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

Taylor, Frederick W. and Edwards, R. Lawrence and Wasserburg, G. J. and Frohlich, Cliff (1990) Seismic recurrence intervals and timing of aseismic subduction inferred from emerged corals and reefs of the Central Vanuatu (New Hebrides) Frontal Arc. Journal of Geophysical Research B, 95 (B1). pp. 393-408. ISSN 0148-0227. doi:10.1029/JB095iB01p00393.

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The recognition and dating of corals that have been killed by tectonic uplift allow us to date paleoseismic uplifts in the Vanuatu island arc. We recognize corals that record paleouplifts by their similarity to those known to have died during contemporary sudden uplifts and date them (1) by counting annual coral growth bands (only if part of the coral is alive at the time of collection) or (2) by newly developed techniques for obtaining ^(230)Th ages by mass spectrometry. The mass spectrometric method produces isotopic ages with precisions of ±3 to ±9 years (2σ) in the 0–1000 years B.P. time range. The ^(230)Th ages in this time range appear to be accurate. Samples whose ages are known by counting coral growth bands give ^(230)Th ages that are indistinguishable from their growth band ages. By dividing the average increment of uplift for the latest Holocene uplifts by the mean Holocene uplift rate, we can estimate average seismic uplift recurrence intervals for the past 6000 years. The results for each of four central Vanuatu arc segments are (1) North Santo emerged 1.2 m in 1866 A.D. and 0.6 m 107 years later in 1973 A.D. The average coseismic uplift of 0.9 m and mean Holocene uplift rate of 4.3 mm yr^(−1) suggest a longer recurrence interval of 212 years. (2) South Santo emerged 0.29 m in 1946 and 0.26 m 19 years later in 1965, including the related 1971 event. Here the mean Holocene uplift rate is 5.5 mm yr^(−1). The uplift data suggest a longer average recurrence interval of about 51 years. (3) North Malekula emerged 1.23 m near 1729 A. D. and 1.05 m 236 years later in 1965. The mean Holocene uplift rate of 2.7 mm yr^(−1) and mean coseismic uplift of 1.14 m for dated events suggest a longer recurrence interval of 422 years. (4) Part of southernmost Malekula has uplifted continuously or episodically by about 0.35 m from about 1957 until at least mid-1983 A.D. The maximum uplift of 2.7 mm yr^(−1) occurs near a nest of small earthquakes. Both the earthquakes and rapid uplift suggest that interplate slip beneath south Malekula may be continuous, rather than episodic. Episodes of 0.35 m uplift would have to recur every 130 years to maintain the 2.7 mm yr^(−1) uplift rate. In contrast, we find no evidence of interseismic vertical movements for the other three blocks. The most reasonable interpretation of these results is that the seismic recurrence intervals and processes for accommodation of slip are quite different on adjacent arc segments. We have used the most widely accepted moment magnitude relationship to evaluate the accumulated seismic slip caused by large earthquakes occurring since 1920. In all four arc segments this analysis suggests that the seismically radiated moments account for less than one-third to one-tenth of the slip associated with plate convergence. The similarity between the paleoseismic record of uplifts and the contemporary record of coseismic uplifts suggests that this analysis can be generalized to times before 1920. For the northern three segments of central Vanuatu, aseismic slip probably occurs in the same years as large earthquakes because the contemporary coral record records uplifts only in years having large historic earthquakes. This suggests that aseismic slip is not continuous and does not occur at rates which vary slowly over the course of the earthquake cycle. The south Santo segment may have the highest proportion of seismic slip because the mean recurrence interval of 51 years is shortest and the mean Holocene uplift rate of at least 5.5 mm yr^(−1) is the fastest.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1990 American Geophysical Union. Received July 28, 1988; revised April 13, 1989; accepted June 20, 1989. We particularly thank R. Campillo, C. Reichenfeld, C. S. Cheney, J.-L. Saos, A. Macfarlane, D. Sogari, the crews of the vessels Vauban and Lopevi, P. Vuti, J.-L. Laurent, Michel Caffin, C. Jouannic, P. Maillet, L. Gilpin, M. Strecker, Alfred Stevens, Stan Combs, Chief Kalman of Brenwe, many other people from villages on Santo, Malekula, and neighboring islands, and the government of Vanuatu for assistance rendered during field work leading to this paper. We thank A. L. Bloom and B. Isacks for helping to inspire this project and for continuing encouragement. The ^(230)Tb analyses were performed at the Lunatic Asylum at Caltech with advice and support from J.H. Chen and D.A. Papanastassiou. This document was written while R.L.E. was supported by the Department of Geology and Geophysics at the University of Minnesota. This work was supported by NSF grants EAR 7919912, EAR-8507983, EAR-8618406, EAR-8843928, and EAR-8904987 to The University of Texas at Austin, EAR-7815188 to Cornell University, EAR-8904705 to the University of Minnesota, the Government of Vanuatu, and ORSTOM. The University of Texas at Austin Institute for Geophysics contribution 787. California Institute of Technology, Division of Geological and Planetary Sciences contribution 4690 (643); University of Minnesota, Department of Geology and Geophysics contribution 1122(7).
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University of Minnesota Department of Geology and GeophysicsUNSPECIFIED
Government of VanuatuUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences4690
Lunatic Asylum Lab643
University of Minnesota Department of Geology and Geophysics1122
University of Texas at Austin Institute for Geophysics787
Issue or Number:B1
Record Number:CaltechAUTHORS:20140115-140311482
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Official Citation:Taylor, F. W., R. L. Edwards, G. J. Wasserburg, and C. Frohlich (1990), Seismic recurrence intervals and timing of aseismic subduction inferred from emerged corals and reefs of the Central Vanuatu (New Hebrides) Frontal Arc, J. Geophys. Res., 95(B1), 393–408, doi:10.1029/JB095iB01p00393.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43391
Deposited By: Ruth Sustaita
Deposited On:15 Jan 2014 23:02
Last Modified:10 Nov 2021 16:37

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