Age determination on a catastrophic rock avalanche using tree-ring oxygen isotope ratios - the scar of a historical gigantic earthquake in the Southern Alps, central Japan

Dendrochronological analysis using the oxygen isotope composition (¹⁸O/¹⁶O) of cellulose was performed to reveal the age of an ancient rock avalanche that scattered huge granitic rock clasts on the bank of the Dondokosawa River in the east of Mount Hououzan, the Akaishi Range, central Japan. The samples for dating were disc-cut wood logs or increment cores of Japanese cypress (Chamaecyparis obtusa (Sieb. & Zucc.) Endl.) and Hemlock tree (Tsuga sp.) buried in lacustrine sediments. Cellulose was extracted directly from tree-ring cross-sectional thin wood plates. Cellulose rings of each disc were sampled and their oxygen isotope ratios were measured with a pyrolysis-type elemental analyser and an isotope ratio mass spectrometer. We compared the inter-annual variations in oxygen isotope ratios of cellulose for samples with those of a 470-year-long tree-ring δ¹⁸O chronology (AD450–919) independently constructed from Japanese cypress. The estimated death year of the barkless Japanese cypress is around AD885, and that of the barked hemlock tree is AD888. If the rock avalanche was induced by strong ground motion from a large earthquake, these ages are relevant to a large historical earthquake that occurred in this area. The candidates are the AD 887 Goki-Shichido Earthquake (Ninna Earthquake), which was one of the gigantic ocean-trench earthquakes documented along the Suruga and Nankai Troughs off central Japan, and another large intraplate earthquake remotely triggered by this ocean-trench earthquake.