Temporal variations of seismicity parameters in the Central Alborz, Iran

Temporal changes of b-value, fractal (correlation) dimensions of epicenters (D (e) ) and occurrence time of earthquakes (D (t) ) and relations between these parameters were calculated to investigate precursory changes before 28 May 2004, Baladeh-Kojour earthquake (M (w) = 6.3) of Central Alborz, Iran. 2086 events with M (N) a parts per thousand yen 1.7 were selected for our analyses. A wide range of variation was seen in these parameters: b-value similar to 0.6-1.11, D (e) similar to 0.97-1.64, and D (t) similar to 0.13-0.93. The results showed decreases in all fractal parameters several months before the main shock. This decrease, which might have arisen due to clusters of events occurred between 2002-2003, was followed by a systematic increase, corresponding to the increased level of low-magnitude seismicity. It seems that changes in fractal parameters may be precursors of Baladeh-Kojour earthquake which was caused by seismic activation and quiescence. Furthermore, a positive correlation between b-value and D (e) was detected before the main shock (D (e) = 0.87 + 0.7b) and during aftershock sequences (D (e) = 2b +/- 0.09), which was further on changed to a negative one (D (e) = 2.56-1.32b).