Seismic heterogeneity and anisotropy of the Honshu arc from the Japan Trench to the Japan Sea

Abstract

S U M M A R YWe determined P-and S-wave tomography and P-wave anisotropic structure under the Honshu arc from the Japan Trench to the backarc area under the Japan Sea using 310 749 P-and 150 563 S-wave arrivals from 4655 local earthquakes recorded by 982 seismograph stations. Arrival times from 1451 suboceanic earthquakes relocated with sP depth phases enable us to determine the structures under the Pacific Ocean and Japan Sea, which expand the study region from the land area to the whole arc from the Japan Trench to the Japan Sea with a width of more than 500 km. The results show strong heterogeneities above the subducting Pacific slab under the Pacific Ocean and most large thrust-type earthquakes occurred in the high-velocity areas where the Pacific slab and the overriding continental plate may be strongly coupled. Low-velocity (low-V ) zones are imaged in the mantle wedge with significant along-arc variations under the volcanic front. The mantle-wedge low-V zone extends westwards under the Japan Sea and it is connected with the subducting Pacific slab at depths of 150-200 km under the backarc.The results indicate that the H 2 O and fluids brought downwards by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under the Honshu arc. The predominant fast velocity direction (FVD) is E-W in the mantle wedge while it is N-S in the subducting Pacific slab. The anisotropy in the mantle wedge is the result of deformation caused by the subduction of the Pacific plate and the induced mantle-wedge convection, while the FVD pattern in the middle of the mantle wedge argues for the 3-D mantle flow or the specific alignment of the olivine in the partial-melting mantle. The N-S (trench-parallel) FVD in the subducting Pacific slab represents either the original fossil anisotropy when the Pacific plate formed or the trench-parallel crystallographic and shaped preferred orientation in the subducting slab due to the slab bending. The present results shed new light on the structural heterogeneities and seismic anisotropy under the Honshu arc, which may improve our understanding of the dynamic processes of subduction zones.

Static slip model of the M w 9.0 Tohoku (Japan) earthquake: Results from joint inversion of terrestrial GPS data and seafloor GPS/acoustic data

“…In addition, based on previous seismic imaging [13], the dip angles of the rupture surface were defined to increase progressively from 6° at the top to 23° at the bottom of the fault. Overall, the geometry of the rupture is consistent with the slab contours of Huang et al [14], and should be more reasonable than other widely used planar faults.…”

Coseismic deformation due to the 2011 Tohoku-oki earthquake: influence of 3-D elastic structure around Japan

“…The rigidity ratio in our SLAB model is 2.4, while Kyriakopoulos et al 's (2013) model used 3.7. According to studies of seismic tomography (e.g., Nakajima et al 2001;Huang et al 2011), velocity anomalies (V P and V S ) in the slab compared to the surrounding mantle are ~6 %, implying that even our contrast might be on the high end. Kyriakopoulos et al 's (2013) larger contrast may lead to an overestimate of the effect of elastic heterogeneity.…”

“…The rigidity for HOM is assumed to be 35 GPa (deformation for homogeneous model is independent of rigidity). Considering seismic velocity structure beneath Japan and V P -and V S -anomalies of the slab for guidance (e.g., Nakajima et al 2001;Matsubara et al 2008;Huang et al 2011), values for the rigidity of the crust, mantle, and slab in LYR and SLAB are set to 35, 65, and 85 GPa, respectively, and the Poisson's ratio is set to 0.25 for all materials. Note that average rigidity for the model increases in order of HOM, LYR, and SLAB.…”

Spatio-temporal distribution of seismic moment release near the source area of the 2011 Tohoku-Oki earthquake

“…The behavior of earthquakes seems to vary with depth. In addition, seismic tomographic images show that the depth of the forearc Moho along the Japan Trench is approximately 30 km (Zhao et al, 1994Satoh et al, 2001;Miura et al, 2005;Shinohara et al, 2005;Huang et al, 2011), which suggests the possible location of the brittle-ductile transition zone. We therefore separate the earthquakes into up-dip (<30 km, near trench) and down-dip (≥30 km) parts in the following analysis.…”