While certain models had foreseen the possibility of a magnitude 7.5, it was the sequence of events that surprised experts during the earthquake that struck Japan on January 1. Because this violent shock was preceded by a seismic swarm that lasted three years. A situation that does not normally result in such a strong earthquake.
This will also interest you
[EN VIDÉO] When a tsunami in Japan destabilizes a glacier in Antarctica The tsunami caused by the March 11, 2011 earthquake in Japan did not have devastating effects…
As Japan now mourns more than 200 deaths and another hundred missing, scientists are now trying to understand the origin of the earthquake that struck the country on the first day of 2024. Because certain observations indicate that this is a very special event.
Undoubtedly, Japan is a region with a very high risk of earthquakes. This is not a surprise to anyone, especially not to the Japanese authorities, who are doing everything they can to ensure the safety of the population. However, the severity of the earthquake on January 1, 2024 was surprising.
Tens of thousands of small shocks over three years
It was not the magnitude of 7.5 that surprised the experts, but the sequence of events that we have been observing in the Noto Peninsula region for several years. In fact, earthquakes of magnitude greater than 7 are relatively common in the country. The peninsula has also experienced several magnitude 6 earthquakes over the past 20 years, and a model created in 2014 suggested the possibility of a magnitude 7.6 earthquake in the area. What is unusual is that it happened after a swarm of earthquakes.
It has been three years since the peninsula was rocked by tens of thousands of small to medium magnitude earthquakes, most of which were imperceptible to the population. However, the Japanese seismological network is extremely dense and scientists have been able to track this seismic evolution over time. In fact, the west coast is riddled with faults that have developed and moved under the stress of multiple tectonic plates meeting on the opposite coast. This intense, but not very strong, seismicity that propagates over time (hence the term “seismic swarm”) indicates that the rock compartments are continuously moving in relation to each other, regularly adjusting constraints and gradually releasing stresses. So in itself this is a good sign, because instead of tectonic stresses accumulating, they are released periodically, a bit like the valve in a pressure cooker. Normally this behavior prevents the triggering of a strong earthquake. And this is the problem. Why did a magnitude 7.5 earthquake occur in this region despite this earlier seismic swarm?
An unusual sequence of seismic events
In Science magazine, Zachary Ross, a geophysicist at the California Institute of Technology, says he is at a loss. “I can't think of another example in the world where a seismic swarm preceded an event of this magnitude,” he admits. So what happened?
As usual, this dramatic episode shows us that we are still far from understanding how seismic zones work in detail and that the processes there are certainly much more complex than we thought.
Japanese seismologist Aitaro Kato explains that seismic swarms usually occur when the heat of rocks at depth increases the pressure of the fluids (gas, water or magma) that lubricate the faults, allowing the rock compartments to gradually slide. few. This process generally lasts several days, months, or years and is characterized by a significant number of small to medium-sized earthquakes without any major rupture occurring. However, this type of seismicity was not necessarily expected in this region, where there is neither volcanic nor geothermal activity to a priori generate sufficient fluid pressure to trigger a swarm. However, this has been observed since November 2020.
Deep landslides that would have destabilized surface faults?
Recent studies suggest that fluids rise from the Earth's mantle into the Earth's crust and slide along deep faults. However, recently scientists have noticed an increase in the strength of the tremors. Previously less than 4, seismic stations recorded a magnitude 5.4 earthquake in June 2022 and a magnitude 6.5 in May 2023. An unusual behavior that could now be seen as a precursor to the January 1st earthquake. For Takuya Nishimura of Kyoto University, movements related to the pressure of fluids at depth may actually have caused additional stress in faults closer to the surface, ultimately leading to a sudden rupture.
However, the exact connection between the seismic swarm and the January 1 earthquake remains to be clarified. Understanding the processes that intervened in this sequence could make it possible in the future to distinguish whether a seismic swarm appears to be a precursor to a larger rupture or not. But there is still a long way to go.