Microseismic Constraints on the Mechanical State of the North Anatolian Fault Zone 13 Years After the 1999 M7.4 Izmit Earthquake

Abstract

The 17 August 1999 Mw7.4 Izmit earthquake ruptured the western section of the North Anatolian Fault Zone and strongly altered the fault zone properties and stress field. Consequences of the co- and post-seismic stress changes were seen in the spatio-temporal evolution of the seismicity and in the surface slip rates. Thirteen years after the Izmit earthquake, in 2012, the seismic network Dense Array for North Anatolia (DANA) was deployed for 1.5 years. We built a new catalog of microseismicity (M < 2) by applying our automated detection and location method to the DANA data set. Our method combines a systematic backprojection of the seismic wavefield and template matching. We analyzed the statistical properties of the catalog by computing the Gutenberg-Richter b-value and by quantifying the amount of temporal clustering in groups of nearby earthquakes. We found that the microseismicity mainly occurs off the main fault and that the most active regions are the Lake Sapanca step-over and near the Akyazi fault. Based on previous studies, we interpreted the b-values and temporal clustering (a) as indicating that the Akyazi seismicity is occurring in high background stresses and is driven by the Izmit earthquake residual stresses, and (b) as suggesting evidence that an intricate combination of seismic and aseismic slip was taking place on heterogeneous faults at the eastern Lake Sapanca, near the brittle-ductile transition. Combined with geodetic evidence for enhanced north-south extension around Lake Sapanca following the Izmit earthquake, the seismicity supports the possibility of slow slip at depth in the step-over.