Microseismic Constraints on the Mechanical State of the North Anatolian
Fault Thirteen Years after the 1999 M7.4 Izmit Earthquake
Abstract
The 17 August 1999 $M_{w}$7.4 Izmit earthquake ruptured the western
section of the North Anatolian Fault Zone (NAFZ) 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 dense seismic array 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
\textit{i}) as indicating that the Akyazi seismicity is
occurring in high background stresses and is driven by the Izmit
earthquake residual stresses, and \textit{ii}) as
suggesting evidence that intricate seismic and aseismic slip was taking
place on heterogeneous faults at the eastern Lake Sapanca, near the
brittle-ductile transition. Geodesy shows enhanced north-south extension
around Lake Sapanca following the Izmit earthquake, therefore, the
seismicity supports the possibility of slow slip at depth in the
step-over.