[S-SS24_1PM2] Seismicity

[SSS24-05] Aftershock distribution in the northern source region of the 2011 Tohoku earthquake by long-term OBSs

*Masanao SHINOHARA¹, Tomoaki YAMADA¹, Kazuo NAKAHIGASHI², Kimihiro MOCHIZUKI¹, Yuya MACHIDA¹, Takashi SHINBO³, Yoshio MURAI⁴, Ryota HINO⁵, Yoshihiro ITO⁶, Toshinori SATO⁷, Kenji UEHIRA³, Hiroshi YAKIWARA⁸, Hajime SHIOBARA¹ (1.ERI, Univ. of Tokyo, 2.Kobe Univ., 3.NIED, 4.Hokkaido Univ., 5.Tohoku Univ., 6.DPRI, 7.Chiba Univ., 8.Kagoshima Univ.)

The 2011 Tohoku earthquake occurred at the plate boundary and many aftershocks followed. To obtain a precise aftershock distribution is important for understanding of mechanism of the earthquake generation. In order to study the aftershock activity, we carried out extensive sea floor aftershock observation using more than 100 ocean bottom seismometers just after the mainshock. Deployment and recovery of the OBS were repeated, and we obtained the data from OBSs just after the mainshock to the middle of September, 2011. A precise aftershock distribution for approximately three months in the whole source area, with an emphasis on depths of events, was obtained from the OBS data. In the southern source region, an aftershock distribution until September, 2011 was also estimated. Totally urgent OBS observations located 1210 aftershocks (Shinohara et al., 2011, 2012). After the urgent aftershock observation using short-term OBSs, we continued the observation using long-term OBSs to monitor seismic activities in the source area. We deployed 40 LT-OBSs in the whole source region in September 2011 and have completed recovery of the LT-OBSs until November, 2012. In this presentation, we concentrate seismic activities in the northern source region using the data from the urgent aftershock observation and long-term seafloor observation.We selected events whose epicenter is located below the OBS network form the JMA earthquake catalog, and P and S-wave arrival times were picked from the OBS data. Hypocenters were estimated by a maximum-likelihood estimation technique with one dimensional velocity structures. Thickness of sedimentary layer changes at each OBS site was evaluated and the estimated travel times by the location program were adjusted. We will report precise seismic activities in the northern source region with spatial and temporal variation. From preliminary analysis, seismic activity in off-Miyagi region was still low until the end of the long-term observation.