Precursory seismic quiescence before 1 May 2003 Bingöl (Turkey) earthquake: A statistical evaluation

Abstract

The variations of seismicity rate changes prior to the occurrence of 1 May 2003 Bingöl earthquake has been analyzed by statistical method, with the observation of pointing up precursory seismic sequence. We investigated the significance of seismic quiescence for the region 39.6ºE-40.6ºE and 38.2ºN-39.6ºN. The analysis was carried out on the instrumental catalogue of the Boğaziçi University, Kandilli Observatory and Earthquake Research Institute (KOERI), covering the period from 1970 to 2005. The catalogue is homogeneous for duration magnitude, MD, including 1926 crustal earthquakes of magnitude equal and greater than 1.8, with depths less than 70 km. For the calculations, we used the completeness magnitude as 3.2 and the number of events exceeding this magnitude is 915. In the first step, we declustered the catalogue using the Reasenberg algorithm. 43% of the events were removed and the number of these events was reduced to 522. The analysis was carried out making use of the ZMAP software package. The methodology we use involves the gridding method at the nodes of a 0.02º grid spacing. By ZMAP, the number of events associated with each grid point was selected as 50 after a few tests. The data are subdivided in bins of 28 days for each sample and we used a mowing time window TW=5 years for the imaging of space and time rate changes. Finally, using the declustered catalogue spatial variability of Z-values is mapped for every one year between 1990 and 1998. It is not observed any significant seismic quiescence in time interval 1990 and 1993 around the 2003 main shock. After 1993, we observed a clear quiescence in and around the main shock epicenter of 1 May 2003 Bingöl earthquake with Z level between 2 and 3. This quiescence period is best revealed between 1995 and 1998. To identify the starting time and duration of precursory quiescence it is plotted cumulative number of events corresponding LTA(t) function for a circle of 7.79 km radius centered on Bingöl main shock. A decrease of the seismicity rate is found with Zmax=2.5 level at 1997.6. Thus, we found that Bingöl earthquake on May 1, 2003 (2003.33) followed an outstanding seismic quiescence starting 5.73 years before main shock. Based on