Evaluation of present day seismicity in the Aegean Region using Kaltek Method

The relative size distribution of earthquakes is an essential input parameter needed to perform probabilistic seismicity analysis. In this analysis, the basic well-known equation of Gutenberg-Richter relation (LogN=a-bM), one of the well-fitted empirical relations in seismology, the frequency of occurrence of earthquakes as a function of magnitude is explained. Many researchers accepted that the b-value in this equation reflects the region seismicity. For the calculation of a and b parameters, many methods are used such as Maximum likelihood, least-square, weighted least-square, Kaltek method, etc. We used newly developed Kaltek method. This method is constituted one assumption and one hypothesis for the calculation of b-values. Assumption, The a-value in the Gutenberg–Richter relation demonstrated exponential distribution of the earthquakes that are zero magnitude. Hypothesis, Under this assumption; the a-value calculated from the whole region data set can be accepted as a constant value for the calculation of new b-value belonging to each subregion, which are included by the main region. On the other hand, the number of earthquakes that have zero magnitude is equal to the constant value for each subregion or every point of the whole region.
   In this study, the spatial distributions of seismicity and seismic hazard were assessed for Aegean Sea and its surrounding area. For this purpose, earthquakes that occurred between 1964 and 2010 with magnitudes of M≥4 were used in the region (32–42°N and 20–30°E), selected from International Seismological Centre (ISC) catalogues. For the estimation of seismicity parameters and its mapping, the Aegean Sea and surrounding area are divided into (0.25⁰Nx0.25⁰E, r=0.25⁰) 1,681 circular subregions. The a and b-value from the Gutenberg–Richter frequency magnitude distributions is calculated by the classic way using the leastsquares technique. In this calculation, the minimum, maximum and average a-values are found to be equal to a_min=1.08, a_max=10.98 and a_avr=5.22, respectively, in the 1,681 subregions. Variance and standard deviation of the a-value are estimated to be v=2.0 and q=1.4. we calculated new b-values for every subregion taking a constant a-value which is equal to a_avr=5.22 according to Kaltek procedure.
   Our results and the seismicity map obtained from the Kaltek method showed very good consistency with the tectonic and earthquake activity in the region. The minimum, maximum and average b-values are determined as b_min=0.65, b_max=1.30 and b_avr=1.05, respectively. From the map view of the b-value, we distinguished two aseismic zones of different sizes in the Aegean Sea. These zones are characterized by high b-values (b=1.15–1.25). First zone locates off the Crete Island between 23.4-25.6⁰E and 35.4-36.4⁰N. The second zone is placed between 23.4-26.6⁰E and 37.0-38.4⁰N on the Cycladic units in the Aegean Sea.