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Yerli
Department of Earth Sciences
University of Southern California
Los Angeles, California 90089-0740
hartleb{at}earth.usc.edu
(R.D.H., J.F.D.)
Department of Geology
Istanbul Technical University
Maslak 80626, Turkey
(H.S.A., B.Y.)
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 23 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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