Spatial-temporal analysis of seismicity before the 2012 Varzeghan, Iran, Mw 6.5 earthquake


Abstract: In this work, the spatial and temporal variations of seismicity in northwestern Iran have been evaluated with a special focus on seismic precursors of the 11 August 2012 Mw 6.5 Varzeghan earthquake. The precursors are defined by the z-value test and the generalized fractal dimensions of earthquake epicenters. The investigation applies earthquakes that have occurred since 2006 in a region that includes the main shock and collected by the Iranian Seismological Center. In order to eliminate the effect of explosions in the processing, we removed the explosion from the used catalog by using the normalized ratio analysis of daytime to nighttime events. Having done the preprocessing procedures, we removed the events with magnitudes of less than 2.5. As a result of our analysis, a period of seismic quiescence has been identified, which started about 3 years before the Varzeghan main shock. In a nice coincidence with these results, significant changes have been observed in the generalized fractal dimensions and the related spectra prior to the occurrence of the Varzeghan earthquake. The changes indicate that the seismic activities of the studied area have had increasingly dense clustering in space since about mid-2009, which suggests the regional preparedness for the occurrence of the main quake. Furthermore, the analysis did not exhibit any significant seismic quiescence anomaly at the beginning of 2013.

Keywords: Varzeghan earthquake, z-value, seismic quiescence, generalized fractal dimensions

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