Variations of seismic b-value at different stages of the seismic cycle along the North Anatolian Fault Zone in northwestern Turkey

We studied spatiotemporal b-value variations along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with a focus on the combined 1999 Izmit and Düzce rupture and the eastern Sea of Marmara. We used a local seismicity catalog of the Izmit-Düzce region covering a time span from 2.5 years prior to the Izmit until 14 months after the Düzce mainshock and a four-year hypocenter catalog in the eastern Sea of Marmara. We consistently calculated moment magnitudes to ensure a homogeneous dataset and applied strict quality criteria. This allows studying variations of b-values throughout the region and at different stages of the seismic cycle. With a standard gridding technique b-value maps, depth sections and time series were calculated which reveal a very heterogeneous b-value distribution in the study area. The variety of b-value observations cannot be interpreted unambiguously, given that the b-value most likely depends on a combination of fault-zone characteristics like local stress conditions, heterogeneity of the crust and damage distribution. By presenting a comprehensive set of possible interpretations we point out that a biased discussion of the results towards stress or another individual parameter may lead to erroneous conclusions. Furthermore, the applied data discretization scheme influences the appearance of the final b-value distribution leading to potential misinterpretations.