Spatial variations of seismicity rate precursor to the Erzurum (Turkey) earthquake sequences on March 25 and 28, 2004

We focused in this study on the detection of seismic quiescence situation prior to the Erzurum (Turkey) earthquake sequences. We used the instrumental catalogue of KOERI from 1970 up to 2005 for the region 38.8º-41.0ºN and 40.2º-43.2ºE. The catalogue includes 1481 crustal events of MD magnitude equal and greater than 2.2, with depths less than 70 km. The completeness magnitude for Erzurum region is 3.0 and the number of events exceeding this magnitude level is 1000. In the first step, we declustered the catalogue using the Reasenberg algorithm. 28 % of the events were removed and the number of these events was reduced to 719. Spatial variation of statistical Z-value is mapped with gridding method at the nodes of a 0.02º grid spacing for every one year between 1995 and 2001 by ZMAP software. We found that the MW=5.5 Erzurum earthquakes on March 25 (at 21:30:50 UTC) and 28 (06:51:09 UTC), 2004 followed an outstanding seismic quiescence with a decrease of the seismicity rate, Zmax=4.0 level at 1999.2, starting 5.04 years before the main shocks near the earthquake regions.