Seismotectonic setting at the North Anatolian Fault Zone after the 1999 Mw=7.4 Izmit earthquake based on high-resolution aftershock locations

摘要

The most recent devastating earthquakes that occurred along the NorthAnatolian Fault Zone (NAFZ) in northwestern Turkey were the 1999 Izmit(Mw=7.4) and Düzce (Mw=7.1) events. In this study we present a catalogof Izmit aftershock hypocenters that was deduced from a network covering theentire 140 km long rupture of the mainshock. 7348 events with a locationaccuracy better than 5 km are analysed. Aftershocks were observed along theentire ruptured segment along a 20 km wide band of activity. Events areclustered in distinct regions and dominantly occur at 5 to 15 km depth. Theeastern termination of the Izmit rupture is characterized by a sharp andsteeply dipping boundary exactly where the Düzce mainshock initiated 87days after the Izmit event. Relocation of the events using double-differencetechnology results in 4696 high-resolution hypocenters that allow resolvingthe internal structure of the seismically active areas with a resolution of300 m (horizontal) and 400m (vertical). Below the Akyazi Plain, representinga small pull-apart structure at a triple junction of the NAFZ, we identifyplanes of activity that can be correlated with nodal planes of EWextensional normal faulting aftershocks. Along the easternmostKaradere-Düzce segment we identify the down-dip extension of theKaradere fault that hosted about 1 m of right-lateral coseismic slip. At theeasternmost rupture we correlate a cloud-type distribution of seismicactivity with the largest aftershocks in this area, a subevent of the Izmitmainshock and the Düzce mainshock that all have an almost identicalfocal mechanism. This part of the NAFZ is interpreted as a classical exampleof a seismic barrier along the fault.