Mass spectrometric techniques have recently yielded significant increases in precision and sensitivity over previous methods for measuring 230Th abundance in corals. To assess the accuracy of 230Th ages, three corals from Vanuatu, whose ages were known from counting annual growth bands, were analyzed. For each sample, the date of growth determined by 230Th analysis (A.D. 1969 ± 3, 1932 ± 5, and 1806 ± 5; 2σ uncertainties based on analytical error) was indistinguishable from the date determined by counting bands (A.D. 1971-1973, 1935-1937, and 1804-1810), indicating that the 230Th dates are accurate. 230Th dates were also determined for two adjacent emerged heads from Santo Is., Vanuatu, which were thought to have died when they were raised above sea level during coseismic uplift. The dates (A.D. 1864 ± 4, 1866 ± 4) were the same, indicating that the heads died at the same time and consistent with the idea that they were killed by coseismic emergence around A.D. 1865. The difference between this date and the date of the only major historically documented earthquake that caused uplift (A.D. 1973, Ms = 7.5), suggests a seismic recurrence interval of 108 ± 4 y for Santo. Analogous emerged corals from Malekula Is., Vanuatu yielded 230Th dates that were similar to each other (A.D. 1729 ± 3, 1718 ± 5) and are inferred to have died during coseismic emergence around A.D. 1729. In conjunction with the date of the only large historically documented earthquake that caused uplift (A.D. 1965, Ms = 7.5), the recurrence interval for Malekula is 236 ± 3 y. If similar emerged corals can be found, this appraoch may be extended back in time and to other localities because it appears that such features can now be dated both accurately and precisely.
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We thank J.H. Chen and D.A. Papanastassiou for advice and support, and K.E. Sieh for stimulating discussion. This document was written while the senior author was at and supported by the Department of Geology and Geophysics at the University of Minnesota. Constructive comments from two anonymous reviewers were beneficial. We thank the crews of the vessels "Vauban" and "Lopevi". The assistance of L. Gilpin, D. Sogari, P. Vuti, J.-L. Laurent, and the people of the villages of Brenwe, Leviamp, and Wounpouku are gratefully acknowledged. J.-L. Saos, J. Bonnemai-son, R. Campillo, J. Recy, and C. Reichenfeld are thanked for support with fieldwork. The age determinations were done in the Lunatic Asylum at Caltech, supported by NASA Grant NAG 9-43. The Government of Vanuatu and ORSTOM are thanked for field support. Fieldwork was funded by NSF grant EAR 79-19912 to F.W.T. This is contribution number 1113(2) from the Department of Geology and Geophysics, School of Earth Sciences, University of Minnesota; 751 from the Institute for Geophysics, University of Texas at Austin; and 4632 (623) from the Division of Geological and Planetary Sciences, California Institute of Technology.