Seismotectonic setting of the Karadere–Düzce branch of the North Anatolian Fault Zone between the 1999 Izmit and Düzce ruptures from analysis of Izmit aftershock focal mechanisms

We investigate aftershock focal mechanisms along the eastern part of the Izmit Mw = 7.4 August 17, 1999 rupture zone during the time period August 22, 1999–October 17, 1999. Two spatial clusters of aftershock activity are analyzed representing the Karadere Fault (KF) and the Düzce Area (DA). Based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors < 2 km, we determine fault plane solutions for 221 events. The high number of focal mechanisms at the eastern Izmit rupture zone could be determined only due to the low magnitude-detection threshold of the seismic network and allows to resolve the local deformation pattern with unprecedented precision. Focal mechanisms along the Karadere Fault allow us to identify dominantly dextral strike–slip mechanisms with normal faulting components on NE–SW trending fault planes. Focal mechanisms in the Düzce Area predominantly exhibit NE–SW extensional normal faulting but also a substantial part of strike–slip faulting. Further subdivision of the data set slightly decreases for the misfit for deeper (z > 10 km) events. North and east of the easternmost end of the Karadere Fault we observe a high variance in stress field orientation correlated with lower b-values. While the Karadere Fault reflects a predominant dextral strike–slip regime with normal faulting components, the Düzce Area further to the East that also hosted the forthcoming Mw = 7.2 mainshock 87 days after the Izmit earthquake can be subdivided into a dominantly NE–SW extensional normal faulting regime below the Düzce Basin (DB) and a first-order strike–slip regime along the western Düzce Fault (DF). We conclude that the Düzce Basin was set under tension by the Izmit rupture and partly released the slip deficit by extensional faulting on Karadere Fault parallel to the coseismic displacement. At the same time this area and in particular the Düzce Fault that bounds the Düzce Basin to the south reflects mostly strike–slip events representing a major asperity along the North Anatolian Fault Zone (NAFZ) before initiating the Düzce rupture 87 days after the Izmit event.