Scientists in Singapore will uncover the Sumatran earthquake of 1861 that lasted 32 years

The new study relies on an unexpected scribe of Earth’s tectonic shifts: corals.

Some types of coral, such as Porites, like fingers, grow outward and upward until they stay just below the surface of the water. If the water rises, the coral quickly grows back. As the water drops, the coral exposed to the air dies, while the submerged portion continues to grow outwards.

Because these corals are layered, like trees growing in concentric circles, scientists can use their skeletons to map changes in relative water levels over time.

“They basically act like natural tide gauges,” Hill said.

Changes in sea level can come from factors driven by climate change, such as melting glaciers, or from shifts in landscape elevation. Off the west coast of Sumatra, it is caused by the movement of tectonic plates.

In this zone, the Australian tectonic plate plunges under the Sunda plate, but is trapped along the zone just below the Indonesian island arc. When plates collide, they pull on the ground above them. This flexes the surface, which pulls the edge of the plate lower into the sea, but causes the rest of the plate to rise.

If the ‘tension’ builds up so high that an earthquake hits the region, the ground will suddenly shift, reversing the effect and sending some areas of the coast upwards. The shift occurred after an 8.7-magnitude earthquake hit Sumatra in 2005.

“When coral reefs rise during an earthquake, the entire ecosystem is left exactly where it is,” wrote study co-author Aron Meltzner in a blog about his 2005 field experience as a Ph.D. students. at CalTech.

Forked corals, sea urchins, mussels, crabs, and fish are all dead or dead, visible on nearly dry land.

Meltzner, now a geologist at Nanyang Technological University in Singapore, returns to studying the coral reefs around Sumatra year after year to decipher the many records they hold. In a 2015 study, he and his colleagues documented the sudden shift in ground motion that led to the giant earthquake of 1861.

Prior to 1829, land near Simeulue Island was sinking by about one to two millimeters annually, according to coral data. But then the rate spiked suddenly, with the ground sinking by 10 millimeters a year until an 1861 earthquake ripped through the region. The team initially thought the changes were due to a shifting area where two tectonic plates interlocked, but they weren’t sure of the exact cause.

In 2016, Mallick of Nanyang Technological University turned new attention to coral data. By modeling the physics of the subduction zone and the movement of fluids along the fault, the researchers found that the rapid changes were caused by the release of inherent strain – the beginning of slow-motion earthquakes.

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