## 4 Discussion  
We believe that employment of the fault-block models seems to be the most promising approach, which allows for more realistic quantitative characterization of the seismic deformation process [19], and propose the generalized two-segment keyboard model [22] as a geomechanical concept capable of explaining the SDC and kinematics in the subduction regions. It is also clear, that validation of such model is only possible through onshore GNSS observations, thus making the problem weakly constrained and unstable, given the lack of data on particular properties of the model. To verify the above assumptions, we performed a numerical simulation of series of seismic cycles using initial and generalized formulations of keyboard model with stochastic behavior of parameters governing the critical amount of accumulated elastic energy and the amount of relaxed energy. During the simulations the model parameters were constrained to values that are typical for the Kuril subduction system (\(E_{1}\) = 29 GPa, \(E_{2}\) = 80 GPa, \(d_{i}\) = 200 km, frontal block length = 100 km, rear block length = 200 km, effective viscosity of the contact layer and the asthenosphere = 10\({}^{19}\) Pa-s). We also performed a comparison of the obtained model parameters with ones calculated earlier using the continuum medium model [19] and with geophysical data. We assumed that the modeled coseismic slip distribution in the source zone of the 2006 Simushir earthquake [19] according to the asperity model [12] represents the preseismic mechanical coupling that was ruptured at the time of the main shock. Such suggestion helps us to construct a model of coupling distribution in the source zone of the 2006 Simushir earthquake (Figure 11). This model allows us to calculate the characteristic time of stress accumulation (i.e., seismic cycle duration) in the source zone of the 2006 Simushir earthquake (Table 3) using known coseismic slip distribution and magnitude and direction of the plate convergence vector [41]. In Table 3 and Figure 12 we present the results of modeling the dynamics of seismogenic blocks in the middle part of the Kuril island arc for 1400 years (10 times the mean duration of SDC for Kuril-Kamchatka subduction zone [32]). To simplify the model, the block sizes are assumed to be the same (100 km \(\times\) 100 km). According to Global CMT Project data [42], large subduction earthquakes of the Kuril Islands are characterized by a focal length of 120 to 250 km. For example, the spatial dimensions of the source zone of the First Simushir earthquake was estimated on the basis of the aftershock distributions recorded in the first 2 months after the earthquake. This megathrust earthquake was the largest event in the central Kuril Islands since 1915 with the source zone of about 230 km \(\times\) 150 km. Coseismic motions in the source zone affected three seismogenic blocks with a maximum slip along the contact zone up to 12 m [19]. The rheological parameters of the medium were selected during the further modeling according to the results of the previous studies of the First Simushir earthquake [19]. As a result of the simulation, we estimated the total duration of the seismic cycle, which lay between 113 and 330 years with an average value of 196 years and the duration of the postseismic stage, which corresponds to a range of 11-19 years with an average value of \(\sim\)15 years. These estimates are in good agreement with the real values of the seismic cycle associated with the 2006 Simushir earthquake, which is 226 years [43] and more than 10 years [19], respectively. For the case of the used rheological parameters of the medium during the realization of seismic cycles, episodes of simultaneous displacements of two adjacent seismogenic blocks were observed, which confirms the possibility of the formation of extended source zones, such as of the First Simushir earthquake (about 200 km). Figure 11: Model of the mechanical coupling distribution in the source zone of the 2006 Simushir earthquake.