Of Sumatran quakes and corals

Coral growing up towards the light
CC Image:Shayan Sanyal

The devastating tsunami that originated off the coast of Sumatra in 2004 was not the first, nor will it be the last, that this region of the world experiences. It was set off by a magnitude 9.3 earthquake that occurred as two tectonic plates slid past each other.

Earthquakes of this size have occurred many times in this region, but predicting when and where they will strike is nearly impossible. A new paper in the journal Science used measurements of corals that grew over the last 700 years to construct a detailed timeline of past motion with an eye towards the future.

Coral are useful for this purpose because they will grow right up to sea level, but not above, and the coral shape can confirm that it has reached the surface. As a result, they provide an excellent measure of local sea level. When you consider that global sea level has not changed dramatically in the past 1000 years, this local sea level must actually reflect changes in the elevation of the land that the coral is growing on.

The paper produced a detailed timeline, showing that this area has experienced major earthquakes about every 200 years. More importantly, these earthquakes occurred just when the land surface reached its current elevation.

In 2007, a magnitude 8.4 earthquake yielded about 73cm of vertical movement at the southern end of the region they examined. This matches the history they reconstructed for the past 700 years, in which a large earthquake releases strain from the southern end, and is followed by an even larger earthquake within 35 years that releases an additional 100-250cm of strain over the entire 300km long region.

Beyond predicting that this area is due for a big quake, the authors also posit that the timing of earthquakes here may be common for tectonically active regions of the world. It is commonly understood that plates are constantly trying to slide past each other; the longer they go without sliding, the more strain builds up, and the bigger the bang when they do go.

The authors propose that this pattern of big quakes followed by even bigger quakes is caused by heterogeneity in the crust. Heterogeneity leads to stronger and weaker regions; inevitably, the weaker regions will break and release much of the pent up energy first. But they can't release it all until the stronger regions finally go, leaving nothing to stop the weaker regions from releasing the last of their strain too. This yields a substantially larger final earthquake as the entire region fails at once.

Science, 2008. DOI: 10.1126/science.1163589