Are there earthquake precursor signals that can provide information about the approach of a strong earthquake? Can many low energy earthquakes be considered as harbingers of strong earthquakes or not?
These are the questions that scientists have always asked themselves and that, in a study just published in the Journal of Geophysical Research, a team of researchers from the Sapienza University of Rome, the National Institute of Geophysics and Volcanology (INGV) , the National Research Council (CNR) and the University of Athens have tried to respond.
Earthquakes are mechanical instabilities of the earth's crust produced by the progressive accumulation of stress in the subsoil over the centuries. Most earthquakes are small, low energy (magnitude), and the vast majority of them do not evolve into a large, disastrous earthquake. Seismologists have always debated whether there are precursor signals capable of providing information about the approach of a strong earthquake, and therefore whether many low-energy earthquakes can be considered warning signs of strong earthquakes or not.
To answer these questions, researchers have studied the seismicity of California over the last thirty years by combining theoretical models with statistical analyses: the research has highlighted that the so-called foreshocks, i.e. mild and moderate earthquakes that can precede larger earthquakes violent, they tend to spread over larger areas, have magnitudes with greater variability and are more numerous and energetic than swarms, i.e. those groups of earthquakes characterized by limited magnitudes that do not evolve into a strong earthquake.
On the contrary, swarms and foreshocks are indistinguishable from the point of view of duration, intensity and frequency of events.
The results, supported by statistical tests, therefore suggest that in the presence of groups of earthquakes that are numerous and extended over significant surfaces, the probability that a minor seismic activity can culminate in a major event is higher than in other conditions.
The research goes even further, attempting to explain the observations.
The hypothesis is that the rock volumes under stress progressively begin to destabilize each other over more or less extended periods and areas, producing clusters of small events. The larger the area over which they occur, the higher the probability that an earthquake will be generated capable of involving the system of unstable faults in its entire extension: it would therefore be a cascade feedback mechanism, in which the history of the release of energy in previous events is able to determine future earthquakes, beyond the local stability conditions of the faults.
If the results of this research were confirmed, then the hopes of being able to estimate the probability of a large seismic event starting from the characteristics of the previous seismicity would be limited; on the contrary, a characterization of the state of stability of the fault systems would be necessary in order to understand what the chances are of a small swarm of evolving into a real seismic sequence.
In support of this hypothesis there is numerous evidence of large earthquakes that occurred without being preceded by foreshocks or even in the presence of a decrease in seismic activity, as in the case of the Amatrice earthquake in 2016, and the failure of numerous statistical tests about the hypothesis that foreshocks behave as precursors reliably and not sporadically.
The research results push us to overcome the concept of "foreshocks" to shift attention to the stability conditions of the rock volumes in which seismicity occurs.
Study citation:
Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are foreshocks fore‐shocks? Journal of Geophysical Research: Solid Earth, 129, e2023JB027337.
What can small earthquakes tell us about major ones?
Are there precursory signals providing information about approaching large earthquakes? Are foreshocks fore-shocks or just clustered seismic events
without any prognostic value about incoming large-scale instability?
Answering such questions is the main goal of a research article recently published in the Journal of Geophysical Research by a team of researchers from the University Sapienza of Rome, of the National Institute of Geophysics and Volcanology (INGV), the National Research Council (CNR) and the National and Kapodistrian University of Athens. Earthquakes are mechanical instabilities mostly occurring within the shallow part of the lithosphere and produced by the progressive accumulation of stress over time.
The overwhelming majority of earthquakes are small and do not evolve into a large disastrous earthquake. Since the dawn of earthquake science seismologists debate whether there exist precursory signals providing information about future earthquakes or not.
To find a possible solution to the puzzle, researchers investigated seismicity in Southern California occurring during the last thirty years combining theoretical models with statistical analysis. The research highlighted that foreshocks, i.e. small and moderate magnitude earthquakes preceding mainshock tend to spread over larger areas, have magnitudes with greater variability and are more numerous and energetic than swarms (clusters with no large shocks). Conversely, swarms and foreshocks share the same distribution of duration, intensity and frequency of events.
The results, supported by statistical tests, suggest that extended clusters with high magnitude fluctuations have higher chances to flow into a seismic sequence. The work goes beyond statistical analysis, attempting to explain the observations. The proposal is that stressed rock volumes progressively become more and more globally unstable and trigger each other seismic activity over more and more extended time intervals and areas, producing clusters of small events. The larger the correlated area, the higher the chances that a run-away earthquake can involve the whole unstable fault system.
Therefore, a cascade feedback mechanism acts on the basis of previous stress release to produce future seismic activity. If these outputs were confirmed, ultimate seismic prediction would be intrinsically unpredictable and efforts should be devoted to characterizing the state of stability of fault systems in order to quantify the probability of major seismic events. In support of this hypothesis there is evidence of large earthquakes occurring without being preceded by any seismic anomaly or even in the presence of a decrease in seismic activity, as in the case of the Amatrice earthquake in 2016, as well as the failure of several statistical tests. This research overcomes the concept of "foreshocks" to shift attention to the large-scale stability conditions of crustal volumes which ultimately may provide information about future seismic activity.
Reference: Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are foreshocks fore‐shocks? Journal of Geophysical Research: Solid Earth, 129, e2023JB027337.
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