Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan
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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.
KeywordsLocalized extensional tectonics Earthquake swarms 2011 Tohoku-Oki earthquake
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). In Northeast Japan, the Pacific plate is subducting along the Japan Trench under the North American plate at a rate of nearly 8–9 cm/year (DeMets 1992). The NE Japan arc is recognized as Chilean type, characterized by compressional subduction tectonics, increased seismic coupling and the occurrence of megathrust earthquakes (Conrad et al. 2004). Indeed, the overriding plate contraction in a roughly E–W direction is widely observed in the NE Japan arc based on a national geodetic survey (e.g., Sagiya et al. 2000). Geological data on slip rates of active faults indicate that significant E–W contraction across the NE Japan arc occurs on different time scales (Wesnousky et al. 1982). Earthquake focal mechanisms also reveal that the maximum principal stress (σ 1) is oriented in a WNW–ESE direction and reverse faulting occurs in this compressional tectonic regime throughout NE Japan (e.g., Townend and Zoback 2006; Terakawa and Matsu’ura 2010). However, recent dense seismic observations reveal that the σ 1 axes are oblique to plate convergence directions on the fore-arc side (Yoshida et al. 2015a).
The 2011 Tohoku-Oki earthquake was an unprecedented megathrust earthquake, monitored by the dense nationwide, high-sensitivity seismograph network (Hi-net) and global navigation satellite system (GNSS). A large number of studies have been reported since the earthquake, based on multiple lines of evidence identified in geological, geophysical and geochemical data (e.g., Somerville 2015; Hasegawa and Yoshida 2015). The amount and quality of various data can advance our understanding of earth and planetary sciences, including megathrust earthquakes in the subduction zone. Especially, geophysical findings of crustal heterogeneities and geological records of crustal deformation provide a clue to the mechanism for the generation of localized extensional tectonics in an overriding compressional island-arc crust. In this review, 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.
As mentioned previously, the stress pattern in NE Japan is WNW–ESE compression due to the Pacific plate subduction direction, with intermediate principal stress (σ 2) oriented in the N–S direction, resulting in an N–S reverse-faulting stress regime prevailing throughout the whole region. To clarify, it is known that stress orientations rotate due to the release of shear stress in a major earthquake, and the degree of rotation can place some constraints on the ambient level of stress in the crust surrounding the mainshock (e.g., Hardebeck and Hauksson 2001). Several researchers estimated the stress induced by the 2011 Tohoku-Oki earthquake using inverting focal mechanism data (e.g., Hasegawa et al. 2012; Yoshida et al. 2012). At depths shallower than 20 km, the stress field in the Northwest Tohoku district changed significantly; σ 1 rotated counterclockwise and has a NE–SW direction, whereas the minimum principal stress (σ 3) in the Southeast Tohoku district aligned with the plate convergence direction. However, in other areas of the Central Tohoku district, there seems to be no significant change in the stress regime (Yoshida et al. 2012).
Vertical crustal deformation
Discussion and conclusions
Most of the earthquake swarms that occurred in the Southeast Tohoku district after the Tohoku-Oki earthquake are atypical of NE Japan, because the focal mechanism is indicative of normal faulting. Actually, the Iwaki earthquake occurred in the central part of the seismogenic zone, on April 11, accompanied by surface ruptures along about 30 km of normal faulting with offset of ~2.0 m (Tsutsumi and Toda 2012). In addition, an extensional stress regime prevailed even before the 2011 Tohoku-Oki earthquake. This area has previously been shown to contain active normal faults; for example, the Itozawa and Yunotake faults, are concordant with surface ruptures associated with the Iwaki earthquake. A paleoseismic trench dug across the Itozawa fault exposed evidence for a penultimate earthquake that occurred sometime between 12,620 and 17,410 cal year B.P. (Toda and Tsutsumi 2013).
Imanishi et al. (2012) insisted that the recurrence of displacement along a low-angle seaward-dipping weak alignment extending from the seismogenic zone to the plate boundary contributes to the development of a normal-faulting stress field in an overall reverse-faulting regime in NE Japan. On the other hand, regional bending deformation of the overriding plate is proposed as a possible mechanism for creating an extensional stress regime in the fore-arc side of NE Japan (Hashimoto and Matsu’ura 2006; Yoshida et al. 2015b). Upper plate bending would facilitate large tensional stresses locally in the shallower part of the overriding plate that exceeds horizontal compression due to subduction (e.g., Turcotte and Schubert 2002). The effect of plate bending in the subduction zone is expected to affect areas over several 100 km. As mentioned above, long-term extension and uplift has been prevalent along the Pacific coast in the Southeast Tohoku district. However, remarkable uplift has been observed on length scales of several tens of kilometers (Fig. 6).
The strength and mechanical behavior of the crust is critical to the tectonic evolution and crustal deformation in subduction zones. Especially, fluids residing in brittle crust have a significant weakening effect on the mechanics of rocks and affect the long-term structural and compositional evolution of the crust (e.g., Hickman et al. 1995). Beneath the seismogenic zone of the earthquake swarms, a prominent, low seismic velocity, low electrical resistivity anomaly with a width of 20 km is defined at depths of ~15 km, indicating the possibility that aqueous fluid networks exist in the lower crust. It is therefore likely that rheological heterogeneities in the crust beneath the seismic source region would control the localized crustal deformation due to anelastic deformation under E–W compressional regional stress (e.g., Hasegawa et al. 2005). Iio et al. (2004) anticipated that uplift with slight contraction occurs above the weakened lower crust based on models having different assumptions of the viscosity, which can explain the vertical crustal deformation within the range of several tens of kilometers on different time scales in the Southeast Tohoku district.
Following the 2011 Tohoku megathrust earthquake, shallow, normal-faulting earthquake swarms occurred along the Pacific coast in the Southeast Tohoku district. I have reviewed geophysical information on the structural heterogeneities in the crust and the upper mantle, and crustal deformation at different timescales in and around the seismic source region. Independent geophysical observations indicate that aqueous fluid networks were present beneath the source region, prior to the Tohoku-Oki earthquake. Localized extensional tectonics in an overall reverse-faulting regime would be caused by upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. However, other seismic source regions with low seismic velocity and low resistivity in the lower crust (e.g., Ichihara et al. 2011; Ogawa et al. 2014) have not been observed to develop localized extensional tectonics on the back-arc side of the NE Japan arc. Future research into the stress regime relationship with crustal heterogeneities due to aqueous fluids is needed to address these issues.
This manuscript was improved by the careful review of G. F. McCrank and anonymous reviewers. This was supported by JSPS KAKENHI Grant Number 15H02998.
The author declares that they have no competing interests.
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