When an 8.5-magnitude earthquake struck the Indonesian island of Sumatra in February 1861, it caused spasms on the ground, raising a wall of water that fell on nearby beaches and killing thousands of people.
Now, it appears that this tragic event wasn’t a single incident: in fact, it marked the end of the longest earthquake ever recorded, which had been underground for 32 years. These types of earthquakes, known as slow slip events, usually occur over days, months, or even years. But a recently described event continued More than doubled From the previous record, according to the information put forward by scientists in natural earth sciences.
“I wouldn’t believe we’d find a slow-slide event for so long, but we did,” he says. Emma Hill, author of the study and geodesy scientist at the Earth Observatory Nanyang University of Technology, in Singapore.
The discovery of such a protracted earthquake could help scientists understand the astonishing variety of ways our turbulent planet moves — and the deadly potential that these silent events can have in causing even more violent earthquakes.
Like its more accelerated relatives, slow earthquakes release energy accumulated by changes in plate tectonics. But instead of releasing that energy in an earth-shaking explosion, slow earthquakes release tension over time and so are not dangerous in and of themselves. However, subtle changes caused by underground can put pressure on adjacent areas along the fault, which could increase the risk of a major tremor in the vicinity.
Other regions in Indonesia have also proven to be a cause for concern. The island of Engano in the south, he says, “is sinking very, very quickly”. Reshav Malik, first author of the new study and a doctoral candidate at Nanyang Technological University in Singapore. Although Reshav cautions that the data comes from only one location, he notes that a slow earthquake may have already occurred near the island.
“It’s not just an isolated event in the 19th century,” Reshav says. “We are watching this happen now.”
The clues are written in coral
The new study is based on the unexpected writings of Earth’s tectonic shifts: coral reefs.
Some types of corals, such as coral reefs of the genus Burrites, shaped like a finger, grows to remain just below the water line. If the water rises, the coral rises again at an accelerated rate. If the water goes down, the coral exposed to the air dies, while the submerged part continues to grow. These corals pile up in layers, like trees growing in concentric rings, and this allows scientists to use their skeletons to map relative changes in water level over time.
“They basically act as natural tidal gauges,” Emma Hill says.
Changes in sea level can come from factors driven by climate change, such as melting glaciers, or changes in landscape elevation. On the western coast of Sumatra, changes in landscape elevation reveal an underground battle between plate tectonics.
In this region, the Australian tectonic plate Diving under the Sunda board, but it is confined along an area directly under the arc of the Indonesian islands. When the plates collide, the descending plate is pushed up into the ground. This flexes the surface, which pulls the edge of the plate to the sea floor, but causes other areas of the plate to rise.
If the accumulated pressure reaches a point where the area is shaken by the earthquake, the ground changes abruptly, reverses the impact and some coastal areas rise. This kind of change happened when it happened 8.7-magnitude earthquake I arrived in Sumatra in 2005.
“As corals rose during the earthquake, the entire ecosystem followed the movement,” the study co-author wrote. Aaron Meltzner, on one Articles About his field experiences in 2005, while studying for his Ph.D. at Caltech. Branching corals, sea urchins, crustaceans, crabs, and “the strangest fish or others” have either died or died close to land.
Aaron Meltzner, now a geologist at Nanyang Technological University in Singapore, has returned to studying coral reefs around Sumatra to uncover the many records they contain. on one a study In 2015, Aaron and his colleagues documented the sudden shift in Earth’s motions that led to the massive earthquake of 1861.
Based on coral reef data, scientists knew that prior to 1829, the soil near Simulo Island sank about 1 to 2 millimeters per year. But then that rate suddenly increased, with the earth sinking by as much as 10 mm per year, until an earthquake in 1861 devastated the area. The team initially thought the shift was due to a moving region where two tectonic plates connect to each other, but the researchers weren’t sure of the exact causes.
In 2016, Reshav Malik of Nanyang Technological University again looked at coral data from another perspective. By modeling the physics of the subduction zone and the movement of fluids across the fault, the researchers found that this abrupt shift was caused by the release of accretion stress — the onset of a slow earthquake.
Earthquakes of different flavors
Slow earthquakes were only recognized in the late 1990s, when they were detected in the Pacific Northwest region of North America and in the Nankai region off the coast of Japan. Their slow release of energy means that the changes they cause on the surface are very subtle, so they were only discovered when the GPS technology It has evolved enough to map these small types of changes.
However, the more places researchers look, the more slow earthquakes they discover, from the coasts of New Zealand to Costa Rica and even Alaska. “We see seismic landslides everywhere,” he says. Lusail Brohat, a geophysicist at the Ecole Normale Supérieure (ENS) in Paris, France, who is not part of the study team.
Slow earthquakes take on many different flavors. In Cascádia and Nankai, slow earthquakes occur with remarkable regularity – every time 14 months or more in Cascadia, and every Three to six months in Nankai. In both places, these long-duration earthquakes are also accompanied by a series of small earthquakes or tremors.
Lucille Brouhat compares these operations to a person walking on a wooden floor. “We’re walking and the wood is cracking all around us,” Lucille says. “All the cracks will be orgasms.”
Over the years, scientists have found that the duration of slow earthquakes can also vary greatly. In Alaska, for example, researchers discovered an event that lasted at least nine years, and only realized they were experiencing a slow earthquake when surface changes stopped in 2004, says Reshav Malik. The recently discovered event near Sumatra is extending the potential periods of slow earthquakes longer than ever before.
“Many people are suggesting that these long, slow events are possible,” he says. Laura Wallace, a geophysicist from the University of Texas at Austin and from GNS Science in New Zealand, who is not part of the study team. However, continuous monitoring of Earth’s movements near subduction zones has only occurred in the past two decades, which means that “in reality, we’re only looking at a small moment in time,” says Laura.
Save the recording
Understanding these slow events is critical to understanding the potential risks they pose to severe tremors. Many of the strongest earthquakes on record were preceded by slow landslides, including بما 9.1 magnitude earthquake In the Indonesian Sumatra Adaman Islands in 2004, A devastating 9.1-magnitude earthquake In Tōhoku in Japan in 2011 and A devastating earthquake of magnitude 8.2 At Iquique in Chile in 2014.
“It’s the hot topic in this field,” he says. Noel Bartello, a geophysicist specializing in slow earthquakes at the University of Kansas, who is not part of the study team. But demonstrating that slow-slip events can actually trigger large geological tremors is challenging, because not all slow quakes result in large tremors.
“The evidence is growing, but it is still limited to a few case studies,” Noel says.
Part of the problem is that it’s not easy to spot a long-term tremor when it occurs. Noel explains that the slow earthquake in the new study occurred along a shallow portion of the fault underwater and far from land. But traditional GPS stations are useless under the sea, since their signals Do not penetrate too much into the water. Few places on Earth have a natural record of coral reef movement in Indonesia.
There are instruments that can help, but they are expensive, says Noel, who plans to investigate similar surface slow-slide events on the Pacific Northwest Coast using instruments that use optical fibers to measure surface deformations.
“Although observation is often considered one of the least inviting things that scientists can do, it is essential to understand our planet in all its complexities,” Emma Hill says.
“Whenever we think we understand tectonics, the Earth throws us another surprise,” Emma says. “The longer the data sets we collect, the more surprises like this one will be.”
This article was originally published in English on Location nationalgeographic.com