[J08-2] Imaging and interpreting lithospheric structures using seismic and geodetic approaches II

[J08-2-05] Structural variation in the rupture zone of the 2011 Tohoku-oki earthquake and its implications for depth-dependent seismic-slip behaviors

^○Shuichi Kodaira, Yasuyuki Nakamura, Yojiro Yamamoto, Koichiro Obana, Gou Fujie, Tetsuo No, Yuka Kaiho, Takeshi Sato, Seiichi Miura (JAMSTEC, Yokohama, Japan)

Seismological data of the 2011 Tohoku-oki earthquake (Mw 9.0) have revealed a clear depth-dependent variation in the source location between high- and low-frequency seismic energy radiation. However, structural variation, which may control seismic energy radiation, in the rupture zone have not been well studied. In this study, we therefore examined depth-varying structural characteristics by using seismic reflection data acquired along five profiles in the rupture zone. The resultant seismic images are characterized as a low-velocity frontal prism, a reflective zone at the trenchward tip of the continental block, and subducted horst-and-graben structures. The frontal prism is imaged as a low-velocity (Vp 2.0–3.5 km/s) wedge-shaped unit north of 37.5N, and abruptly transforms to a channel-like sedimentary unit south of 37.5N. Compiling the distribution of the frontal prism and the fault slip of the 2011 earthquake and the 1896 Sanriku earthquake, which is well-known as a tsunami earthquake, suggests that the coseismic slip to the trench may have occurred where the frontal prisms well developed. In 30 to 80 km landward from the trench, reflective zones with thickness of ~ 5 km were imaged at the base of the overriding block just above the subducted oceanic basement. Growth of subducted horst-and-graben structures (the throws of the normal faults associated with the horst-and-graben are larger by up to ~2 km) is imaged beneath the reflective zone. By comparing the seismic images and seismicity, we conclude that 1) in the area where a well-developed low-velocity frontal prism is imaged, seismicity along the plate interface is very low but the aftershocks of the 2011 earthquake occurred within the uppermost mantle and in the overriding plate, 2) in the area where growth of the subducted horst-and-graben structures are imaged, low levels of short-period seismic energy during the Tohoku-oki and weak background seismicity along the plate interface are observed.