Lack of Spatiotemporal Localization of Foreshocks before the 1999 Mw 7.1 Duzce, Turkey, Earthquake

We use a matched-filter technique to detect small seismic events before the M-w 7.1 Duzce earthquake that are not included in the regular catalog. The study employs extensive waveform dataset recorded by a 10-station fault zone array near the epicenter of the Duzce earthquake, deployed about three months before the event. We use 3134 earthquakes within 20 km of the Duzce epicenter listed in the local catalog as templates to scan through waveforms recorded within similar to 65 hours before the Duzce earthquake. The analysis reveals 262 newly detected events in this time interval, which is similar to 5 times more than the 55 events listed in the original catalog. Most of the events occur to the west of the Duzce epicenter, which was initiated between a more active western and relatively quite eastern fault segments. The results do not indicate a localizing foreshock process that accelerates in time and/or involves progressive concentration of activity around the mainshock hypocenter during the preceding 65 hour period. Instead, we find that the Duzce source region becomes less active during the similar to 20 hours immediately before the mainshock. Our results, together with other recent studies, suggest that progressive acceleration and localization of foreshocks around the mainshock epicenter is not a general phenomenon.