Abstract:
The presence of seismic clusters has a significant impact on the study of the seismic regime in each area. In the present study the space-temporal distribution of the aftershock sequence observed after the earthquake on April 2002, with MW4.6. in the village of Krumovo, city of Plovdiv, Maritsa seismogenic zone is analyzed. The spatial and temporal clustering of aftershocks is a dominant non-random element of the seismicity, so when the clusters are removed, the remaining activity can be modelled as a Poisson process. Space distribution of earthquakes described that the main shock and aftershock activity are clustering almost perpendicular to a defined structure. The properties of aftershock sequences allow time-dependent prediction of aftershock probabilities. On the assumption that aftershocks are distributed in time as a non-stationary Poisson process it use the maximum likelihood method for estimating the parameters (K, c and p) of the modified Omori formula. For testing the goodness of fit between the aftershock occurrence and different statistical models a transformation from the time scale t to a frequency-linearized time scale t is applied. The aftershock activity observed after the earthquake is studied for two different time intervals. There is a relatively good correlation between the theoretical and the observed distribution for both analyzed cases. Akaike’s information criteria is applied to compare models for the distribution of aftershock activity in time. The attenuation of aftershock activity is well described by the modified Omori formula and multipole activity with two secondary series was established. About 378 days after the main shock the cumulative number of aftershocks increases rapidly with t, showing a significant deviation from the prior trend. No large earthquake occurred in the region at that time. Therefore, this change in slope could be treated as a transition from aftershock activity to background seismicity.