Analysis of partially emerged corals and reef terraces in the central Vanuatu Arc: Comparison of contemporary coseismic and nonseismic with quaternary vertical movements

Abstract

In the central Vanuatu arc, living and recenfiy deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns. As corals grow, the density of the aragonite coral skeletons varies on an annual cycle, producing annual growth bands similar to tree tings. Using coral growth bands, we can determine the year coral surfaces died due to emergence. We interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on January 5, 1946 (M S = 7.3), August 11, 1965 (M S = 7.5), October 27, 1971 (M S = 7.1) and December 29, 1973 (M S = 7.5). The 1965 and 1973 events caused maximum uplifts of 120 and 60 cm, respectively, in the frontal arc. Also related to these events are uplifts of 10 cm and 6 cm in the back arc on Pentecost and Maewo islands, which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence. Similar secondary zones of uplift occurred with the great 1960 Chile and 1964 Alaska earthquakes. The amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half-space. However, the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half-space. At various places in 1957, 1969-1970, 1977, and 1978-1981 there was about 5-10 cm of emergence not associated with major earthquakes, which may indicate nonseismic tectonic uplift. However, oceanographically lowered sea levels, as in E1 N/rios, may have determined the times when corals died and recorded these events. Nevertheless, the accumulation of emergence, its persistence, the limited geographic extent of each event, and occurrence in areas of rapid Holocene uplift suggest that the causes of the uplifts are tectonic. These events suggest that in some areas a third or more of the total accumulated uplift in central Vanuatu takes place as aseismic motion. However, in some areas we find only coseismic emergence. In central Vanuatu, contemporary coseismic vertical deformation, Holocene uplift, and topography have remarkably similar patterns. This suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years. Apparently, the topography, structure, and seismotectonics are controlled by the subduction of the d'Entrecasteaux ridge, a major bathymetric feature underthrusting this part of the arc. The influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend, and thus it interacts for a long time with a single portion of the arc system. Our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the Santo-Malekula interval of the arc, and our present results further support this conclusion. Each block has uplifted at different times, by different amounts, at different rates, and tilted in a different direction. Bo...

Seismic recurrence intervals and timing of aseismic subduction inferred from emerged corals and reefs of the Central Vanuatu (New Hebrides) Frontal Arc

“…Coral colonies living at the shallowest possible depths (usually intertidal) are partially or completely killed by tectonic emergence of even a few centimeters [Taylor et al, 1981[Taylor et al, , 1987. Therefore the age of the the last skeletal material accreted to the dead surface of a tectonically emerged coral records the time of the tectonic event.…”

“…Therefore the age of the the last skeletal material accreted to the dead surface of a tectonically emerged coral records the time of the tectonic event. Using this concept, Taylor et al [1981Taylor et al [ , 1987 analyzed the annual growth bands (similar to tree rings) of coral colonies partially killed by tectonic emergence events to determine a record of contemporary uplift in Vanuatu. The largest emergence of about 1.2 m occurred in 1965 on part of northern Malekula.…”

“…When earthquake sequence includes more than one event, the moment values in the [Minster and Jordan, 1978] does not include the expected contribution from back arc spreading on the Fiji plateau [e.g., Falvey, 1975]. Boundaries between frontal arc segments are taken from Taylor et al [1980Taylor et al [ , 1987 and are supported by independent studies including Ebel [1980]. Chatelain et al [1986] proposed the additional boundary limiting the southern side of the SM (south Malekula) block.…”

“…Several hundred meters of frontal arc uplift in central Vanuatu may be caused by subduction of the aseismic d'Entrecasteaux ridge (DR) (Figure 1). DR subduction appears to produce significant structural and morphological changes across the entire frontal and back arcs [Collot et al, 1985;Taylor et al, 1985Taylor et al, , 1987. For example, Taylor et al [1980,1987] found that the frontal arc at Santo and Malekula is divided into at least four arc segments: (1) north Santo (NS), (2) south Santo (SS), (3) north Malekula (NM), and (4) south Malekula (SM) (Figure 1).…”

High-precision 238U–234U–230Th disequilibrium dating of the recent past: a review

“…This, combined with dating and characterisation of lagoon sediments from Heron Reef, opens up the opportunity for high age-resolution reconstruction of cyclone history and for an accurate assessment of the trend and frequency of cyclone occurrence in the GBR over the past millennium. Taylor et al (1987) and Edwards et al (1988) studied historical earthquake frequency and co-seismic uplift rates in Vanuatu by dating the tops of partially emerged coral heads killed as a result of co-seismic uplift above sea level. Two adjacent emerged massive corals from Santo Island yielded analytically indistinguishable dates of AD 1864 AE 4 and 1866 AE 4, suggesting that the heads died as a result of uplift around AD 1865.…”