Radiation and Generation of Short- and Long-Period Ground Motions from the 2011 Off Tohoku, Japan,Mw9.0 Earthquake

Ground motion from the M w 9.0 March 11, 2011, Off-Tohoku earthquake recorded by dense seismic networks in Japan, K-NET and KiK-net, clearly demonstrated the high-frequency seismic wavefield radiating from the earthquake source and developing longperiod ground motion in sedimentary basins. The photographic sequence of the visualized wavefield demonstrated the process in which the high-frequency seismic waves radiated from large slips at the top of the subducting Pacific Plate at relatively deeper depth of 25-50 km, which caused multiple large shocks of large (>1000-2000 cm/s 2 ) ground acceleration and several minutes lasting ground motions over a wide area along the Pacific Ocean side of northern Japan. An efficient seismic wave propagation along the subducting Pacific slab and ground motion amplification in a superficial thin low-velocity layer overlying rigid bedrock also enhanced high-frequency (>5 Hz) ground motions very drastically. However, the dominant frequency of the strong ground motion recorded in nearfield station was too high such as to cause serious damage to wooden-frame residences having relatively longer-period resonance period ( T = 1-2 s); The velocity response in this frequency band was only about one third to one half of those observed in severely damaged area during the destructive M w 6.9 1995 Kobe earthquake. The 2011 Off-Tohoku earthquake also produced long-period ground motion in sedimentary basins such those at Tokyo’s population center but observation of the long-period ground motion within T =6-8 s was rather weak and of a level comparable to that of an M 7.9 Tonankai earthquake occurring along the Nankai Trough in 1944. This was because the surface wave in this period band was not generated efficiently by the relatively deeper slip over the source fault of the Off-Tohoku earthquake.