Thousands of GPS data reveal an earlier stage announcing great earthquakes | Sciences

It’s the holy grail for seismologists and geologists: finding reliable evidence about when, where, and how forcefully the Earth will shake next. So far in this century, more than a million people have died from the earthquake, not to mention the astronomical cost to infrastructure and the economy, particularly in the poorest countries. Now, French scientists have discovered a precursor phase that starts hours before a major earthquake. As detailed in the magazine…

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It’s the holy grail for seismologists and geologists: finding reliable evidence about when, where, and how forcefully the earth will shake next. So far in this century, more than a million people have died from the earthquake, not to mention the astronomical cost to infrastructure and the economy, particularly in the poorest countries. Now, French scientists have discovered a precursor phase that starts hours before a major earthquake. As detailed in the journal SciencesThey achieved this by analyzing small movements recorded by the Global Positioning System (GPS). These researchers believe that deploying detection networks around major faults can help find that holy grail.

In the 1970s, euphoria broke out among seismologists. The accumulation of data on earthquakes, new theoretical models, and experiments in the laboratory made us dream of discovering the phenomena and mechanisms that heralded the earthquake. As University of California professor Roland Borgmann says, everything indicates that “earthquakes are usually preceded by precursor processes.” But the excitement faded: “When scientists looked further and got better observations of these precursors, they found that while they did occur occasionally, they could not really be distinguished from similar processes that often occurred at other times and places.” Julián García Mayordomo, an expert in earthquake geology from the Institute of Geology and Mining (IGME), also recalls the complexity: “Large earthquakes occur at a depth of 15 or 10 kilometers in the crust, where we have never been able to look. In addition, the main fault that produces earthquakes of magnitude 6.5 or 7 is a plane that can be tens of kilometers long and 15 kilometers deep. It is a huge area in which many occur. From the processes. It is absolutely impossible to control it. There are too many variables, which makes the phenomenon highly unpredictable.”

“Great earthquakes happen at a depth of 15 or 10 kilometers in the earth’s crust, where we have never been able to look at them.

Julián García Mayordomo, earthquake geologist from the Institute of Geology and Mining

But scientists Quentin Pelletre of France’s Côte d’Azur University and Jean-Mathieu Noquet of the Institute of Planetary Physics in Paris have found a way to detect a signal of a future earthquake amid all the noise. His idea was to take advantage of Global Navigation Satellite Systems (GNSS), such as the American GPS or the European Galileo System. The entire planet is dotted with geodetic stations (there are 400 in the Iberian Peninsula) which include a series of sensors of interest to geologists. One of them is a GNSS unit that relies on triangulation with GPS or Galileo satellites (as well as with Russia’s GLONASS network or China’s Beidou network) to determine its location. Installed on the ground, the location of these stations, given in millimeters, is essential for mapping. But the seasons move and are not always in the same place: their position changes throughout the year due to global phenomena, such as continental drift, or local phenomena, such as dam construction, land surveys or crushing. A major earthquake can also move them from one place and it is recorded by GPS.

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What these two French scientists did was analyze the positioning data of more than 3,000 geodetic stations while the earth shuddered with 90 earthquakes of magnitude greater than 7 (those in Turkey measured 7.8 and 7.5) so far this century. More importantly, they also collected and analyzed GPS data for the 48 hours before each of these large tremors. His initial hypothesis was that earthquakes begin with a precursor phase characterized by a slow displacement, without tremors, at the fault point where the epicenter of the next earthquake will be.

The precursor phase is the time window during which tectonic masses begin to move relative to each other, first slowly and then gradually accelerating.

Jean-Mathieu Noquet, of the Institute for Planetary Physics in Paris

“Earthquakes are sudden landslides along the faults that separate two tectonic masses,” recalls Nücke, co-author of this paper. before the two rock blocks crashed. “The initial phase is the window of time during which the tectonic masses begin to move relative to each other, first accelerating slowly and gradually to eventually reach a rapid sliding velocity. The rapid sliding results in the seismic waves that cause the damage observed during large earthquakes,” explains the French scientist. Although there is some consensus on the existence of this initial stage, there is no its main characteristics, such as duration. For some it only lasts a few seconds, for others it can be seen as a series of small earthquakes over weeks or months. “On the contrary, our study indicates that slippage gradually accelerates over a few hours, about two hours,” he adds.

