when An 8.5-magnitude earthquake struck the Indonesian island of Sumatra in February 1861, shook the earth and caused a wall of water to wash away the nearby banks and kill thousands of people.
Now it seems that the tragic incident was not an isolated incident: the truth is that It was the end of the longest documented earthquake to date, which occurred over a span of 32 years. These types of earthquakes, known as slow slip events, can occur over days, months, or years. But the recently described phenomenon lasted more than twice as long as the previous record holder, as stated in an article published in natural earth sciences.
“I didn’t believe we’d find such a long slow event, but we did,” says study author Emma Hill, a geodesic scientist at the Earth Observatory at Nanyang Technological University in Singapore.
Finding such a slow earthquake promises the help of scientists Understand the staggering diversity of the ways our turbulent planet moves and the deadly potential of these silent phenomena to cause much stronger earthquakes.
Like fast phenomena, slow earthquakes release energy stored by the movements of tectonic plates. But instead of releasing it into an earth-shaking storm, slow earthquakes release tension little by little over time and are not a danger on their own. However, subtle changes below the surface can increase pressure in adjacent areas along the fault, which could increase the risk of a larger nearby earthquake.
Other regions of Indonesia are already a cause for concern. Reshav Malik, first author of the new study and a doctoral candidate from Nanyang Technological University in Singapore, explains that the southern island of Enganu is “sinking very quickly.” Although he warns that the data is only coming from one location, they suggest that a slow earthquake may have already occurred near the island.
“This is not a single isolated phenomenon of the nineteenth century,” Mallik says. “We’re seeing how that happens now.”
The clues are written in coral
The new study is based on A An unexpected record of terrestrial tectonic movements: coral reefs.
Some types of corals like Burrites, It grows outward and upward until just below the surface of the water. If the water rises, the coral grows again. If the water goes down, the coral exposed to the air dies, while the submerged part continues to grow. While these corals grow in layers, like trees that grow in concentric rings, Scientists can use the skeletons to map changes in water level over time.
“They basically act as normal tidal gauges,” Hill says.
Changes in sea level may be the result of factors driven by climate change, such as melting glaciers or changes in landscape elevation. On Sumatra’s west coast, the latest types of changes reveal an underground battle between tectonic plates.
In this region, the Australian tectonic plate sinks below the Sunda Plate, but is trapped in an area just below the arch of the Indonesian island. When the plates collide, the descending plate pulls the ground above it. This causes the surface to bend, which pulls the plate boundary and sinks it into the sea, but causes other parts of the plate to rise.
If so much pressure builds up that the area is shaken by the earthquake, the ground shifts abruptly, reversing the effect and causing some coastal areas to rise. Such movement occurred after an 8.7-magnitude earthquake in Sumatra in 2005.
“As coral reefs rose during an earthquake, the entire ecosystem remained completely in place,” one of the study authors Aaron Meltzner wrote in a blog about his experience in the field in 2005 as a PhD student at the California Institute of Technology. Coral reefs, sea urchins, mollusks, crabs and “unfortunate exotic fish” were 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 in Sumatra year after year, revealing the many records they contain. In a 2015 study, he and his colleagues documented the sudden change in Earth’s movements that led to the massive earthquake of 1861.
Prior to 1829, soil near Simulo Island sank one to two millimeters per year, according to reef data. But the rhythm suddenly rose and the earth began to sink 10 mm a year until an earthquake in 1861 struck the area. At first, the team thought the change was due to a shifting region where two tectonic plates collided, but they weren’t sure of the exact cause.
In 2016, Malik from Nanyang Technological University analyzed coral reef data with fresh eyes. By modeling the physics of the subduction zone and the movement of fluids along the fault, the researchers discovered that the rapid change was caused by the release of accumulated stress – the onset of a slow earthquake.
Different types of earthquakes
Slow earthquakes have only been known since the late 1990s, when they were first observed in the Pacific northwest from North America and the Nankai region off the coast of Japan. Its weak release of energy means it can cause subtle movements on the surface, so it wasn’t detected until GPS technology had improved enough to chart these small changes.
However, the more places researchers have studied since then, the more slow earthquakes they discover, from the shores of New Zealand to Alaska. “We see seismic landslides everywhere,” says Lucile Prohat, a geophysicist at the Ecole Normale Normale Superior (ENS) in Paris, France, who was not part of the study team.
Slow earthquakes can take many forms. In Cascadia and Nankai, slow earthquakes occur with incredible regularity, about every 14 months in Cascadia and every three to six months in Nankai. In both places, these long-lasting earthquakes are accompanied by a series of small earthquakes or earthquakes.
Bruhat compares the process to someone walking on a wooden floor. “You move and the wood creaks around you,” he says. “Every squeak will be jerks.”
Over the years, scientists have also discovered that the duration of slow earthquakes can vary greatly. In Alaska, for example, researchers discovered a phenomenon that lasted at least nine years and only realized it was a slow earthquake after the surface stopped moving in 2004, Mallick says. The new phenomenon near Sumatra extends the potential periods of slow earthquakes more than ever before.
“A lot of people have suggested that there might be a longer and larger slow-slip phenomenon,” 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 continuous monitoring of Earth’s movements near subduction zones has only occurred in the past two decades or so, which means “we only observe a brief moment in time,” he says.
Understanding these slow phenomena is critical to understanding the potential risks they pose in causing larger tremors. Slow landslides preceded many of the strongest earthquakes documented to date, such as the catastrophic magnitude 9.1 earthquake in Sumantra Andaman, Indonesia.And the In 2004, the devastating 9.1-magnitude Tohoku earthquake in JapanAnd the In 2011, the devastating 8.2 earthquake in Iquique, ChileAnd the in 2014.
“It’s a hot topic in this area,” says Noel Bartlow, a slow seismic geophysicist at the University of Kansas who was not involved in the study. But proving that slow slip events can cause major geological earthquakes has been difficult. Not all slow earthquakes cause a large earthquake.
“The evidence is growing, but it is still limited to a few case studies,” he says.
Part of the problem is that spotting a long-lived earthquake right away isn’t easy. In the new study, Bartlow explains, the long earthquake penetrated a shallow section of the fault underwater and away from the ground. But conventional GPS stations are useless on the sea floor, because their signals don’t penetrate far underwater. Few places on Earth contain a natural record of such movements, such as the coral reefs in Indonesia.
But they are expensive, Bartlow says. He plans to search for similar, shallow, slow-sliding events off the Pacific Northwest coast using instruments that use optical fibers to measure surface tension.
Although observation is often considered one of the “least attractive things” that scientists can do, Hill says it is vital to understanding our planet and all its intricacies.
“When we think we understand tectonics, the Earth sends us another surprise,” Hill says. “The more long-term data sets we collect, the more surprises like this will be.”
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