How Long Will the Earthquake Last? The Earth's Crust tells us
How long will the earthquake last? It is one of the questions that researchers often find themselves facing at the beginning of each seismic sequence. A new analysis of earthquake aftershocks shows that extensional tectonic settings are characterized by longer durations and a greater number of aftershocks than compressional environments. The study, Longer aftershocks duration in extensional tectonic settings, conducted by a team of researchers from the Institute for electromagnetic detection of the environment of the National Research Council (CNR-IREA), the National Institute of Geophysics and Volcanology (INGV) and Sapienza University of Rome, has been published on Scientific Reports by Nature.
"This work", explains Carlo Doglioni, president of the National Institute of Geophysics and Volcanology (INGV) and Professor of Sapienza University, "shows that in the areas where the earth expands, seismic sequences, despite having magnitude on average lower than compressional environments, last longer because the crustal volume moves in favour of gravity. Therefore, these seismic sequences end when the collapsed volume finds again its gravitational balance". Conversely, in compressional environments, the involved volume must move against gravity and, therefore, the energy able to continue raising the hangingwall of the faults is exhausted more quickly.
"From a comparative analysis of ten seismic sequences", says Pietro Tizzani, Researcher of CNR-IREA, "five included in extensional tectonic contexts and five in compressional ones, it was possible to demonstrate that, irrespective of the magnitude of the selected seismic events, extensional earthquakes last longer than those that develop in compressional environments". This study also explains why the earthquakes nucleated along the Italian Apennines, which are mostly characterized by extensional tectonics, are followed by an impressive number of aftershocks, also persistent over time. For example, the Amatrice-Norcia seismic sequence began about 15 months ago and about 80,000 aftershocks occurred.
This interpretation of seismicity can have significant applications in the management of the post-event emergency; an approximate estimate of the duration of aftershock can already be obtained according to the type of the tectonic context in which the seismic sequence is inserted. Moreover, it confirms that the energy stored for tens or thousands of years is different depending on the tectonic setting and is mainly gravitational for the extensional earthquakes and elastic for the compressional ones. "Understanding the different mechanisms and the phenomena associated with the various geodynamic environments", concludes Doglioni, "can lead to a more useful classification of earthquakes and this represents an essential step to understand their nature and temporal evolution".
Longer aftershocks duration in extensional tectonic settings - E. Valerio, P. Tizzani, E. Carminati e C. Doglioni - Scientific Reports 7, Article number: 16403 (2017); doi:10.1038/s41598-017-14550-2
Department of Earth Science, Sapienza University of Rome
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