This study contributes paleoseismological data on the recent history of surface-rupturing earthquakes on the southern (main) branch of the North Anatolian fault zone (nafz, Turkey), west of the Bolu basin. We focused on the fault segment that ruptured during the 1967 Mudurnu Valley earthquake (Ms 7.1), which preceded the devastating earthquakes of 1999 in the sequence of westward-migrating nafz earthquakes since 1939.

Geomorphological mapping was carried out in a search for trenching sites on the central part of the 22 July 1967 earthquake segment. In the absence of locations in sedimentary environments best suited for paleoseismological interpretation of faulted Holocene deposits, we trenched fluvial channel deposits at a terrace of the gravel- dominated Mudurnu River.

The most conservative interpretation of the trench stratigraphy and faulting evidence suggests that at least one paleoearthquake (most probably two) occurred after a.d. 1693. The 1967 earthquake segment has ruptured at least once since the historical earthquake of a.d. 1668, which was previously considered to be a likely candidate for the penultimate event, and before 1967. It is not possible to confidently assign the penultimate event to one of the post a.d. 1693 historical earthquakes in the broader area around the Mudurnu Valley (e.g., the earthquake of a.d. 1719) because the historical data published so far do not provide conclusive information about when past ruptures of the Mudurnu Valley branch of the nafz (a secluded area) did or did not take place.

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