The Nankai subduction zone has repeatedly generated great earthquakes in excess of Mw 8. Because great earthquakes at convergent plate margins are believed to occur both at the plate interface and on mega-splay faults, seismic reflection data were acquired to reveal the structure of these seismogenic faults. However, we cannot clearly interpret the evolution of such a mega-splay fault system from the reflectivity images alone because of the low signal-to-noise ratio at the landward side of the active mega-splay fault, which may be due to rock consolidation as well as multiple reflections. Furthermore, the transition zone between the landward mega-splay fault (inner wedge) and the seaward décollement (outer wedge) is unclearly imaged because of the topographic relief at the outer ridge. The structures and pressure conditions of the transition zone are critical to evaluate coseismic rupture propagation close to the trough axis that may lead to tsunami generation. The 1605 Keicho earthquake (Mw7.9) is well known as a tsunami earthquake in the Nankai Trough, and was characterized by coseismic rupture close to the trough axis. The tsunami of the 2011 Tohoku-oki earthquake was also generated because of rupture propagation close to the trench.
Kamei et al. [2012 EPSL, 2013 GJI] has applied frequency-domain Waveform Tomography (WT) to controlled source Ocean Bottom Seismometer (OBS) data, and retrieved high-resolution P-wave velocity images of the mega-splay fault system. By exploiting recorded seismic waveforms beyond their first arrivals, the WT method achieves a much higher resolution than that of conventional traveltime tomography methods, and resolves the transition zone between mega-splay fault and seaward décollement.
Here, we explore the evolution of the mega-splay fault and its relationship to the décollement based on pore pressure distribution. We applied the methodology in Tsuji et al. [2008, JGR] in order to estimate the pore pressure around the Nankai mega-splay fault from the WT-derived seismic velocity model by integrating logging data and laboratory-derived data. Our results suggest that a high pore pressure zone at the footwall side of the deep mega-splay fault continues to the seaward region close to the trough axis. The over pressure zone of the footwall of the basal mega-splay fault (between the mega-splay fault and crustal surface) has almost constant values. This high pore pressure distribution indicates the possibility of coseismic rupture propagation from the deep mega-splay fault to the seaward trough region.
In this presentation, we further show crustal stress variation derived from seismic anisotropy analysis for the same OBS data as well as walkaround VSP data [Tsuji et al., 2011 Geophysics, 2011 G-cubed]. In these studies, we estimated principal stress orientation and its magnitude within the Nankai accretionary prism. Finally we show the studies of monitoring seismic velocity variation using seismic interferometry [e.g., Minato et al., 2012 GRL] to introduce our recent research objective (i.e., monitoring stress and pore pressure).
References
Kamei, R., G. Pratt, and T. Tsuji, Waveform Tomography Imaging of a Megasplay Fault System in the Seismogenic Nankai Subduction Zone, Earth and Planetary Science Letters, 317-318, 343-353, 2012.
Kamei, R., G. Pratt, and T. Tsuji, On acoustic waveform tomography of wide-angle OBS data - Strategies for preconditioning and inversion, Geophysical Journal International, 194, 1250-1280, doi:10.1093/gji/ggt, 2013.
Minato, S., T. Tsuji, S. Ohmi, and T. Matsuoka, Monitoring seismic velocity change caused by the 2011 Tohoku-oki earthquake using ambient noise records, Geophysical Research Letters, 39, L09309, 2012.
Tsuji, T., R. Hino, Y. Sanada, K. Yamamoto, J-O. Park, T. No, E. Araki, N. Bangs, R. von Huene, G. Moore, and M. Kinoshita, In situ stress state from walkaround VSP anisotropy in the Kumano basin southeast of the Kii Peninsula, Japan, Geochem. Geophys. Geosyst., 12, Q0AD19, doi:10.1029/2011GC003583, 2011.
Tsuji, T., J. Dvorkin, G. Mavko, N. Nakata, T. Matsuoka, A. Nakanishi, S. Kodaira, and O. Nishizawa, Vp/Vs ratio and shear-wave splitting in the Nankai Trough seismogenic zone: Insights into effective stress, pore pressure and sediment consolidation, GEOPHYSICS, 76, No.3, WA71-WA82, 2011.
Tsuji, T., H. Tokuyama, P. Costa Pisani, and G. Moore, Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan, J. Geophys. Res., 113, B11401, doi:10.1029/2007JB005002, 2008.
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