When a mega-earthquake of magnitude 8.5 struck off the Indonesian island of Sumatra in February 1861, it triggered tremors, raising a wall of water that collapsed onto nearby shores and killed thousands of people.
Now it seems that this tragic event was not an isolated one: It actually marked the end of the longest earthquake ever recorded, which lasted a full 32 years in the bowels of the earth. Such earthquakes, called slow-moving earthquakes, have been known to occur over days, months or years. But recently an event was recorded that lasted more than twice as long as the past record holder, scientists report in the journal Nature Geoscience.
“I wouldn’t have believed we would find a slow-slip event of this duration, but we found it,” says study author Emma Hill, a geodesist at Singapore’s Nanyang Technological University Earth Observatory.
The discovery of such a slow-moving earthquake promises to help scientists understand the amazing variety of ways our restless planet moves – and the deadly possibility that these quiet events could trigger much more powerful earthquakes.”
Like their speedy counterparts, slow earthquakes release energy stored by tectonic plate shifts. But instead of releasing it in a rumbling explosion, slow earthquakes slowly release the stress over time, and so are not dangerous in themselves. Nevertheless, subtle shifts in the subsurface potentially stress neighboring zones along the fault, which could increase the risk of a larger earthquake nearby.
Other areas of Indonesia are already giving cause for concern. The southern island of Enganno “is sinking too fast,” said Rishav Mallick, first author of the new study and a PhD candidate at Nanyang Technological University in Singapore. This indicates that there may already be a slow earthquake near the island.
“It’s not just an isolated incident in the 1800s,” Mallick says. “We see it happening right now.”
Changes in sea level can occur as a result of climatic factors, such as melting glaciers, or as a result of changes in the elevation of the landscape. Off the west coast of Sumatra, the latter type of change demonstrates an underground struggle between tectonic plates.
In this zone, the Australian tectonic plate plunges beneath the Sunda Plate, but it is stuck in an area directly beneath the arc of the Indonesian islands. When the plates collide, the sinking plate presses on the land above them. This bends the surface, which pulls the edge of the plate lower into the sea, but causes other parts of the plate to rise.
If the tension becomes so intense that an earthquake occurs in the region, the land will shift sharply, throwing the tension back and sending some coastal areas upward. Just such a shift occurred after an 8.7 magnitude earthquake in Sumatra in 2005.
Slow earthquakes have only been recognized since the late 1990s, when they were first observed in the Pacific Northwest of North America and in the Nankai region off the coast of Japan. Their slow release of energy means they cause barely noticeable shifts on the surface, so they were not detected until GPS technology improved enough to capture such small changes.
But the more places researchers have studied since then, the more slow earthquakes they’ve found, from the coast of New Zealand to Costa Rica and even Alaska. “We see aseismic slip everywhere,” says Lucille Bruhat, a geophysicist at the École Normale Supérieure (ENS) in Paris, France.
Sluggish earthquakes have different signs. In Cascadia and Nankai, slow earthquakes occur with surprising regularity: in Cascadia, they occur about every 14 months, and in Nankai, every three to six months. In both places, these long-lasting earthquakes are also accompanied by a series of small tremors known as shocks.
Understanding these slow events is crucial to understanding the potential risks they pose by triggering stronger earthquakes. Slow events have preceded many of the most powerful earthquakes ever recorded, including Indonesia’s cataclysmic Sumantra-Adaman earthquake of magnitude 9.1 in 2004, Japan’s devastating Tohoku earthquake of magnitude 9.1 in 2011 and Chile’s devastating Iquique earthquake of magnitude 8.2 in 2014.
