The aftershock processes of strong earthquakes in the Western Caucasus

The aftershock processes of the four strong earthquakes that occurred in the Western Caucasus from 1991 to June 2013 are considered. The main shocks of these earthquakes include the first Racha earthquake (April 29, 1991, Ms = 6.9); second Racha earthquake (June 15, 1991, Ms = 6.2); Oni earthquake (September 7, 2009, Ms = 5.8); and East Black Sea earthquake (December 23, 2012, Ms = 5.6). Based on the simulations with the LPL relaxation model and the ETAS model of triggered seismicity, the differences in the properties of the aftershock processes and the characteristics of the fault zones accommodating the main shocks are revealed. The nonrelaxation character of the aftershocks from the East Black Sea earthquake is established. It is hypothesized and validated that this is a result of the violation of the fluid-dynamic equilibrium in the fault zone due to the destruction of the gas hydrate layer by the main shock and strong aftershocks.