Paleogeodetic records of seismic and aseismic subduction from central Sumatran microatolls, Indonesia
We utilize coral microatolls in western Sumatra to document vertical deformation associated with subduction. Microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track the long-term aseismic deformation that occurs during the intervals between earthquakes (paleogeodetic data). This paper focuses on the twentieth century paleogeodetic history of the equatorial region. Our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface. The site closest to the trench rose 90 cm, whereas sites further east sank by as much as 35 cm. Our model reproduces these paleogeodetic data with a 2.3 m slip event on the interface 88 to 125 km from the trench axis. Our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence. Likewise, interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence. Among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or "silent event" in 1962, 27 years after the 1935 event. Paleogeodetic deformation rates in the decades before, after, and between the 1935 and 1962 events have varied both temporally and spatially. During the 25 years following the 1935 event, submergence rates were dramatically greater than in prior decades. During the past four decades, however, rates have been lower than in the preceding decades, but are still higher than they were prior to 1935. These paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century.
Additional Information© 2004 American Geophysical Union. Received 11 January 2003; revised 11 November 2003; accepted 24 December 2003; published 14 April 2004. This research was supported by NSF grants EAR-9628301 (KS), EAR-9804732 (KS), 9628716 (RLE), 9903443 (RLE), 0207686 (RLE), EAR-9804970 (SNW), and EAS-9903301 (KS) and inspired by Fred Taylor (University of Texas at Austin), who initiated the use of coral microatolls for paleoseismic study. We are grateful to Hiroo Kanamori for many useful discussions and for helping us quantify the uncertainties in our paleogeodetic data. Mark Simons and Tom Heaton provided valuable comments for improving the interpretation of the data. We thank Hery Harjono, Jan Sopaheluwakan, Suparka, and Hariadi Permana from the Indonesian Institute of Sciences (LIPI) for providing essential administrative support for conducting research in Indonesia. We appreciate the Fishery Department of Bung Hatta University and Firdaus, the owner of Padang Diving, for the use of their boats. We also thank our boat crews, LIPI field assistant Dudi Prayudi, and diver Imam Suprihanto, who made our field expeditions safe and productive. Finally, we wish to acknowledge our debt to Paul Tapponnier, whose painstaking and thorough review of the first submitted manuscript led to this much more succinct and readable version.
Published - Natawidjaja2004p04306_Journ_Geophys_Res.pdf