Large earthquakes could warn hours before they occur. The retrospective analysis of 90 powerful earthquakes (of magnitude equal to or greater than 7) has made it possible to identify precursor signals of tremors up to two hours before they occur. The finding opens the door to the prediction of earthquakes, one of the great objectives of geology until now incomplete.
“The ambition of our study was to answer the most fundamental question in seismology: are earthquakes a chaotic or deterministic phenomenon?” Quentin Bletery, a geophysicist at the Géoazur Laboratory in France, and one of the authors of the work, details in an email to this newspaper. “Our results strongly support the second option,” continues the expert, whose finding rules out the unpredictability of the chaotic option, and suggests that earthquakes do not just happen suddenly.
The study provides evidence that two hours before an earthquake begins, the nearby land surface deforms slowly but rapidly. If properly measured, these movements can be used to predict when and where the tremor will occur. The results of the study open the door to developing a warning system that minimizes human and economic losses, although “we are still a long way from being able to predict any earthquake,” says Bletery.
The search for precursor signals to earthquakes is a matter that geology has been studying, without success, since 1970. Several studies have observed that before some powerful earthquakes a slow displacement of the fault occurs, that is, a slow movement of the ground in the area of ??the tremor, but none have been able to establish a causal relationship between these thrusts and earthquakes.
“These events occur constantly along the faults [fractures in the earth’s crust whose movement causes tremors] without being followed by earthquakes,” says the French researcher, “and what is more important, they do not directly precede them.” In other words, observing said movement does not allow us to know when an earthquake is going to occur, and it does not even guarantee that it will happen.
In the paper published this Thursday in the journal Science, Bletery and his colleague Jean-Mathieu Nocquet have refined the search for these movements. French scientists have analyzed, using GPS measurements, how the ground moved at 3,000 points near the epicenters of 90 earthquakes of magnitude greater than or equal to 7 moments before they occurred. In addition, they have measured only the movements that occur in the direction in which the earthquake will later displace the ground. In other words, they have only looked at the points close to the core of each earthquake, in the moments before it occurred and in the direction in which they knew the fault was going to move.
The researchers have calculated the average of the signals of the 90 earthquakes, and have seen how in the two hours prior to the events the displacement of the fault that occurs has an exponential acceleration. That is to say, that the movement of the ground is faster and faster, until the earthquake explodes. This acceleration is what for Bletery confirms that her finding is, indeed, a precursor sign of earthquakes, and that “the causal relationship [between both events] is clear.”
However, questions still remain. The study has not specifically analyzed each earthquake, but to obtain a detectable signal it has needed to average the more than 3,000 observations of the 90 events. “If we look at each event one by one, the signal is dominated by noise and we cannot interpret the results,” acknowledges the French seismologist, so “we cannot conclude if this happens before each earthquake, although it would be the most logical explanation.”
In addition, “it is not clear if these accelerations of slow displacements are exclusively associated with large earthquakes,” he points out in a comment on the article also published this Thursday in Science Roland Bürgmann, a researcher at the Berkeley Seismological Laboratory, in the United States, who has not participated in the study. “It will be important to explore the frequency with which similar episodes are a false start”, that is, they are not followed by earthquakes, he develops.
In any case, and even if this precursor signal is confirmed, this does not mean that we are already capable of predicting earthquakes before they happen. The study has analyzed tremors that occurred in the past, and has identified the precursor phenomenon using data such as the location, magnitude and direction of movement of the earthquake, which will be precisely what the prediction of future events must predict.
“A meaningful earthquake prediction has to clearly define the expected time, location, and magnitude of the future event,” says Berkeley’s Bürgmann, something that currently available instrumentation is unable to identify. The objective of the work was not that, recalls Bletery, but “to show that prediction is not physically impossible.”