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Megathrust and Sea-Level History of the Sumatra Region from Coral Microatolls

UTIG Brownbag Seminars

Megathrust and Sea-Level History of the Sumatra Region from Coral Microatolls

By:
Aron Meltzner

When: Wednesday, March 23, 2011, 12:00 p.m. to 1:00 p.m.
Where: Seminar Conference Room, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758
image from Aron's talk

Abstract
Coral microatolls, with their ability to provide century-long or multi-century continuous records of high precision paleo-elevation data with remarkably precise ages, allow for unprecedented resolution in the reconstruction of the timing and magnitude of paleo-sea-level changes and past megathrust ruptures.

Along the Sunda megathrust off the west coast of Sumatra, two persistent barriers to earthquake rupture have been identified, by using coral microatolls to infer uplift of outer arc islands, and hence slip on the plate interface, for modern and prehistoric earthquakes. Simeulue, a 100-km-long island off the west coast of northern Sumatra, straddles the boundary between the 2004 (MW 9.1) and 2005 (MW 8.6) Sunda megathrust ruptures and acted as a barrier to both ruptures. Historical and coral paleoseismic records suggest that central Simeulue has behaved as a persistent barrier over at least the past 1100 years. Farther south near the Equator, the Batu Islands patch of the megathrust has been a barrier to rupture in great earthquakes from the north (1861, 2005) and south (1797). Both of these rupture barriers lie approximately atop fracture zones in the downgoing slab and coincide roughly with prominent breaks in the structural fabric of the overriding plate. Nevertheless, an understanding of the relationship between such physical features and rupture terminations has remained elusive.

To the east of Sumatra, near Belitung Island on the Sunda Shelf, coral microatolls are providing the most precise data yet obtained on past sea levels in a tectonically stable part of Southeast Asia. Although still preliminary, initial results appear to contradict the previously preferred models: whereas the various glacial isostatic adjustment (GIA) models predict a relative sea-level highstand around 5 ka or no highstand at all in the vicinity of our study area, our data suggest that relative sea level at 5.0-6.4 ka was already falling rapidly from a highstand ca. 6.6-6.8 ka. Our new data will be critical to calibrating GIA models and will allow for an improved understanding of past sea-level change; this will, in turn, enable us to answer lingering questions on the tectonic history of the Sunda megathrust.