The work curated by INGV, United States Geological Survey and University of Berkeley suggests a reinterpretation of the model according to which extensional earthquakes such as those of the central Apennines could be generated by gravitational collapses, and not by elastic dislocations
A new study, conducted by a team of researchers from theNational Institute of Geophysics and Volcanology (INGV), the United States Geological Survey (USGS) and Berkeley Seismology Lab of University of California Berkeley, Offers a new reading of the model “Gravimoto” as a mechanism of earthquake genesis.
The research, entitled “Do graviquakes exist?” and just published in the scientific journal 'Bulletin of the Seismological Society of America (BSSA)', analyzed the model “Gravimoto” proposed in 2015, according to which, in areas of the planet subjected to crustal extension (such as, for example, in the Apennines) seismic events would be essentially generated by gravitational collapses, and not by the elastic dislocations that characterize every other type of seismic rupture. According to the model "Gravimotion”, each collapse would serve to fill a void formed deep in the Earth's crust during the interseismic period (that is, during the period of time between two large earthquakes that occur on the same fault).
For extensional, or 'normal', faults only, the model “Gravimotion” is presented as an alternative to the 'classical' model of earthquake genesis theorized in 1910 by Harry Fielding Reid, based on observations of the fault that generated the great San Francisco earthquake of 1906: according to this model, earthquakes are generated by the release of elastic energy accumulated over centuries or millennia (the so-called 'elastic rebound').
“With our study we have carried out a rigorous evaluation of the Gravimoto model, first proposed in 2015 and also used for the study of the 2016 Central Apennines seismic sequence”, explains Luca Malagnini, Research Director of INGV and first author of the article. “Our critical reinterpretation of the model was based on seismological theory, on the mechanical behavior of the upper crust in the presence of fluids, on geodynamic evidence and on the analysis of crustal deformations induced by strong normal fault earthquakes”.
The analysis presented in the study just published highlights how the hypothesis of a co-seismic gravitational collapse of the Earth's crust is not actually supported by geodetic data and independent seismological observations: the seismograms and ground deformations observed during the 2016 seismic sequence, for example, are not compatible with what is predicted by the model "Gravimotion” but, on the contrary, appear consistent with the classical elastic dislocation model.
“The classical theory, formalized thanks to the studies of numerous authors over the decades, has given a fundamental contribution to modern earthquake modeling”, he adds Malagnini. “The approach of seismology to the study of earthquakes must be to reproduce every observation made during a seismic event (for example, the waves of a seismogram) using models of seismic rupture based on continuum mechanics and the theory of wave propagation”.
"These models allow us to accurately calculate the displacement, strain and stress fields associated with fault rupture, to generate realistic 'synthetic' seismograms and to interpret in detail the seismograms we observe during an earthquake. Our recent results remind us of the importance of applying theories and models validated by independent and convergent geophysical observations to the interpretation of natural phenomena, and in particular seismic ones.", concludes the researcher.
Cover image: ©MetroUK
Useful links:
National Institute of Geophysics and Volcanology (INGV)
United States Geological Survey (USGS)
University of California, Berkeley
Figure: Schematic of the Gravimoto and Elastoquake hypotheses compared (from Bignami et al., 2020, redrawn and modified), showing the assumed imbalance between co-seismic uplift and subsidence.