Changes of seismic velocity around the Karadere-Duzce branch of the north Anatolian fault from coda waves generated by repeating earthquakes

We analyze temporal variations of seismic velocity along the Karadere-Duzce branch of the north Anatolian fault from repeating earthquakes in aftershocks of the 1999 Mw7.4 Izmit and Mw7.1 Duzce earthquakes. We identified about 30 sets of repeating earthquakes, each containing 5-16 events. Relocation of events in each set suggests that most rupture approximately the same fault patch at different times. We use a sliding cross-correlation window technique (Niu et al., 2003) to measure travel time difference and evolving decorrelation in waveforms generated by each set of repeating events relative to the waveform of the first event. We find systematic changes between the early S coda waves generated by events before the Duzce mainshock and those generated after it. The largest co-seismic velocity change is observed at station VO that was deployed inside the rupture zone of the Izmit earthquake and recorded up to 1g ground acceleration during the Duzce mainshock. The amplitude of delay times seems to follow a logarithmic decay after the Duzce mainshock, suggesting a possible recovery from the mainshock damage. The observations are likely to reflect changes of material properties in the shallow structure of the fault zone. Updated results will be presented in the meeting.