Dating earthquakes with high-precision thorium-230 ages of very young corals

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, M_s = 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, M_s = 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.