Abstract

Paleoseismologic data from trenches excavated across the central part of the North Anatolian fault at Alayurt, Turkey, reveal evidence for at least four, and possibly five, surface ruptures during the past 2000 years, as well as one much older event. These surface ruptures, as recognized on the basis of upward fault terminations and colluvial gravel layers, include (1) the historic 1943 Mw 7.7 Tosya earthquake; (2) an older event that is not well dated, but which we interpret as the great 1668 earthquake, which historical accounts suggest ruptured this part of the fault; (3) a late-eighth- to early-thirteenth-century surface rupture; (4) a surface rupture that occurred between the first and third centuries A.D., possibly the historic A.D. 236 event; (5) a possible surface rupture that occurred between the late fourth and early eleventh centuries A.D.; and (6) a much older event that occurred sometime between 4600 and 3550 B.C. Our documentation of a late-eighth- to early-thirteenth-century surface rupture at Alayurt is particularly important because, when taken in context with available historical and paleoseismologic data, it suggests the occurrence of a brief interval during which large earthquakes ruptured most of the North Anatolian fault. This interval resembles two other short-lived clusters of activity in the sixteenth to eighteenth and twentieth centuries. These brief intervals of activity are separated by much longer periods of relative quiescence that range from 250 years to ≤800 years (if there was not a late-fourth- to early-eleventh-century event) or ∼600 years (if there was a late-fourth- to early-eleventh-century event). The factor of 2–3 variation in inter-event times suggests quasiperiodic earthquake occurrence, which we attribute to the structural simplicity and relative isolation of the central North Anatolian fault from other earthquake-producing faults.

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