Structural heterogeneities around the megathrust zone of the 2011 Tohoku earthquake from tomographic inversion of onshore and offshore seismic observations

Abstract

We performed a three-dimensional seismic tomography around the coseismic slip area of the 2011 Tohoku earthquake. By combining data from the aftershock period collected by ocean bottom seismographs (OBSs), OBS data from previous studies off the Miyagi coast, and land seismic data, we modeled the detailed seismic velocity structure along the plate boundary from the Japan Trench to near the coastline. Our results indicate that V P , V S , and V P /V S along the plate boundary change drastically about 60 km landward from the trench axis. Trenchward of this boundary, velocities are consistent with a fluid-rich environment (low V P and V S and high V P /V S ) associated with the occurrence of sediment compaction and opal-to-quartz metamorphism. This area also corresponds to the contact between the slab and upper crust or between the slab and the sediment layer. A comparison of our results with numerical simulations and geological studies suggests that thermal pressurization might have occurred near the trench axis during the 2011 Tohoku earthquake. To the west of this boundary, where V P , V S , and V P /V S have moderate values, our model showed small-scale heterogeneous structures in the subducted oceanic crust near the hypocenter of the Tohoku earthquake with regions of low and high V P /V S on the updip and downdip sides of the hypocenter, respectively. We interpret the former as corresponding to the strong coupling patch and the latter as the localized fluid-rich zone. Small-scale heterogeneities thus may affect the nucleation and rupture processes of large earthquakes.

Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku‐oki earthquake: A comprehensive search using template events

“…Cross sections (depth distribution) of faulting types across the large coseismic slip area off Miyagi Prefecture and north of the large coseismic slip area off Iwate Prefecture (rectangular areas, Figure b) show that normal faulting earthquakes in the offshore region were relocated at shallower depths than in the original catalogue (Figures d and f). This is consistent with depth determinations by sP depth phase [ Gamage et al , ] and temporal ocean bottom seismometer (OBS) data analyses for the earthquakes in the area [e.g., Hino et al , ; Obana et al , , ; Yamamoto et al , ]. We also found that most of the normal faulting events (green) in the horizontal range of 0–150 km are located in the upper plate after the Tohoku‐oki earthquake (Figures d and f).…”

Preceding seismic activity and slow slip events in the source area of the 2011 Mw 9.0 Tohoku-Oki earthquake: a review

“…Smaller stars show hypocenters of the M 7.3 foreshock and M>7 aftershocks that occurred just after the mainshock earthquake above the large slip area. However, the result from Zhao et al [41] cannot be confirmed from the tomographic image obtained using data from this network [45]. This is because the spatial extent of the OBS network was too small for confirmation; only the southern asperity patch was covered by the network.…”

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

“…5h). Studies of seismic tomography do not support V P /V S ratios corresponding to such a high Poisson's ratio except in the toe part of the overriding plate (Matsubara and Obara 2011;Yamamoto et al 2014).…”