Seismic evidence for reactivation of a buried hydrated fault in the Pacific slab by the 2011 M9.0 Tohoku earthquake

Abstract

We employ seismic tomography to estimate detailed 3D seismic velocity structures in the focal area of an intraslab earthquake (M7.1), which occurred on April 7, 1 month after the 2011 Tohoku earthquake (M9.0) near its source area. The results show a low‐velocity zone around the focal area of the M7.1 event, and that the aftershock activity is limited to the upper 15 km of the oceanic mantle. The lateral extent of the low‐velocity zone is comparable to the distribution of aftershocks, suggesting a concentration of fluids in the aftershock area. The angle between the aftershock alignment and the dip of the slab surface is estimated to be ∼60°, which is consistent with the dip of an oceanward‐dipping normal fault observed at the outer‐trench slope. These observations suggest that the M7.1 intraslab event occurred as a result of reactivation of a buried hydrated fault that formed prior to subduction. The upper ∼15 km of the oceanic mantle may be locally hydrated by bending‐related tensional faulting at the outer‐trench slope.

Multiscale Seismic Tomography

“…Nakajima et al (2011) determined a detailed tomography in the source zone of this intraslab earthquake. Nakajima et al (2011) determined a detailed tomography in the source zone of this intraslab earthquake.…”

Ground Shaking and Seismic Source Spectra for Large Earthquakes around the Megathrust Fault Offshore of Northeastern Honshu, Japan

“…Both events were probably triggered by stress changes resulting from the large fault slip in the great 2011 event Ohta et al, 2011). Nakajima et al (2011) suggest that the thrust event possibly involved reactivation of a preexisting hydrated fault with overpressurized fluids or serpentine minerals that had originally formed during bending near the trench. We also consider another outer-trench slope normal-faulting event (M w 7.0, 14 November 2005) and its preceding 2005 Miyagi-Oki interplate event (M w 7.3, 16 August 2005) located in the down-dip region of the megathrust, along with seven other underthrusting events located in megathrust domains A, B, or C. To suppress path effects, we use the empirical Green's function (EGF) method (Irikura, 1986), extracting reliable average source spectra that we can relate to the type of faulting and position along the megathrust.…”

Focal depths and mechanisms of Tohoku-Oki aftershocks from teleseismic P wave modeling

“…The focal depths of the 36 events in common with Table 1 differ *7 km on average with a maximum value of 19 km for event 27. Nakajima et al (2011) estimated event 27 to be *18 km deeper than the plate interface (*10 km deeper than the plate interface in this study) and argues this to be seismic evidence for reactivation of a buried hydrated fault in the Pacific slab. Lay et al (2013) estimated the event 65 consisted of three subevents, with the first one at 52.5 km depth (44.5 km in this study) and regarded it as indicating compressional stress accumulating at the uppermost mantle of the Pacific plate around the outer-rise regions.…”