Mid‐late Holocene paleoseismicity of the eastern Clarence Fault, Marlborough, New Zealand

Abstract

The middle Clarence valley, or eastern, section of the active Clarence Fault bounds the steep southeast flank of the Inland Kaikoura Range, Marlborough, New Zealand. This section of the fault extends c. 45 km northeastwards from its junction with the Elliott Fault to Mead Stream, where the active trace of the fault appears to abruptly end. Along the active trace, 41 separate right‐lateral offsets of c. 3–40 m were recorded with uncertainties of 10–50% (mean 25%) using tape measurement of displaced topographic features (e.g., gully walls, landslide margins, and ridge crests). Single‐event right‐lateral displacements of 7 ± 2 m are inferred and indicate earthquake magnitudes of Mw 7.2–7.9. Probable rupture lengths have been in excess of 45 km, which suggests that rupture of the middle Clarence valley (eastern) section of the fault extended to the west onto the central Clarence and/or Elliott Faults. Stream incision through the uphill‐facing fault scarp at Ravine Stream and Dead Horse Gully has exposed progressively faulted peat and charcoal horizons. Elsewhere, there are several exposures of unfaulted sediments. Twelve radiocarbon samples dated from these horizons constrain the timing of the four most recent surface‐rupture earthquakes on the Clarence Fault in the study area. The most recent surface rupture took place at c. 1700–1900 cal. yr BP (rounded to the nearest hundred years), with two surface ruptures since c. 3400 cal. yr BP, three since 6700 cal. yr BP, and most likely a fourth surface rupture at c. 6600–6900 cal. yr BP. These data suggest a recurrence interval for surface‐rupture earthquakes of c. 1700 yr, and imply a lateral slip rate of c. 4 mm/yr (i.e., c. 7 m of slip every c. 1700 yr).