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 a nd after the earthquake. Since this timescale is too long for seismome ters but not suitably long for Global Positioning System (GPS), we rel y primarily on strainmeter data taken similar to 200 km southwest of t he epicenter. To confirm that the recorded strain changes are not loca l disturbances, we compare them with crustal movements derived by high time resolution analyses of GPS data in the same period. The transiti on to the long term afterslip was gradually achieved by slow fault sli p 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 ove restimate the coseismic displacement if we look at GPS data time serie s composed of daily solutions. The result presented here indicates tha t a single earthquake could have multiple aspects in its slowness, vis ible with different seismological and geodetic tools, and we need to u se all such data to understand fully such a hybrid earthquake.