Earthquake lasted 32 years and scientists want to know how


When an 8.5 magnitude earthquake hit the Indonesian island of Sumatra in February 1861, it caused convulsions in the earth, shaking a wall of water that crashed into the nearby shores and killed thousands of people.

Now, it seems that this tragic event was not an isolated incident: it actually marked the end of the longest earthquake on record, which crept through the basement for 32 years. Known as a slow-slip event, these types of earthquakes are known to unfold over days, months, or years. But the newly described event lasted more than twice as long as a former record holder, scientists report Geoscience of nature.

“I wouldn’t have believed we would find a slow-slip event this long, but this is where we found it,” says the study’s author. Emma Hill, geodesist at Nanyang Technological UniversitySingapore Earth Observatory.

The discovery of such a slow earthquake promises to help scientists understand the astonishing variety of ways our restless planet moves – and the lethal potential of these silent events to trigger much more powerful earthquakes.

Much like their high-speed cousins, slow-motion earthquakes release energy accumulated by the movements of tectonic plates. But instead of triggering it in a gust that shakes the ground, slow earthquakes slowly release tension over time, and therefore aren’t dangers in themselves. Yet the subtle movements of the subsoil potentially stress adjacent areas along a fault, which could increase the risk of a larger tremor nearby.

Other parts of Indonesia are already of concern. The southern island of Enganno “is sinking a little too fast”, says Rishav Mallick, first author of the new study and holder of a doctorate. candidate at Nanyang Technological University in Singapore. Although he warns that the data is coming from one location, it does suggest that a slow-motion earthquake may already be underway near the island.

“It’s not just an isolated event in the 1800s,” says Mallick. “We are seeing it happening right now.”

Clues written in coral

The new study leans on an unexpected scribe of Earth’s tectonic changes: the coral.

Certain types of coral, such as finger Porites, grow outward and upward until they linger just below the surface of the water. If the water rises, the coral quickly grows back upwards. If the water falls, the coral exposed to the air dies, while the submerged part continues to grow outward. Because these corals grow in layers, like trees growing in concentric rings, scientists can use their skeletons to map relative changes in water level over time.

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

Changes in sea level can be caused by factors related to climate change, such as melting glaciers, or changes in the height of the landscape. Off the west coast of Sumatra, these latter types of changes expose an underground battle between tectonic plates.

In this area, the Australian tectonic plate dive under the plate Sunda, but it gets stuck along an area directly under an arc of Indonesian islands. As the plates collide, the descending slab pulls on the earth above. This flexes the surface, which pulls the edge of the plate lower in the sea, but pushes other parts of the plate up.

If the voltage rises so high that an earthquake tears the area apart, the land will suddenly shift, reversing the effect and sending some coastal areas upward. Such a change occurred after a magnitude 8.7 earthquake seized Sumatra in 2005.

“As the reef moved up during the earthquake, the whole ecosystem was left exactly in place,” co-author of the study Aron Meltzner written in a blog about his experiences in the field in 2005 while a doctoral student at CalTech. Branched corals, sea urchins, crustaceans, crabs and “an occasional hapless fish” all lay dead or dying, exposed on the nearly dry land.

Meltzner, who is now a geologist at Nanyang Technological University in Singapore, has returned to study the corals around Sumatra year after year to unravel the many records they hold. In a 2015 study, he and his colleagues documented the sudden earth change movements leading up to the giant earthquake of 1861.

Prior to 1829, the ground near Simeulue Island flowed about one to two millimeters each year, based on coral data. But then the rate suddenly jumped, with the earth sinking at up to 10 millimeters per year until the 1861 earthquake tore the area apart. The team initially thought the change was due to a changing region where two tectonic plates interlocked together, but they weren’t sure of the precise cause.

In 2016, Mallick from Nanyang Technological University took a fresh look at coral 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 a release of accumulated strain – the start of a slow-motion earthquake.

Earthquake flavors

Slow earthquakes have only been recognized since the late 1990s, when they were first spotted in the Pacific Northwest of North America and the Nankai region off the coast of Japan. Their release of languid energy means they cause subtle changes on the surface, so they weren’t discovered before. GPS technology had improved enough to trace these tiny changes.

Yet the more researchers have searched since then, the slower the earthquakes have been, from the coasts of New Zealand to Costa Rica and even Alaska. “We are seeing seismic landslide everywhere,” says Lucille Bruhat, geophysicist at the Ecole Normale Supérieure (ENS) in Paris, France, who was not part of the study team.

Slow motion earthquakes take on many different flavors. In Cascadia and Nankai, slow earthquakes strike with remarkable regularity, About 14 months in Cascadia and each three to six months in Nankai. In both places, these long-lasting earthquakes are also accompanied by a series of tiny earthquakes called tremors.

Bruhat likens the process to a person walking on a wooden floor. “You move and the wood cracks around you,” she said. “All the cracks would be tremors.”

Over the years, scientists have also found that the duration of slow earthquakes can vary widely. In Alaska, for example, researchers discovered an event that lasted for at least nine years, only realizing they were examining a slow quake after the crawling surface stopped in 2004, Mallick says. The new event near Sumatra pushes the possible durations of slow earthquakes further than ever.

“Many people have suggested that these larger, longer, slow-sliding events are possible,” says Laura Wallace, a geophysicist at the University of Texas at Austin and GNS Science in New Zealand, who was not part of the study team. But the continuous monitoring of ground movement near the subduction zones has only taken place for the last two decades or so, which means that “we’re really only looking at a small snapshot in time,” he says. it.

Keep tabs

Understanding these slow events is essential to understanding the potential risks they pose to trigger larger tremors. Slow landslides preceded many of the strongest earthquakes on record, including the cataclysmic in Indonesia Magnitude 9.1 Sumantra-Adaman earthquake in 2004, the devastating effects of Japan Magnitude 9.1 earthquake in Tōhoku in 2011, and destroyer of Chile Iquique magnitude 8.2 earthquake in 2014.

“It’s a hot topic right now on the pitch,” says Noel Bartlow, a geophysicist specializing in slow earthquakes at the University of Kansas who was not part of the study team. But precisely demonstrating that slow-slip events can actually trigger greater geological thrills has long been a challenge. Not all slow earthquakes lead to a great shaking.

“The evidence is kind of growing, but it’s still limited to a few case studies,” she says.

Part of the problem is that it’s not easy to catch a long-lasting temblor in the act. The long earthquake in the new study crept along a shallow section of the fault, which lies underwater far from the earth, Bartlow explains. But traditional GPS stations are useless on the seabed since their signals do not penetrate very far in the water. And few places on Earth have a natural record of such movements similar to Indonesian corals.

There are instruments that can help, but they’re expensive, Bartlow says. She plans to search for similar shallow slow-slip events off the Pacific Northwest coast with instruments that use fiber optics to measure surface deformation.

While surveillance is often considered one of the “less sexy things” scientists can do, says Hill, it is vital to understanding our planet in all of its complexities.

“Every time we think we understand tectonics, the Earth will surprise us again,” says Hill. “The more we collect these very long datasets, the more surprises like this one we’ll have.”

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