The longest known earthquake lasted 32 years
A devastating earthquake that rocked the Indonesian island of Sumatra in 1861 has long been seen as a sudden rupture on a previously calm fault. But new research reveals that the tectonic plates beneath the island had been slowly and quietly roaring against each other for 32 years before the cataclysmic event.
This decades-long silent earthquake – known as the “slow slip event” – was the longest such streak ever to be detected. It was too subtle and gradual to be noticed during its evolution, but a new study indicates that it may have precipitated the massive earthquake of 1861 by at least 8.5, which in turn triggered a tsunami that killed thousands of people. The new study could help scientists today monitor dangerous earthquakes more effectively.
As the most well-known type of earthquake, we experience an earthquake on the surface of the earth, slow-sliding earthquakes occur when two segments of the crust move against each other. Some faults are now monitored for slow slip with seismic instruments or GPS technology, but tracking such events on distant faults (or before the 1990s, when GPS became available) is very difficult. The most recent slow-slip events that scientists have studied have lasted for hours, days, or weeks, only a few of which have lasted for several years. The existence of a decades-long slow slide “indicates that the subduction zones are more diverse than we have necessarily appreciated,” says Kevin Furlong, a geoscientist at Pennsylvania State University, who was not involved in the news. research. (Subduction zones are areas where the oceanic crust slides under the continental crust.)
Near the Indonesian island of Simeulue off the coast of Sumatra, coral growth patterns record up and down movement along the fault involved in the 1861 earthquake, providing a rare window into the past. . Corals cannot grow when exposed to air. So when the local sea level changes due to tectonics, these changes are visible in the skeletal growth records of the corals, says Rishav Mallick, a doctoral student at Nanyang Technological University in Singapore and lead author of the new study. , which was released this month. in Geoscience of nature. Simeulue’s corals hold an almost annual history of vertical movement at the fault from 1738 to 1861.
The corals reveal that Simeulue had been sagging, or sinking, for 90 years at a steady rate of one or two millimeters per year, which is consistent with the bottom movement of the fault. But around 1829, it suddenly started to sink five to seven times faster – a few years down to an inch, Mallick says. This indicated that the fault had started to move in a slow-slip earthquake. “It’s a very clear change,” he says. This “rapid” subsidence continued until the earthquake of 1861.
The study highlights the complexity of the subduction zones, says Furlong. For a long time, he notes, “the assumption was that between large earthquakes, the system was simple”: two sections of crust jam against each other at the fault, creating tensions up to the point. ‘that has –rift– they break free with an earthquake.
Slow-sliding events complicate this picture. They could even trigger larger, detectable earthquakes by relieving stress on part of a fault, but adding stress to neighboring sections, Mallick says. “It’s like a bunch of sources,” he explains. “So if one releases, the others have to take that load.”
The 2004 Indian Ocean earthquake and tsunami that killed more than 220,000 people were preceded by a few years of slow landslide in the Andaman Islands, Mallick says. However, a slow slide cannot yet help predict larger earthquakes because the duration of the slide varies greatly. There are no faults that have been monitored by GPS for 32 consecutive years, so modern monitoring may not pick up on events as lasting as the Indonesian slow slide of the 19th century. And not all faults are well monitored. This is especially true for subduction faults under the ocean, which require special monitoring of the seabed rather than GPS.
If the slow sliding motion is missed, researchers can miscalculate where the stumps are on a fault and how strong of an earthquake that fault can potentially produce. “Once we can better define the locked region,” says Furlong, “we can better define the magnitude of an earthquake that can occur.”