Abstract
In Northeast Japan, it has been recognized that trench-normal compressional stresses, aligned in the approximate direction of plate convergence, tend to dominate stress fields over a broad region. However, a particularly notable event was the shallow, normal-faulting earthquake swarms with a T-axis oriented in the E–W or NW–SE directions that occurred immediately after the 2011 Tohoku-Oki earthquake near the Pacific coast in the Southeast Tohoku district. The stress tensor inversion represents the pre-Tohoku-Oki earthquake stress field in this area as a normal-faulting stress regime with the minimum principal horizontal stress oriented in a roughly NW–SE direction. Additionally, the stress regime varies with depth from normal faulting at shallow depths (<15 km) to thrust faulting at greater depths. Seismic tomography and magnetotelluric soundings defined a geophysical anomaly with low seismic velocity and low resistivity clearly visible beneath the swarm activity, strongly supporting the existence of an interconnected network with fluid-filled porosity. The upper boundary of the conductor is in good agreement with an extensional–compressional stress transition zone. A plausible explanation for these drastic changes in the stress regime is upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. Additionally, remarkable upwarping and localized extensional tectonics during the late Pleistocene reflect the long-term rheological heterogeneities in the crust beneath the seismic source region.
Highlights
Convergent plate boundaries are classified into two groups according to whether their back-arc regions are or not actively spreading (Mariana type) or (Chilean type), respectively (Uyeda and Kanamori 1979)
The results indicate the following three features (Fig. 2): (1) the σ1 axis tends to be oriented E–W or WNW–ESE, approximately parallel to the direction of the plate subduction, (2) the σ1 orientation has an N–S direction in the Northeast Tohoku district, corresponding to the forearc side, and (3) in the Southeast Tohoku district, normal-faulting stress regime with the minimum principal horizontal stress oriented in the NW–SE or N–S directions is predominant, but reverse-faulting stress regime occupies in the deeper portion of the seismogenic zone (>15 km)
The stress field in the source region of the earthquake swarms changed from reverse or strike-slip faulting to normal faulting due to static stress change associated with the Tohoku-Oki earthquake (e.g., Asano et al 2011), the stress regime is characterized as normal-faulting type at depths shallower than 15 km in the Southeast Tohoku district, even before the 2011 Tohoku-Oki earthquake (Imanishi et al 2012; Yoshida et al 2015b)
Summary
Convergent plate boundaries are classified into two groups according to whether their back-arc regions are or not actively spreading (Mariana type) or (Chilean type), respectively (Uyeda and Kanamori 1979). I focus on seismotectonics before and after the Tohoku-Oki earthquake, the details of seismic velocity structure and electrical resistivity structure in the seismogenic zone, and vertical crustal deformation for the late Pleistocene and the present time.
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