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How long will an earthquake last? This is one of the questions that researchers face at the beginning of every seismic sequence. A new analysis reveals that earthquake aftershocks last longer and are more numerous in extensional zones rather than in compressional zones.

The study, entitled “Longer Aftershocks Duration in Extensional Tectonic Settings,” was conducted by a team of researchers at the National Research Council Institute for Environmental Electromagnetic Monitoring (CNR-IREA) at the National Institute for Geophysics and Volcanology (INGV) and Sapienza University of Rome. The study has been published by Scientific Reports.

“The study,” explains Carlo Doglioni, President of the National Institute for Geophysics and Volcanology and Sapienza Professor, “demonstrates that, notwithstanding the fact that seismic sequences have lower magnitudes than in compressive zones, they last longer where the earth dilates because the volume of the earth’s crust moves with the force of gravity. Thus, aftershocks only end when the collapsed volume find a new gravitational equilibrium.” In compressive zones, instead, the volume moves against the force of gravity and the energy that raises seismic faults is dissipated more rapidly.

“A comparative analysis of ten seismic sequences,” explains CNR-IREA Researcher Pietro Tizzani, “including five extensional zone tectonics and five compressive zone tectonics has revealed that extensional earthquakes last longer.”

This study also explains why earthquakes in the Italian Appennine Mountain Range, which are prevalently extensional, are followed by long sequences of significant aftershocks. For example, 15 months have passed since the beginning of the Amatrice-Norcia Seismic Sequence and there have been 80,000 aftershocks. This interpretation of earthquakes may have important applications in post-seismic emergency management, as we may formulate an estimate of aftershock duration based on the type of area in which the earthquake occurred. Moreover, the study also confirms that the nature of the energy, which accumulates over the course of centuries, differs on the basis of the type of tectonic zone. It is prevalently gravitational in extensional zones and elastic in compressive zones.

“The comprehension of the various mechanisms and the phenomenology associated with different geodynamic environments,” Doglioni concludes, “can point us towards a more useful and detailed classification of earthquakes, a fundamental step to understand their nature and temporal evolution.”



Longer Aftershocks Duration in Extensional Tectonic Settings - E. Valerio, P. Tizzani, E. Carminati and C. Doglioni - Scientific Reports 7, Article N. 16403 (2017); doi:10.1038/s41598-017-14550-2