Twenty years ago, scientists discovered a new type of seismic event, the functioning of which is still poorly understood. These are slow earthquakes. Unlike classic earthquakes which only last a few seconds, slow earthquakes last several weeks or even months.
Generally, when we hear the word “earthquake” we immediately think of an earthquake, of tremors shaking the ground and causing damage, more or less important. Earthquakes are in fact the result of a sudden release of energy associated with a sudden movement along a fault plane. They are intimately linked to plate tectonics and the accumulation of stresses experienced by the rocks of the earth’s crust.
Classic earthquakes characterized by a brief and violent rupture
These earthquakes, which we will call “classic” here, then occur in a recurring, more or less regular manner. This recurrence is illustrated by the seismic cycle, which includes a stress loading (accumulation) period. During this so-called interseismic period, the fault is blocked and the rocks will deform in response to the tectonic load. Beyond a certain threshold, however, the rocks can no longer endure additional elastic deformation: this is rupture. The blocks slide abruptly along the fault. This is the cosismic period. The amount of energy released during this movement will define the magnitude of the earthquake.
While the seismic cycle of a fault can range from approximately 10 to 1,000 years, the duration of an earthquake is only a few seconds, generally less than a minute. Earthquakes are therefore extremely brief events, which can turn out to be very violent.
A redefinition of earthquakes
However, over the past twenty years, we have heard more and more about slow earthquakes. Their discovery also called into question the very fixed view of the seismic cycle described below, and questioned the idea that nothing happened, by definition, during the stress loading period.
Slow earthquakes were first identified in 2001 at the Cascades subduction zone off Canada. Quickly, others were detected in Japan, New Zealand, Mexico… Always, therefore, on subduction zones, which are characterized by the sinking of an oceanic plate under a continental plate.
Subduction zones are well known for their seismogenic potential. About 80% of the most powerful earthquakes on the planet occur in the various subduction zones that run through the planet.
The contribution of GPS to understanding slow earthquakes
But thanks to progress in the precision of GPS measurements and the development of networks of instruments that allow continuous monitoring over vast regions, scientists have been able to observe the occurrence of other types of events, which had gone unnoticed until now. there.
In practice, GPS measurements are used to quantify ground deformation. In the case of a subduction zone, if the GPS stations located on the continental plate are all moving at the same speed and in the same direction as the plunging oceanic plate, this means that there is no deformation and that the two plates glide quietly over each other in a regular manner. But when we record a speed contrast between certain stations, this means that the continental plate is deforming, in response to a blocking of the fault at depth. We then know that the tectonic stresses accumulate. At the time of the rupture, the GPS stations then record, for only a few seconds, a reverse movement, which corresponds to the release of the tension accumulated in the continental plate. This very brief episode corresponds to the earthquake.
Earthquakes that go unnoticed
But from 2001, scientists began to observe this behavior over much longer periods: the stations thus recorded a change of direction not over a few seconds, but over several weeks, even several months! From there was born the definition of slow earthquakes.
Depending on the amount of movement recorded by the GPS, it is therefore possible to establish a magnitude for this new type of earthquake. Thus, it appeared that some are of magnitude greater than 7. However, because the release of energy is spread over several weeks or months, these events go completely unnoticed. A slow earthquake of magnitude 7.5 was thus recorded in Mexico, without being felt by the population, while the “classic” earthquake in Haiti in 2010, of magnitude 7.3, caused considerable damage and the death of 200,000 people.
What is a slow slide? © GNS Science, YouTube
Although data is beginning to accumulate on slow earthquakes, there are still too few of them to fully understand this phenomenon. The link with classical earthquakes remains to be clarified. Do slow earthquakes act as a safety valve, releasing accumulated pressure and thus preventing the occurrence of more destructive earthquakes? Or, on the contrary, do they have the capacity to destabilize a blocked area nearby, thus triggering a classic earthquake earlier than expected?
The installation of new GPS equipment and the continuous monitoring of the most seismogenic regions should make it possible to answer this question in the years to come.