Heki, K, and Y. Tamura, Short term afterslip in the 1994 Sanriku-Haruka-Oki earthquake,

Geophys. Res. Lett., 24, 3285-3288, 1997.

リストで少し上にあるNatureの論文では一日毎の測位データから1年間の地震後の地殻の動き を求めましたが、この論文では国立天文台江刺地球潮汐観測施設の石英管伸縮計の1時間毎のデータを用 いて地震直後から数日にわたる地殻変動の様子を明らかにしました。その結果地震直後の一日の間に地震 時の瞬間的な断層ずれのおよそ三分の一の量のゆっくりしたすべりが発生していたことがわかりました。 Kawasaki et al. (1995)は1992年に三陸はるか沖で発生した地震の直後に一日程度の時定数の断層すべ りが発生したことを報告していますが、1994年の地震でも同様の時定数のすべりが存在したことになりま す。またGPSのデータを通常より高時間分解能で解析し、伸縮計と調和的な1日以内の地面の動きを捉えた ことも報告しています。

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 ~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 ~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.


日置幸介 (email: heki@ep.sci.hokudai.ac.jp)