A note on seismic activity near the eastern termination of the Izmit rupture in the hours preceding the Duzce earthquake

We use continuous recordings at four stations installed near the eastern termination of the Izmit rupture to analyze the seismic activity there during the 5 hr leading to the Duzce earthquake. The records clearly show the presence of three groups of events. One of the groups originates from the middle of the Izmit rupture and consists of aftershocks of the M 5.7 Spanca Lake earthquake that had occurred a day earlier and was itself, to date, the second largest aftershock in the Izmit sequence. A second group of events is spread over the 30-km-long eastern (Karadere) segment of the Izmit rupture, and a comparison with the number of aftershocks expected, based on decay laws for similar sequences in other regions, suggests that seismic activity was high on the eastern segment in the hours leading to the Duzce earthquake. The third group of events occurs beyond the termination of the Izmit rupture and clusters around the hypocenter of the Duzce earthquake. It shows that, in the little over 5 hr leading to the Duzce earthquake, this destructive event was preceded by six precursory shocks ranging in magnitude from 0.9 to 2.6.