The earthquake lasted 32 years, and scientists want to know how

When an earthquake measuring 8.5 on the Richter scale struck the Indonesian island of Sumatra in February 1861, it shook the ground, shattering a wall of water on nearby beaches and killing thousands of people.

Now, it appears that the tragic event was not an isolated incident: it was, in fact, the end of the longest earthquake ever recorded, which slipped through the Earth’s interior for 32 years. Known as a slow slip event, these types of earthquakes have been known to propagate over days, months, or years. But the newly described event lasted more than twice as long as the previous record holder, scientists at Nature Geoscience reported.

“I wouldn’t have imagined that we would find a slow event for a long time, but we discovered it here,” says study author Emma Hill, a geodesy scientist at the Earth Observatory at Nanyang Technological University in Singapore.

The discovery of such a slow-moving earthquake promises to help scientists understand the surprisingly diverse ways in which our turbulent planet is moving – and the lethal potential of these silent events to cause more powerful earthquakes.

Slow-moving earthquakes, like their high-speed cousins, release energy accumulated from shifts of tectonic plates. But rather than unleashed in an explosion of ground rumbling, slow earthquakes release pressure slowly over time, so they are not risks on their own. However, slight shifts below the surface are likely to carry pressure on adjacent areas along the fault, which could increase the risk of a larger earthquake in the vicinity.

Other regions in Indonesia are already showing cause for concern. Reshav Malik, first author of the new study and Ph.D., says the southern island of Engano is “sinking very, very quickly”. Candidate at Nanyang Technological University in Singapore. While he warns that the data is only coming from one location, they are hinting that a slow earthquake may already be in progress near the island.

“It is not just one isolated event in the nineteenth century,” Mallik says. “We are seeing this happen now.”

Clues are written in coral

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

Some types of coral, such as the toe-like burrite, grow outward and upward so that they stay just below the surface of the water. If the water rises, the coral will quickly shoot up again. If the water falls, the coral exposed to the air dies, while the submerged portion continues to grow outward. Because these corals cluster in layers, like trees that grow in concentric rings, scientists can use their skeletons to map relative changes in the water level over time.

“They basically work like natural tide gauges,” Hill says.

Changes in sea level can come from factors driven by climate change, such as melting glaciers, or from shifts in landscape elevation. Off the western coast of Sumatra, recent types of changes have revealed an underground battle between tectonic plates.

In this region, the Australian tectonic plate is submerged under the Sunda Plate, but it gets attached directly along an arc of the Indonesian islands. When the plates collide, the down plate pulls the ground on top of them. This bends the surface, which pulls the edge of the plate down into the sea, but causes other parts of the plate to rise.

If tensions escalate so high that an earthquake strikes the area, the ground will suddenly shift, reversing the impact and sending some coastal areas up. Such a shift occurred after an 8.7-magnitude earthquake struck Sumatra in 2005.

“As coral reefs move in an earthquake, the entire ecosystem is left completely in place,” wrote Aaron Meltzer, co-author of the study, in a blog about his field experiences in 2005 while receiving his Ph.D. Student at California Institute of Technology. Branching coral reefs, sea urchins, oysters, crabs and “sometimes unlucky fish” are all dead or dying, exposed on nearly dry land.

Meltzner, who now works as a geologist at Nanyang Technological University in Singapore, has returned to study coral reefs around Sumatra year after year to decipher the many records they possess. In a 2015 study, he and his colleagues documented the sudden shift in the earth’s movements that led to the mega-earthquake of 1861.

Before 1829, land near Simolo Island sank about 1 to 2 millimeters per year, based on coral reef data. But then the rate suddenly jumped, as the earth sank at a rate of 10 mm a year until the earthquake in 1861 hit the area. The team initially thought the change was due to a changing region where two tectonic plates lock together, but they weren’t sure the exact cause.

In 2016, Malik from Nanyang Technological University took a fresh look at coral reef data. By modeling the physics of the subduction zone and the movement of fluids along the fault, the researchers found that the rapid change was caused by the release of the accumulated stress – the onset of a slow-moving earthquake.

Earthquake flavors

Slow earthquakes have only been recognized since the late 1990s, when they were initially observed in the Pacific northwest from North America and in the Nankai region off the coast of Japan. The weak energy release means that it causes slight shifts at the surface, so it wasn’t detected until GPS technology had improved enough to draw such subtle changes.

However, the more places researchers have searched since then, the more slow earthquakes they discover, from the coast of New Zealand to Costa Rica and even Alaska. “We see seismic slip everywhere,” says Lucille Bruhat, a geophysicist at Ecole Normale Supérieure (ENS) in Paris, France, who was not part of the study team.

Slow Earthquakes take on many different flavors. In Cascadia and Nankai, slow earthquakes are regularly striking, occurring every 14 months or so in Cascadia and every three to six months in Nankai. At both locations, these long-lasting earthquakes are also associated with a large number of small earthquakes known as tremors.

Bruhat likens the process to someone walking across a wooden floor. “You’re moving and the wood is cracking around you,” she says. “All the cracks will be jerks.”

Over the years, scientists have also found that the duration of slow earthquakes can vary greatly. In Alaska, for example, researchers discovered one event that lasted at least nine years, and realized that they were looking at a slow earthquake after the creeping surface stopped in 2004, Mallick says. The newly discovered event near Sumatra pushes the potential periods of slow earthquakes further than ever before.

“A lot of people have suggested that these larger, longer, slow-slip events are possible,” says Laura Wallace, a geophysicist at the University of Texas in Austin and GNS Science in New Zealand, who was not part of the study team. But the constant monitoring of Earth’s movements near subduction zones has only occurred in the last two decades or so, which means “we’re really only looking at a small snapshot in time,” she says.

Are tracking

Understanding these slow events is critical to understanding the potential risks they pose to triggering larger jolts. The slow slides were preceded by many of the strongest earthquakes ever recorded, including the disastrous 9.1 earthquake in Indonesia in 2004, the devastating 9.1-magnitude Japan earthquake in Tohoku in 2011, and the devastating Chilean earthquake of magnitude 8.2 ECWIC in 2014.

“It’s a hot topic right now in this area,” says Noel Bartlow, a geophysicist specializing in slow earthquakes at the University of Kansas, who was not part of the study team. But proving that slow slip events can actually elicit greater geological tremors has long been a challenge. Not every slow earthquake triggers a major tremor.

“The evidence is growing somewhat, but it is still limited to a few case studies,” she says.

Part of the problem is that catching a long-lived earthquake during the act is not easy. In the new study, Bartlow explains, the prolonged earthquake crawled along a shallow portion of the fault, which is underwater, away from the ground. But conventional GPS stations are useless on the sea floor because their signals do not penetrate the water very much. Few spots on Earth have a natural record of such coral-like movements in Indonesia.

But they are expensive, Bartlow says. It plans to search for similar shallow slow-slip events off the coast of the Pacific Northwest, using instruments that use optical fibers to measure surface stress.

Although observation is often considered one of the “less attractive things” that scientists can do, Hill says, it is imperative to understand our planet in all its intricacies.

“When we think we understand tectonics, the Earth will throw another surprise at us,” Hill says. “The more we collect really long data sets, the more surprises like this we’ll have.”

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