Folded sediment unusual in Sumatran tsunami area

Tsunamis propagate only when earthquakes occur under water and have an up and down component to their motion. Earthquakes where tectonic plate boundaries slide side by side, do not cause tsunamis. Subduction zone earthquakes, those areas where one plate moves beneath another, are prime candidates for tsunami generation, but if the two plates slide smoothly across each other, water is displaced very slowly.

“If a fault is not locked, the two plates just creep along and there is no big upsurge,” says Fisher. “However, if they are locked, the bottom plate drags the top plate along until it snaps back and quickly displaces water.”

An expedition, organized by the Discovery Channel and BBC-TV, explored the 136-mile area of the fault using seismic reflection, a system where a sound source activates beneath the water and researchers record the time the sound takes to reach the underwater receivers. This process provides a detailed map of the sea bottom and the terrain beneath it. The researchers reported their work in a recent issue of Geology.

The researchers, who included Fisher; David Mosher, Geological Survey of Canada – Atlantic who supplied the seismic reflection equipment; James A. Austin Jr., senior research associate and Sean P.S. Gulick, research associate, University of Texas, Austin; Timothy Masterlark, assistant professor, University of Alabama, and Kathryn Moran, associate professor, University of Rhode Island, found that the shape of the upper plate boundary was unusual.

“The fault line does not look as we assumed it did,” says Fisher. “We expected a wedge with one plate going under the other.”

The Sumatran plate boundary looked, in many ways, like the cabling on a suspension bridge. The area near the edge did form a wedge, but the central portion was framed by two peaks with a sway or saddle in the middle, with the farthest part then sloped downward. The central swayback portion was also populated by bumps located about 8 miles apart across its length.

“We also found that this is a blind fault, one that is not visible at the surface because it is covered in a deep layer of silt and sediment,” says Fisher. “The peaks every 8 miles were caused when an earthquake folded the sediment.”

These peaks add to the amount of water displaced when the entire plate edge snaps back. The fold spacing shows that the sediments are from 1 to 3 miles deep. The researchers suggest that the sediment deforms independently from the actual plate boundary.

The researchers conclude that the combination of processes, plate edge movement and snap back and the deformation of the sediment combines to enhance the uplift on a substantial portion of the fault and has important implications for evaluation of the 2004 tsunami and others that occur in this location.