ИСТИНА

The region of Japanese island arc is one of the most seismically active regions of the world due to very high plate convergence rate. On March 11, 2011., the great Mw=9.0 Tohoku earthquake occurred in the central segment of the Japanese subduction zone, the region where the previous megathrust earthquake was in 869. To identify the spatiotemporal variations of surface deformations following the 2011 earthquake we analyzed more than 7 years of continuous GNSS observations from over 1400 stations of GEONET network that covers the entire territory of Japanese islands. Coseismic displacements captured by GNSS stations allowed us to model the slip distribution in the earthquake source and to revealed significant effect of Fossa Magna Graben on the coseismic deformations pattern. The observed surface displacements before, during and after the Tohoku earthquake exhibited good agreement with the expected motions implied by the keyboard model of subduction zones [Lobkovsky and Baranov, 1984] at the appropriate stages of the seismic cycle. At the same time, analysis of the displacement rate variations estimated over 1-year intervals after the Tohoku earthquake showed an intense ongoing postseismic motion of complex mechanism. We tested a hypothesis of significant afterslip in the earthquake source using 1-month cumulative displacements for the first half of the year after the earthquake. The slip distribution in the earthquake source and afterslip process were modelled using open-source software package STATIC1D of F. Pollitz. We also used the open-source software package VISCO1D of F. Pollitz to test the influence on surface deformations of rapid viscoelastic relaxation caused by coseismic slip. Our analysis of the afteslip process showed that its maximal contribution rapidly decreases in the first six months after the earthquake from 1-3 meters to 10-20 centimeters per month and cannot predict the observed long-term displacements.