 1994 at the Japan Trench as a typical interplate thrust event, was followed by year-long afterslip as large as the slip in the high-speed rupture [Heki et al., 1997]. Here we report on the transition between these, inferred from crustal movements during the five days interval before and after the earthquake. Since this timescale is too long for seismometers but not suitably long for Global Positioning System (GPS), we rely primarily on strainmeter data taken similar to 200 km southwest of the epicenter. To confirm that the recorded strain changes are not local disturbances, we compare them with crustal movements derived by high time resolution analyses of GPS data in the same period. The transition to the long term afterslip was gradually achieved by slow fault slip with a time constant of a few hours. The cumulative slow slip in 24 hours amounts to similar to 1/3 of the coseismic slip, i.e. we may overestimate the coseismic displacement if we look at GPS data time series composed of daily solutions. The result presented here indicates that a single earthquake could have multiple aspects in its slowness, visible with different seismological and geodetic tools, and we need to use all such data to understand fully such a hybrid earthquake.