Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia

Abstract

The Tunka fault is a major normal‐oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024–1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947–2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091–7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733–7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2–12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well‐dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19–0.44 mm/yr. Intermediate‐term (100 ka) and long‐term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08–0.16 mm/yr and 0.07–0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.