When an 8.5 magnitude earthquake struck the Indonesian island of Sumatra in February 1861, it shook the land and kicked up a wall of water that crashed onto nearby banks, killing thousands of people.
Now it seems that the tragic event wasn’t an isolated incident: it actually marked the end of the longest earthquake ever recorded, which crawled underground for a whopping 32 years. Known as the slow slip event, this type of tremor is known to unfold over days, months, or years. But the newly described event lasted more than twice as long as the previous record holder, scientists report in Nature Geoscience.
“I didn’t think we’d find a slow-slip event for that long, but that’s where we found it,” says study author Emma Hill, a geodist at Nanyang Technological University’s Earth Observatory of Singapore.
The discovery of such a slow earthquake promises to help scientists understand the surprising variety of motions of our restless planet – and the deadly potential of these silent events to cause much stronger tremors.
Similar to their fast cousins, earthquakes in slow motion release energy that has formed from the shifting of the tectonic plates. But instead of unleashing it in a rattling eruption, slow tremors create inert strain over time and are therefore not a threat in themselves. Still, the subtle shifts in the subsurface may place stress on neighboring zones along a fault, which could increase the risk of a major earthquake nearby.
Other areas of Indonesia are already a matter of concern. The South Island of Enganno is sinking “a little too quickly,” says Rishav Mallick, first author of the new study and Ph.D. Candidate at Nanyang Technological University in Singapore. While they are careful to ensure that the data is only from one location, they do point out that a slow-motion quake may already be underway near the island.
“It’s not just an isolated event in the 19th century,” says Mallick. “We see this happening right now.”
Notes written in coral
The new study draws on an unexpected writer of the Earth’s tectonic shifts: corals.
Some types of coral, such as the finger-like porites, grow outward and upward until they linger just below the surface of the water. If the water rises, the coral quickly shoots up again. When the water sinks, the exposed coral dies while the submerged part continues to grow outward. Because these corals pile up in layers, like trees growing in concentric rings, scientists can use their skeletons to record relative changes in water levels over time.
“They basically act like natural tide meters,” says Hill.
Changes in sea level can result from climate change-related factors such as melting glaciers or changes in the elevation of the landscape. Off the west coast of Sumatra, the latter types of changes reveal an underground battle between tectonic plates.
In this zone, the Australian tectonic plate dips beneath the Sunda plate, but remains stuck along a zone just below an arch of Indonesian islands. When the plates collide, the descending plate tugs on the land above. This bends the surface, pulling the edge of the slab deeper into the sea, but lifting other parts of the slab.
When the load becomes so high that an earthquake rips through the region, the land shifts abruptly, reversing the effect and sending up some coastal areas. One such shift came after an 8.7 magnitude earthquake hit Sumatra in 2005.
“When the reef was drawn into the earthquake, the entire ecosystem was left right in place,” study co-author Aron Meltzner wrote on a blog about his 2005 field experience during his PhD. Student at CalTech. Branched corals, sea urchins, shellfish, crabs, and “the occasional bad luck fish” all lay dead or dying on nearly dry land.
Meltzner, who is now a geologist at Nanyang Technological University in Singapore, returned year after year to study the corals around Sumatra to decipher the many records they hold. In a 2015 study, he and his colleagues documented the sudden shift in land movements that led to the huge earthquake of 1861.
Before 1829, the soil near the island of Simeulue sank by about one to two millimeters every year, according to coral data. But then the rate suddenly rose, and the land sank up to 10 millimeters per year until the 1861 quakes ripped through the region. The team initially thought the change was due to a shifting region where two tectonic plates are connected, but they weren’t sure of the exact cause.
In 2016, Mallick of Nanyang Technological University took a new look at the coral data. By modeling the physics of the subduction zone and the movement of liquids along the fault, the researchers found that a rapid change was caused by the release of a built-up tension – the start of an earthquake in slow motion.
Earthquake flavors
Slow earthquakes have only been known since the late 1990s, when they were first sighted in the Pacific Northwest of North America and the Nankai region off the coast of Japan. Their sluggish release of energy means they cause subtle shifts on the surface, so they weren’t discovered until GPS technology improved enough to record such tiny changes.
But the more places researchers have searched since then, the slower earthquakes they have found, from the coasts of New Zealand to Costa Rica and even Alaska. “We’re seeing seismic slip everywhere,” says Lucille Bruhat, a geophysicist at the Ecole Normale Supérieure (ENS) in Paris, France, who was not part of the study team.
Slow motion quakes take on many different flavors. In Cascadia and Nankai, the slow quakes strike with remarkable regularity, moving roughly every 14 months in Cascadia and every three to six months in Nankai. In both locations, these long-lasting tremors are also accompanied by a series of tiny tremors known as tremors.
Bruhat compares the process to a person walking across a wooden floor. “You move and the wood cracks around you,” she says. “All cracks would be tremors.”
Over the years, scientists have also found that the duration of slow quakes can vary widely. In Alaska, for example, researchers discovered an event that lasted at least nine years and didn’t realize it was a slow quake until after the creeping surface came to a standstill in 2004, Mallick says. The newly discovered event near Sumatra is driving the possible duration of slow quakes further than ever.
“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 at Austin and GNS Science in New Zealand who was not on the study team. But continuous monitoring of land movement near subduction zones has only happened in the last few decades or so, which means that “we’re really only looking at a small snapshot of time,” she says.
Keep track of things
Understanding these slow events is critical to realizing the potential risks they pose to causing larger quakes. Slow slips preceded many of the strongest earthquakes ever recorded, including the devastating 9.1 earthquake in Sumantra-Adaman, Indonesia in 2004, the devastating 9.1 earthquake in Thoku, Japan in 2011, and the devastating earthquake in the Strength 8.2 in Iquique in Chile in 2014.
“This is a hot topic in the field right now,” says Noel Bartlow, a geophysicist specializing in slow earthquakes at the University of Kansas who was not on the study team. But providing accurate evidence that slow-slip events can actually cause major geological tremors has long been a challenge. Not every slow tremor leads to a big shake.
“The evidence is growing, but it’s still limited to a few case studies,” she says.
Part of the problem is that catching a long-lived quake in the red is not easy. The long quake in the new study has crept along a shallow section of the fault that’s underwater off land, Bartlow explains. However, conventional GPS stations are useless on the ocean floor because their signals do not penetrate very far through the water. And few places on earth have a natural record of such movements, similar to the corals in Indonesia.
There are tools out there that can help, but they’re expensive, says Bartlow. She plans to use instruments that use optical fibers to measure surface elongation to look for similar shallow slow-slip events off the coast of the Pacific Northwest.
While surveillance is often viewed as one of the “less sexy things” scientists can do, Hill says it is vital to understanding our planet in all its complexities.
“Whenever we think we understand tectonics, the earth will give us another surprise,” says Hill. “The more we collect these really long data sets, the more surprises we will encounter.”
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