Anomalous seismotectonic interrelationships preceding the 1999 Izmit event , NW Turkey

The 1999 Izmit earthquake (17/08/1999, Mw=7.4) was one of the largest earthquakes in the past 100 years in the northwestern region of Turkey. This earthquake occurred along the Izmit-Sapanca fault within the Northern Anatolian Fault Zone. Variations in the generalized fractal dimensions (Dq) of clustering in time and space, the Gutenberg-Richter b-value, and earthquake frequency (N) are evaluated in detail over an 8 year time period preceding the Izmit event. Comparisons of spatial and temporal variations in these parameters reveal the presence of anomalous behavior prior to final rupture that occurs on shorter time scales than previously observed and provide insights into the nature of precursory deformation along the fault zone. Significant correlation is observed between b-value and spatial fractal dimension together with the correlation between time-clustering (DT) and spatial clustering (DS). These correlations change sign over 2 to 3 year time intervals and reveal fundamental changes in the nature of the deformation occurring along the fault zone. Seismotectonic behavior immediately preceding the Izmit event is represented by an excessive rise in b value to a maximum of 2.26 accompanied by relatively small increases in D. The rapid rise of b with D < 1 is associated with clustered and increasingly frequent low magnitude seismicity. A significant drop in b value, considered an earthquake precursor, was not observed prior to