Spatial variations of stress orientation on the Tohoku megathrust show a correlation with spatial variations in coupling, with locked areas stronger than creeping areas in the megathrust zone ( 14). ( 13) proposed that variations in the forearc lithology of the overriding plate were the main cause of along-strike variations in the Tohoku-oki slip behavior. Recently, using residual topography and gravity data, Bassett et al. ( 12) also suggested that the Tohoku megathrust has a relatively homogeneous shear strength. Analysis of heat flow data ( 11) shows a relatively lower apparent coefficient of friction, suggesting a weaker megathrust zone producing the 2011 Tohoku-oki earthquake.

However, some authors have argued that the subduction of rough seafloor relief would cause fault creep and have proposed that the 2011 Tohoku-oki earthquake occurred in an area of relatively smooth subducting seafloor ( 10). Subsequent tomographic studies ( 7– 9) confirmed such a structural feature and suggested that the anomaly may reflect topographic and/or compositional variations on the subducting Pacific slab. Early tomographic studies ( 3, 4) of the Tohoku subduction zone showed that the 2011 Tohoku-oki mainshock hypocenter was located in a significant high-velocity (high- V) area, suggesting the possibility of a mechanically strong patch in the megathrust zone, where tectonic stress accumulated for a long time because of the strong coupling between the subducting Pacific slab and the overriding Okhotsk plate ( 5, 6). The red star in (B) and (C) denotes the mainshock epicenter of the 2011 Tohoku-oki earthquake ( M w 9.0).Īlthough the 2011 Tohoku-oki earthquake is the best instrumentally recorded megathrust earthquake ( M w ≥ 9.0) in the world, its causal mechanism is still a matter of controversy. The red triangles in (A) to (C) denote active arc volcanoes. ( D) East-west and ( E) north-south vertical cross sections of the 4760 earthquakes shown in (C). The three black lines denote locations of the vertical cross sections in Fig. The green circles denote 435 suboceanic events located using OBS data ( 49). The red circles show 1788 events that occurred under the Pacific Ocean and were relocated using sP depth phase data ( 7, 48). The blue crosses denote 2537 events that occurred beneath the seismic network ( 9). ( C) Map view showing the distribution of 4760 local earthquakes used in this study. ( B) Tectonic settings of the study region (blue box). The yellow dashed lines denote depth contours of the UBP. The white line marks the downdip limit of interplate seismicity ( 2). The red beach ball denotes the focal mechanism of the great 2011 Tohoku-oki earthquake ( M w 9.0), whereas the pink beach balls denote focal mechanisms of other megathrust earthquakes ( M w ≥ 7.0) during 1917 to 2017. ( A) Distribution of the 382 seismic stations (blue squares) used in this study. These structural heterogeneities in and around the Tohoku megathrust originate from both the overriding and subducting plates, which controlled the nucleation and rupture processes of the 2011 Tohoku-oki earthquake. Strong coseismic high-frequency radiations also occurred in high-velocity patches, whereas large afterslips took plate in low-velocity areas, differences that may reflect changes in fault friction and lithological variations. Its mainshock nucleated in an area with high seismic velocity, low seismic attenuation, and strong seismic coupling, probably indicating a large asperity (or a cluster of asperities) in the megathrust zone. We constrain the structure of the Tohoku forearc using seismic tomography, residual topography, and gravity data, which reveal a close relationship between structural heterogeneities in and around the megathrust zone and rupture processes of the 2011 Tohoku-oki earthquake. The great 2011 Tohoku-oki earthquake is the best-documented megathrust earthquake in the world, but its causal mechanism is still in controversy because of the poor state of knowledge on the nature of the megathrust zone.

Moment magnitude scale japan 2011