Paleoseismology of the Rueisuei Segment of the Longitudinal Valley Fault, Eastern Taiwan

Paleoseismic trench excavations in the Dafu area of eastern Taiwan have provided data on the rupture history of the Rueisuei segment of the Longitudinal Val- ley fault during the late Holocene. The 1951 Taitung earthquake ruptured the Dafu site, which is characterized by several terrace raises with late Holocene sediments uplifted by the westward thrust fault. Trenches across the northwest-facing fault scarp exposed fluvial and alluvial deposit sediments. Although nearly all of the 23 radio- carbon ages vary somewhat within each layer, the overall age determined for each layer is in good accord with the stratigraphic ordering of these layers. Based on strati- graphic ordering and a statistical comparison of radiocarbon dates using the OxCal program, we estimate that two pre-1951 earthquake surface ruptures at the Dafu site occurred in the periods A.D. 1736-1898 and A.D. 1564-1680. The same OxCal model constrains the past two recurrence intervals to about 165 and 140 yr, although with sizable uncertainties, 55-215 and 90-260 yr, respectively, which are 95% ranges. Through the correlation of three trenches across the Longitudinal Valley fault, we are able to identify evidence for at least three earthquakes with moment magnitudes of about 7.0-7.2 that occurred up to 390 yr prior to and during 1951. Furthermore, based on the radiocarbon dates, the mean recurrence interval is roughly 150 yr (un- certainty is indeterminate), with a minimum vertical uplift rate of 8:5-12:2 mm=yr.