Geology suggests why Indonesia’s 2004 quake was surprisingly strong
Chemical transformations in minerals deep beneath the seafloor could explain why Indonesia’s 2004 mega-earthquake was unexpectedly destructive, researchers report in the May 26 Science.
The magnitude 9.2 quake and the tsunami that it triggered killed more than 250,000 people, flattened villages, and swept homes out to sea across Southeast Asia. It was one of the deadliest tsunamis in recorded history.
“It raised a whole bunch of questions, because that wasn't a place in the world where we thought a magnitude 9 earthquake would occur,” says study coauthor Brandon Dugan, a geophysicist at the Colorado School of Mines in Golden.
The thick but stable layer of sediment where tectonic plates meet off the coast of the Indonesian island of Sumatra should have limited the power of an earthquake, seismologists had predicted. But instead, this quake was the third-strongest on record worldwide.
Dugan spent two months aboard a boat with 30 other scientists collaborating through the International Ocean Discovery Program. The researchers drilled down 1,500 meters below the seafloor in two places off the coast of Sumatra, extracting narrow cylinders of sediment. This sediment is very slowly moving toward the fault where the 2004 earthquake occurred — a zone where one massive tectonic plate slides over another, pushing that plate downward.
Analyzing how sediment changes with depth can give scientists a snapshot of the geological processes at play near the fault zone.
In particular, deep down, the researchers identified a layer of sediment where the water had a lower salinity than the water in the sediment above or below. Since seawater seeping into the sediment would be salty, the evidence of freshwater suggests that the water must have instead been released from within minerals in the sediment.
For tens of millions of years, Dugan proposes, minerals sat on the seafloor taking in water — baking it into their crystal structure. Then, more sediment settled on top. It got toasty under such a thick blanket of sediment, heating up the minerals beneath. The temperature increase triggered a chemical transformation within the sediment, pushing water out of the mineral crystals and into tiny pores between the grains.
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The sediment sampled in this study is still dehydrating. By the time any of it reaches the plate boundary, Dugan says, it’ll be buried under kilometers of more sediment and will probably be completely dehydrated.
At first, the liberated water would have softened the material, actually decreasing the risk of a big earthquake by allowing it to absorb more force, Dugan says. As the sediment got closer to the fault over millions of years, though, the water flowed away, leaving it brittle and unstable — the perfect setup for a mega-quake.
The timing of this sediment dehydration process can make or break a quake. Had the sediment near the fault been in a softened state when the quake struck in 2004, the tremor might not have been as deadly, Dugan says. But since enough time had passed for it to become brittle again, the tectonic plates were able to rapidly slip past each other for a much greater distance during the quake. That massive motion displaced the seafloor itself, setting a tsunami into motion.
“It’s really the tsunamis from these earthquakes that prove to be the deadliest and most dangerous,” says Roland Bürgmann, a seismologist at the University of California, Berkeley who wasn’t part of the study. And quakes that displace the seafloor are far more likely to trigger tsunamis.
The findings could apply to other faults with similarly thick sediment, such as the Cascadia Subduction Zone in the Pacific Northwest, suggests study coauthor Andre Hüpers, a geophysicist at the University of Bremen in Germany.
But more evidence is needed before applying such analysis to faults beyond this one, says Bürgmann. The argument for what happened along the Sumatran fault is compelling, he says. “But nonetheless, it’s only one data point. It doesn’t yet make for a pattern.”
Editor's note: This story has been updated to clarify where the researchers discovered a change in water salinity as they drilled down beneath the seafloor.
A. Hupers et al. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra. Science. Vol. 356, May 25, 2017. doi: 10.1126/science.aal3429.
A. Witze. Intraplate quakes signal tectonic breakup. Science News. Vol. 182, October 20, 2012, p. 5.