Tomotsugu Demachi

Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

Electrical image of subduction zone beneath northeastern Japan

We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ωm, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80–100 km depth and takes an overturned form toward the backarc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent backarc volcanoes in the system.

Extremely early recurrence of intraplate fault rupture following the Tohoku-Oki earthquake

Intraplate earthquakes on a fault in Earth’s upper crust commonly recur in 1,000 years or longer. The 2011 M9 Tohoku-Oki earthquake triggered the activation of intraplate earthquakes in northern Kanto, Japan, including two M6 events on 19 March 2011 and 28 December 2016 located near one another. Here we use displacements captured by satellite radar and the Global Navigation Satellite System to show that the two earthquakes ruptured an identical fault. Detailed fault slip modelling shows that the deformation data for the two earthquakes are well explained by slip along a common fault geometry, and that the majority of the slipped area on the fault overlaps. Strain analysis reveals that the first M6 earthquake was followed by exceptionally large postseismic deformation. Such deformation is consistent with afterslip around the M6 rupture area, which in turn rebuilds the shear stress on the fault enabling the next earthquake. We infer that the rapid and large postseismic deformation of the 2011 Tohoku-Oki earthquake promoted such afterslip and made the second earthquake recur in just 5.8 years. This study suggests a mechanism to explain observations of extreme temporal clustering in palaeoseismic earthquake recurrence studies.A pair of closely spaced intraplate earthquakes in Japan can be explained by postseismic deformation associated with the 2011 Tohoku-Oki earthquake.