Anomalous seismicity preceding the 1999 Izmit event, NW Turkey

Abstract

The 1999 Izmit earthquake (1999 August 17, M-w = 7.4) was one of the largest earthquakes to occur in northwestern Turkey during the past 100 yr. This earthquake occurred along the Izmit-Sapanca fault within the Northern Anatolian Fault Zone. Variations in the generalized fractal dimensions (D-q) of clustering in time and space, the Gutenberg-Richter b-value, and earthquake frequency (N) are evaluated in detail over an 8.5 yr time period preceding the Izmit event. Spatial and temporal comparisons of the variations in these parameters reveal anomalous intermediate term behaviour over shorter timescales than previously observed. Significant correlation is observed between changes in b-value with the spatial and temporal fractal dimension of epicentre distribution. These correlations oscillate back and forth over 2-3 yr time intervals and suggest occurrence of significant instability in the nature of intermediate-term deformation along the fault zone. Seismotectonic behaviour immediately preceding the Izmit event is represented by a significant rise in b-value to a maximum of 2.26 accompanied by relatively small increases in D. The rapid rise of b is associated with an increased frequency of low magnitude seismicity. Although D (spatial and temporal) increase slightly in the 2-3 yr period preceding rupture, they remain less than 1 indicating that seismicity remains clustered prior to rupture. In areas where earthquake frequency or station density do not permit resolution of short-term variations of fractal scaling parameters within a few months of main rupture, an accelerated period of low-magnitude seismicity (high b-value) concentrated along a fault zone (D < 1) may suggest heightened probability of a forthcoming large event.