Abstract

We use a matched‐filter technique to detect small seismic events before the Mw 7.1 Düzce 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 Düzce earthquake, deployed about three months before the event. We use 3134 earthquakes within 20 km of the Düzce epicenter listed in the local catalog as templates to scan through waveforms recorded within ∼65 hours before the Düzce earthquake. The analysis reveals 262 newly detected events in this time interval, which is ∼5 times more than the 55 events listed in the original catalog. Most of the events occur to the west of the Düzce 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 Düzce source region becomes less active during the ∼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.

Online Material: Figures of timing of recorded waveforms, peak ground velocities, foreshock activity, seismicity rate, and temporal evolution of earthquake activity, vertical waveforms, and distribution of template events.

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