北海道十勝沖で発生する地震の近地sP変換波を用いた震源の深さの決定
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概要
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Based on a realistic two-dimensional velocity model, we investigated the origins of local sP converted phases observed for earthquakes occurring beneath the Pacific Ocean off Tokachi in Hokkaido, Japan. The sP converted phases appear about 5-10 seconds after initial P waves at stations with epicentral distances of 100-250km. If we assume that the S to P conversion occurs at the sea floor, the observed sP-P times constrain the hypocenters at shallow depths away from the megathrust plate boundary. If we assume that the S to P conversion occurs at the bottom of the unconsolidated sedimentary layer (<I>V</I><SUB>P</SUB>=1.8km/s), the hypocenters relocated using the sP-P times are distributed on or near the plate boundary. The theoretical amplitudes of sP phases converted at the bottom of the sedimentary layer is about 4 times larger than those of the sP phases converted at the sea floor. Therefore the observed sP phases are not likely to be the ones converted at the sea floor. The theoretical amplitude of sP phases converting at the interface between the consolidated sedimentary layer (<I>V</I><SUB>P</SUB>=2.5km/s) and the basement (<I>V</I><SUB>P</SUB>=3.8km/s) is as large as those of the sP phases converting at the bottom of the unconsolidated sedimentary layer. However, if we assume that the observed sP phases are converted at the interface between the consolidated sedimentary layer and the basement, the hypocenters are relocated deep inside the subducted slab. Judging from the distribution of relocated hypocenters, focal mechanisms, and theoretical amplitudes, it is most likely that the observed sP phases are those converted at the bottom of the unconsolidated sedimentary layer.
- 公益社団法人 日本地震学会の論文