The mystery of "slow earthquakes": Scientists recorded an earthquake that lasted 32 years

The discovery of this very slow earthquake can be very useful for scientists to understand the surprising variety of ways in which our restless planet moves and also the lethal potential of these silent phenomena in their ability to trigger much more powerful tremors.

slow earthquakes
Slow motion earthquakes were only recognised since the late 1990s when they were first spotted in North American's Pacific Northwest and the Nankai Trench region off the coast of Japan.

In February 1861, an earthquake, estimated at around 8.5 Richter, hit the area off the Indonesian island of Sumatra, triggering a gigantic tsunami that killed thousands of people on the island.

It seems that that tragic event was not an isolated case, but rather the final phase of the longest earthquake ever recorded that has crossed the underground for a full 32 years. Known as 'slow-slip events', in English slow-slip event, this type of phenomenon develops over several days, months or even years.

What just described broke the records set in the past, with a duration longer than twice the time. This was reported by scientists in Nature Geoscience.

What do we know about these “slow quakes”?

“I never thought such a long slow slipping event could occur, yet we found traces of it,” says study author Emma Hill, a geodesist at the Earth Observatory at Nanyang University of Technology in Singapore.

The discovery of this very slow earthquake can be very useful for scientists to understand the amazing variety of ways in which our restless planet moves and also the lethal potential of these silent phenomena in their ability to trigger much more powerful shocks.

earthquakes
These long earthquakes release the energy that accumulates with the displacement of the tectonic plates but, instead of causing shocks, gradually release the tension over time, not representing a danger in itself.

These long earthquakes release the energy that accumulates with the displacement of the tectonic plates but, instead of causing shocks, gradually release the tension over time, not representing a danger in itself. However, small underground displacements can increase stresses on adjacent areas along a fall, increasing the risk of a stronger shock nearby.

The discovery of these earthquakes

Slow motion earthquakes were only recognised since the late 1990s when they were first spotted in North American's Pacific Northwest and the Nankai Trench region off the coast of Japan.

Their weak release of energy causes only slight surface movements, which is why they have not been detected until GPS technology has been sufficiently advanced to be able to track them.

But since then, the more researchers look for traces of “slow” earthquakes, the more they find: from the coasts of New Zealand to Costa Rica and even in Alaska. 'We detect asismic slips everywhere,' says Lucile Bruhat, a geophysicist at the École Normale Supérieure (ENS) in Paris, France, who was not part of the study team.

Slow earthquakes in the Cascadia area and in the Nankai Trench region, take place with considerable regularity. Respectively every 14 months or so in the first and every three or six months in the second. In both areas these long-lasting shocks are also accompanied by a series of small minor shocks, better known as tremors.

The findings in two decades of studies

Over the years, scientists, as their studies continued, have also found that the duration of slow earthquakes can vary a lot. In Alaska, for example, as Mallick recounts, an event was discovered that lasted at least nine years.

earthquakes, Pacific ocean
In geology, the Pacific Belt of Fire is called an area characterised by frequent earthquakes and volcanic eruptions.

The seismologists were able to ascertain it only after the slipping of the surface stopped in 2004. The event detected near Sumatra greatly shifts the bar of the possible duration of this type of phenomenon.

“Many people argue that these major slow-swipe events are possible,” says Laura Wallace, geophysicist at the University of Texas at Austin and at GNS Science in New Zealand, who did not take part in the team's studies.

But continuous monitoring of soil movements near subduction zones has only been carried out for about two decades, so 'we are only observing a very short time'.

Why does it become very important to know this type of earthquake?

Understanding these slow motion events is critical to identifying the potential risks they pose in terms of triggering more powerful quakes. This type of phenomenon preceded many of the most terrible earthquakes recorded, including the catastrophic magnitude 9.1 earthquake of Sumatra-Andaman that struck Indonesia in 2004, the devastating 9.1 magnitude earthquake of Tōhoku in Japan in 2011 and the terrible magnitude 8.2 earthquake of Iquique in Chile in 2014.

But to demonstrate precisely that slow-scrolling events can actually trigger violent shocks is still very complex. Not all slow earthquakes lead to very intense earthquakes. To understand more, it is necessary to continue to study the phenomenon. But often to do this it is necessary to use a very complex and expensive instrumentation.