The devastating earthquake that rocked the Indonesian island of Sumatra in 1861 has long been considered a sudden fault in a previously inactive fault. But new research reveals that the tectonic plates beneath the island were slowly and quietly rumbling against each other for the 32 years before the cataclysmic event.
These decades of silent earthquakes, known as “slow slip events”, are the longest ever detected. It was too subtle and gradual to notice during its evolution, but a new study suggests it may have triggered the massive 1861 earthquake of at least 8.5 magnitude, 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 earthquakes we experience shaking the earth’s surface, slow-slip 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 it is very difficult to trace such events to distant faults (or before the 1990s, when GPS was available). The latest slow-slip events scientists have studied have lasted for hours, days, or weeks, and few have lasted for years. The existence of slow slides over decades “suggests that the subduction zones are more diverse than we should appreciate,” said Kevin Furlong, a geoscientist at Pennsylvania State University, who was not involved in the new research. (A subduction zone is an area where oceanic crust slides under continental crust.)
Near the Indonesian island of Simeulue off the coast of Sumatra, coral growth patterns recorded back and forth 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 local sea levels change due to tectonics, these changes are seen in coral skeleton growth records, said Rishav Mallick, a doctoral student at Nanyang Technological University in Singapore and lead author of the new study, published this month. in Natural geoscience. The Simeulue reef has a history of near-annual vertical movement of the fault from 1738 to 1861.
The reef revealed that Simeulue had been sinking or sinking for 90 years at a constant rate of one or two millimeters per year, which is consistent with the movement of the fault floor. But around 1829, it suddenly started sinking five to seven times as fast, a few years down to an inch, Mallick said. This indicates that the fault has begun to move in a slow earthquake. “It’s a very brutal change,” he said. This “rapid” subsidence continued until the great earthquake of 1861.
The study highlights the complexity of subduction zones, Furlong said. For a long time, he noted, “the assumption was that between large earthquakes, the system was simple”: the two pieces of crust interlocked at the fault, creating stresses until—cracks– they let go with a frightening vibration.
Slow events complicate this picture. They may even act as triggers for larger earthquakes and can be detected by removing stress on the fault section but adding stress to adjacent sections, Mallick explains. “It’s like a bunch of springs,” he explains. “So if one lets go, the other has to bear the brunt.”
The 2004 Indian Ocean earthquake and tsunami that killed more than 220,000 people was preceded by several years of slow landslides in the Andaman Islands, Mallick said. However, slow slips cannot yet help predict larger earthquakes because slip durations vary widely. No gap has been monitored by GPS for 32 consecutive years, so modern surveillance may fail to detect long-lasting events such as the slow decline in Indonesia in the 19th century. And not all errors are monitored properly. This is especially true for subducted faults under the sea, which require specialized monitoring on the seabed rather than GPS.
If slow motion is missed, researchers can miscalculate where the stresses on the fault are and the earthquake strength that the fault could potentially generate. “Once we can better define locked areas,” Furlong said, “we can better determine the magnitude of an earthquake that could occur.”
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