With coral reefs as their tea leaves, scientists are forecasting that in the next several decades there will be another major earthquake along the Sunda fault off Sumatra like the one that spawned the catastrophic tsunami of Dec. 26, 2004.

Image A reef exposed in what may be the start of a cycle of quakes. Credit... Science

Kerry Sieh, formerly of the California Institute of Technology and now at Nanyang Technological University in Singapore, and colleagues write in the journal Science that a 2007 quake along a more southerly stretch of the fault represented only a first, partial rupture of that 400-mile section, which had been quiet for nearly two centuries. The researchers say this part of the fault, called the Mentawai section, is likely to be the site of at least one more major rupture.

As evidence, they point to the growth patterns of coral reefs in the region over the last 700 years. When a quake occurs, the seafloor rises up, effectively lowering the sea level so that shallow coral reefs are now above the surface. The reefs cannot grow upward, but their still-submerged portions grow outward.