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5) Shantha S. N. Gamage, N. Umino, A. Hasegawa and S. Kirby: Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks, Gophys. J. Int., 2009, in press.

6) Mochizuki, K., T. Yamada, M. Shinohara, Y. Yamanaka and T. Kanazawa: Weak Interplate Coupling by Seamounts and Repeating M 7 Earthquakes, Science, 321, 5893, 1194-1197, 2008.

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819� 200812�20� ������� Fig.1 The earthquake east off Kanto district (M6.6) on December 20 2008.

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829� |}����£x%& Fig.2 Tsunamis caused by the earthquake east off Kanto district on December 20 2008.

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839� |}���~���xK0Hh/��c��z{ Fig.3 Seismic activity around the epicenter of the M6.6 earthquake.

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849� |}���~��"]âã0H����wéJ^._C|}�����ÊR` Fig.4 Seismic activity and slip distributions of the major earthquakes and the slow slip off Ibaraki and

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859� 01ÄÅÆÇ24È-ÉC�ÊËpC� ÌÛÜ9 Fig.5 Relationships between epicentral distances and observed times of maximum acceleration amplitudes.

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869(a)� ä�âãv�/fgdq7Ç2&¾�aA¨Bbc?9 Fig.6(a) Waveforms of the M6.6 mainshock and the aftershocks.

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869(b)� �dï Fig.6(b) Continued.

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869(c)� Hi-net=efg¯�N.HTAH��Ã�x&¾ÛÜ Fig.6(c) Comparison of waveforms which observed at the Hirata Hi-net seismic station.

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879� |}����£xfCFF Fig.7 Distribution of fCFF by the M6.6 mainshock.