High Precision Thorium-230 Dating of Corals Using Thermal Ionization Mass Spectrometry: Applications to Paleoseismology.
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The recent development of mass spectrometric methods for determining 230Th abundances reduces the analytical error in 230Th ages of corals. Errors of ±3 yrs (2?) for a 17 yr old coral, ±5 yrs at 180 yrs, ±44 yrs at 8,294 yrs, and ±1.1 kyat 123.1 ky (1 ky = 1,000 yrs) were obtained using these techniques. Within the error of the measurements, 230Th ages agree with ages determined by counting of annual growth bands. These measurements indicate that the maximum amount of 230Th incorporated into a coral skeleton during growth is equivalent to the amount of 230Th generated by radioactive decay in <6 yrs. Using these techniques, we have dated two emerged corals from north Malekula Island and two from northwest Santo Island, Vanuatu. By analogy to partially emerged corals that were killed by coseismic uplift on Santo in 1973 (Ms = 7.5) and on Malekula in 1965 (Ms = 7.5), it appears that each pair of emerged corals was killed by an earlier coseismic uplift event. Pairs of emerged coral heads from each of the localities yield similar 230Th ages. This demonstrates that each pair of corals died at the same time and is consistent with the idea that they were killed by the same event (presumed to be coseismic emergence). The 230Th growth dates of the emerged corals (A.D. 1864±4 (2 ?) and A.D. 1865±4 for Santo; A.D. 1729±3 and A.D. 1718±5 for Malekula) in conjunction with the dates of historical earthquakes yield recurrence intervals of 108 yrs for northwest Santo Island and 236 yrs for north Malekula Island. If a slip-predictable model is used, average uplift rates over the past few centuries are similar to uplift rates averaged over the past 6,126 yrs. It may be possible to extend this approach back in time and to other localities because coral features that represent paleoseismic events are preserved in the geologic record and we have the ability to recognize these features in the field. However, the difficulties in recognizing and sampling corals that represent paleoseismic uplifts become increasingly greater with increasing age.