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To make sure the detected signal was correct, they repeated their analysis, powered by artificial intelligence, for another 100,000 time windows, but then there was no earthquake. They did not detect a signal of slow but exponential growth like that seen in the initial phase of a major earthquake.

So far the good news. As the authors themselves are aware, in almost half of the earthquakes they do not find this initial stage. This does not mean that it does not contain it, it may have occurred prior to the time period they analyzed. For various reasons, such as computational cost, they did not revise their analysis beyond the hours before each major earthquake. Another reason could be that the earthquake occurred too far from one of these geodetic stations. Nocquet is convinced that “the development of systematic, extensive, and accurate fault monitoring could have the potential to detect slippage of future precursors of individual events.”

Victor Puente is a researcher in geodesy applied to seismology at the National Geographic Institute. Puente, who appreciates the importance of work based on information from the last 90 major earthquakes, recalls that the French scientists based their analysis on the database of the Geodesy Laboratory in Nevada (USA). Here they have records of not 3,000 stations, but 17,000 stations. If they are all used, the analysis ability will be much greater. “But this lab delivers data in two hours of latency,” Puente recalls. To support its detection system, the alarm will arrive when the earthquake has already happened. In any case, Puente points out, if the results achieved by the French researchers are to be confirmed, this response time should be reduced until the data is acquired in real time. It will be difficult, but possible.”

The more we know about the faults, the better we can figure out the magnitude of earthquakes, and then their intensity at the surface, and in the second step, try to know better when they will occur.”

Jesús Galindo, from the Department of Geodynamics at the University of Granada

Another key for a system like the one proposed in this paper to work is the need for in-depth knowledge of all the faults that could be the origin of a major earthquake. Jesús Galindo, of the University of Granada’s Department of Geodynamics, points out that this is an area of ​​future research to pursue. “As happened with meteorology, with more seasons and better mathematical models, we are already able to predict weather, temperature, heat waves or when it will rain. The same for faults. What we also need is an understanding of how the Earth moves and other physical parameters such as deep structure. The more we know about faults, the more we can model and know in principle what the maximum magnitude of earthquakes is, to know the intensity of earthquakes later, and in the second step, try to know better when they will happen,” he explains.

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“The ultimate key is the data you have near the epicenter,” highlights the professor at the Polytechnic University of Madrid, an expert in geodesy applied to seismic hazards. Regarding the research of French scientists, who are at the forefront of this field, he said, he stresses that it is a great contribution and that it is on the way. But for the sake of saying it In two hours an earthquake will happen We still have a lot.”

BARACA, project to identify earthquake risks in southern Spain

The government research agency has just approved a project to comprehensively investigate the complex set of open rifts between the southeast Eurasian plate and the north African plate. From the northeast of Morocco beyond Alicante, passing through the Alboran Sea, the meeting of the two plates places terrain boundaries under great stress, rifting faults. In-depth knowledge of these faults and identification of seismic hazards is one of the main objectives of this project, BARACA christened.

Researcher from the University of Granada, Jesús Galindo, is one of the researchers of BARACA, which brings together surveyors, seismologists, geologists and engineers from various Spanish universities. “There are very obvious defects, such as those in San Andres, Japan or the Chilean coast, with a single plane and where the deformation is not distributed,” he explains. It is on these faults that the most catastrophic earthquakes occur. “Then, there are areas like here at the Eurasian-Africa contact where there are many small faults, so the deformation is much more distributed. The situation we have is much better because the small faults jump around. No large fault accumulates much energy. It is true that we have many earthquakes, but they are not like those in Japan or Chile or the West Coast of the United States.”

However, there is a relative risk and every few years an earthquake of similar strength to the one Turkey experienced in February can occur. BARACA is a new attempt to anticipate as much as possible.

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