The catastrophic earthquake of 1861 on the Indonesian island of Sumatra is considered to be the immediate cause of an inactive fault. But new research reveals that 32 years before the catastrophic event, tectonic plates under the island were slowly and silently colliding with each other.

These decades of silent earthquakes, known as “slow slip events”, are the longest ever to be detected. It was very precise and systematic during evolution, but a new study suggests that the 1861 earthquake may have been at least 8.5 on the Richter scale, causing a tsunami that killed thousands of people. The new study will help scientists today to more effectively monitor dangerous earthquakes.

Slow-slip earthquakes occur when two parts of the crust move relative to each other, as are the most well-known earthquakes that shake the Earth’s surface. Some errors are now being monitored for slow slip using seismic devices or GPS technology, but it is very difficult to detect such events into remote errors (or before the 1990s, when GPS was available). The latest slow-slip events that scientists have learned lasted for hours, days, weeks, and even years, lasting from a few years to years. The existence of slow slides for decades “suggests that subduction zones are more diverse than we should appreciate,” said Kevin Furlong, a geoscientist at Pennsylvania State University who is not involved in new research. (Subduction zone is the area where the oceanic crust falls under the intercontinental crust.)

Near the Indonesian island of Simelu, off the coast of Sumatra, coral growth patterns have been recorded moving back and forth through an error in the 1861 earthquake, which provides a rare window into the past. Corals cannot grow when they reach the air. Rishav Mallik, a doctoral student at Nanyang Technological University in Singapore and lead author of the new study, said that when local sea levels change due to tectonics, these changes are reflected in the growth records of the coral skeleton. Inside Natural Geoscience. Simulu Reef has a history of the annual vertical movement of the crash from 1738 to 1861.

Reef revealed that for 90 years Simiul has been sinking or sinking at the rate of one or two millimeters per year, which is commensurate with the movement of the floor. But by 1829 it was sinking five or seven times as fast, and a few years down to an inch, Mallick said. This indicates that the fault began to move in a slow earthquake. “This is a very cruel change,” he said. This “rapid” excess continued until the Great Earthquake of 1861.

The study highlights the complexity of subduction zones, Furlong said. For a long time, he wrote, “The assumption was that the system was simpler during a major earthquake”: the two pieces of crust connect to each other and cause stress.Cracks– Let them go with the frightening vibration.

Slow events complicate the picture. They can even act as triggers for large earthquakes, which can be detected by adding pressure to nearby parts by removing the pressure in the wrong section, Mallick explains. “It’s like a bunch of springs,” he explains. “So if one is allowed to go, the other must bear its burden.”

Mallik said the 2004 Indian Ocean earthquake and tsunami that killed more than 220,000 people had slowed the Andaman Islands for years. However, slow slips have not yet been able to predict major earthquakes as the slow duration varies widely. GPS has not observed any gaps for 32 consecutive years, so modern observation may fail to detect long-term events such as the slowdown in Indonesia in the 19th century. Not all errors are properly monitored. This is especially true for submarine subject errors, which require special monitoring at sea rather than GPS.

If slow motion is lost, researchers can miscalculate where the pressures on the fault are and the magnitude of the potential earthquake. “Once the locked areas are well defined, we can determine the magnitude of the earthquake,” Furlong said.