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.

The Chilean subduction zone is one of the most seismically active regions on Earth. Focal zones of earthquakes with magnitude M>8, registered in this region for the last 200 years, cover almost the entire length of the Chilean coast. In 2010 the Darwin seismic gap existing from 1835 between the focal zones of 1960 Great Chilean earthquake and the 1985 Valparaiso earthquake was interrupted by the Maule earthquake Mw = 8.8. We analyzed the data of 8 years of continuous observations at 27 sites of the Chilean GPS network in order to distinguish coseismic and postseismic deformations in the vicinity of the 2010 earthquake. The analysis of postseismic deformations is based on the ground displacement velocities estimated over 1-year intervals. We used the keyboard model of subduction zones [Lobkovsky and Baranov, 1984] combined with the model of viscoelastic relaxation in the asthenosphere and the upper mantle to explain the variety of motions observed in the region of Central Chile. We model viscoelastic relaxation caused by coseismic slip using the open-source software package VISCO1D of F. Pollitz. The coseismic displacements captured by the sites close to the epicenter of the 2010 event amounted to 1 to 3 meters, which characterizes the magnitude of the displacements of seismogenic blocks at the time of the earthquake. All the coseismic displacements are directed toward the ocean, which agrees well with the predicted movements of the blocks in the seismic stage of the seismic cycle. Data from the first two years after the 2010 event show attenuating site offsets toward the ocean over the whole Central Chile region which indicates passage of aftershock stage of the seismic cycle. Over the next 5 years the observed displacements can be explained by the process of restoring the stationary state of stress accumulation for seismogenic blocks in the frontal part of the subduction zone in combination with the continuing motion of the rear block with viscous asthenospheric flow. The duration of the viscoelastic relaxation process for the Maule earthquake is estimated to last for more than 15 years.

The Chilean subduction zone is one of the most seismically active regions on Earth, due to the shallow depth of the seismogenic zone in combination with the high coupling coefficients and high plate convergence rate. On February 27, 2010, the Maule earthquake, which is one of the strongest instrumentally registered megathrust earthquakes, occurred in Darwin seismic gap – a seismically calm zone existed near the coast of Chile since 1835. We use the keyboard model of generation of strong subduction earthquakes [Lobkovky et al., 1991] to analyze the peculiarities of seismic deformation cycle (SDC) related to the Darwin seismic gap and the 2010 earthquake. Keyboard structure of the South American continental margin near the source zone of the Maule earthquake is confirmed by seismological and geological data. Using of keyboard model allows us to associate interseismic, coseismic and postseismic deformations observed by satellite geodesy with the action of particular geodynamic processes and, therefore, to study their features. To achieve this aim, we analyze the data of almost 10 years of continuous observations at 76 stations of the Chilean GPS network deployed along the source zone of the 2010 Maule earthquake. The analysis of variations in surface deformation fields is based on displacement rates fields of GPS stations estimated over 1-year intervals. On the basis of registered coseismic displacements we construct a model of slip distribution in the source zone of the Maule earthquake and determine the magnitudes of instantaneous shifts of seismogenic blocks towards the ocean, which reached 1-3 meters. In the first two years after the 2010 earthquake GPS stations shift toward the ocean over the whole Central Chile region, which indicates passing of aftershock stage of the SDC. Over the next 7 years of observations, the observed displacements can be mainly explained by the process of restoring the stationary state of stress accumulation for seismogenic blocks in the frontal part of the subduction zone in combination with the continuing displacement of the rear massif by a viscous asthenospheric flow. To assess the time of transition of seismogenic zone to stationary state of accumulation of elastic stresses we construct models of frictional afterslip and viscoelastic relaxation in the asthenosphere. The durations of the afterslip and viscoelastic relaxation processes for the Maule earthquake are according to our estimates about half a year and more than 15 years respectively. Understanding of the features of the SDC plays a significant role in the seismic hazard assessment of the Maule and Biobio regions of Chile.

Modern seismotectonic studies are aimed at obtaining a self-consistent explanation of fault zone heterogeneity, the rupture process, recurrence times and rupture mode of large earthquake sequences. In subduction regions large earthquakes are often characterized by very long source zones and complex long-term postseismic processes following the coseismic release of accumulated elastic stresses. A set of mechanical models was proposed to describe the generation of strongest earthquakes based on the idea of the synchronous failure of several adjacent asperities. In this study we propose a model which is based on verified numerical schemes, which allows us to quantitatively characterize the process of generation of strong earthquakes. The model takes into account the fault-block structure of the continental margin and combined the ideas of a possible synchronous destruction of several adjacent asperities, mutual sliding along a fault plane with a variable coefficient of friction and subsequent healing of medium defects under high pressure conditions. The applicability of the proposed model is shown by the example of the recent seismic history of the Kuril subduction zone. Kuril island arc is one of the most tectonically active regions of the world due to very high plate convergence rate. Heterogeneities in the mechanical coupling of the interplate interface in this region lead to the formation of the block structure of the continental margin, which is confirmed by various geological and seismological studies. GPS observations recorded at different stages of seismic cycle related to the 2006--2007 Simushir earthquakes allow us to model geodynamic processes of slow strain accumulation and its rapid release during the earthquake and the subsequent posteseismic process. We use parameters describing the regional tectonic structure and rheology obtained from the inversion of geodetic data to construct a 2D model of generation of large earthquakes in central Kurils. Analysis of paleoseismic data on dates and rupture characteristics of previous major earthquakes shows a good agreement between the modeled and observed seismic cycle features. The predicted horizontal displacements of the seismogenic block at the coseismic stage are consistent with satellite geodetic data recorded during the 2006 Simushir earthquake. The proposed model provides new insights into the geodynamic processes controlling the occurrence of strong subduction earthquakes.