LAUSR

Abstract 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 Duzce rupture and the eastern Sea of Marmara. We used a local seismicity catalog of the Izmit-Duzce region covering a time span from 2.5 years prior to the Izmit until 14 months after the Duzce 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.