Deformation along the northern edge of the westward-moving Anatolian plate is concentrated along the North Anatolian fault. This northward-arching fault extends from the Karliova triple junction in the east, ∼1500 km into the Aegean Sea in the west. A sequence of twentieth-century earthquakes ruptured the fault, displaying a spatiotemporal pattern consistent with a stress triggering mechanism. In 1943, the Mw 7.6 Tosya earthquake ruptured a 280-km-long segment near the center of the fault. Four paleoseismic investigations have previously investigated this segment, and the present study was conducted near its center, in an ∼180-km-long gap between existing studies. A paleoseismic trench revealed a sequence of eight sediment packages abutting a highly developed shear zone. Each of the packages consists of a fine-grained layer overlying a coarse-grained layer. Based on correlation between the age of the base of the coarse-grained layers and existing earthquake records, we infer that the coarse-grained layers were deposited in response to earthquakes because of increased erosion on an adjacent steep slope. The most recent event horizon may correlate to the historical 529 A.D. earthquake. Timing of six older earthquakes is constrained to (2s): 23 B.C.–103 A.D., 609–185 B.C., 971–814 B.C., 1227–968 B.C., 2050–1777 B.C., and 2556–2235 B.C., which correspond to a summed interevent time of 97–912 yr (2s). The earthquake record is relict because the local stream network was incised ca. 1000 A.D., isolating the trench site from its sediment source. A stream near the trench was subsequently offset by 23.5 ± 1.5 m, yielding a right-lateral slip rate of 21.4–25.6 mm/yr and suggesting that the 1943 rupture caused an uncharacteristically small offset.

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