The earthquake of 30 October 2016 was triggered by the previous events of the seismic sequence of Amatrice and these same events limited its magnitude. This is what a brand new study by INGV – ENS, just published in Scientific Reports, suggests
There would be a connection between the magnitude 6.5 earthquake that hit Central Italy on 30 October 2016 and the seismic events of the previous months. These, in fact, would have "anticipated" the occurrence of that earthquake by over a century, but at the same time would have halved the available energy, limiting its magnitude. This is the hypothesis of a research team from the National Institute of Geophysics and Volcanology (INGV) and from École Normal Superior (ENS) of Paris.
During the seismic sequence of Central Italy in 2016, several earthquakes of significant magnitude occurred in the space of a couple of months. The first earthquake of 24 August of magnitude 6.0, with epicenter located in the municipality of Accumoli (RI), was followed by two earthquakes of 26 October, of magnitude 5.4 and 5.9 respectively located in Visso (MC). Four days later, on 30 October, the Norcia (PG) earthquake occurred in the area between the seismic events of 24 August and 26 October, the strongest of the sequence, with a magnitude of 6.5.
"Over the last few decades", explains Nicola Alessandro Pino, INGV researcher and co-author of the study, "in Italy there have been other seismic sequences consisting of earthquakes of similar magnitude, which occurred in the same area within a few seconds, a few days or a few months. These evidences”, continues the expert, “lead us to hypothesize that the rapid succession of earthquakes is not accidental. In fact, for some of these sequences it has been demonstrated that the previous events triggered the ones that followed. The faults interact with each other and alter the state of stress on the neighboring faults, delaying the time of the next earthquake or, on the contrary, bringing them closer to failure".
The stress variations produced by previous events, according to the authors, can therefore move the rupture on an adjacent fault further or closer over time, also limiting the energy emitted.
“Even for the 2016 sequence”, continues Vincenzo Convertito, INGV researcher and co-author of the study, “a cascade effect of previous earthquakes on subsequent ones can be hypothesized. The calculation of the variations caused by the earthquakes of August 24 and those of October 26 on the fault which will then rupture on October 30 show, in fact, that the previous earthquakes modified the stress field on the southern and northern parts of the fault, increasing on the other hand, the effort in the central area is significantly higher, especially in the deeper portion of the fault. From here on the morning of October 30 the breakup will start".
The structure affected by the October 30 event has an area of about 440 km2, twice that actually activated by the earthquake. If it had broken completely in a single event, the energy emitted would have been at least double, producing an earthquake of magnitude 6.7, the authors say.
The study quantifies the time by which the earthquake that occurred on October 110 would have been anticipated in 30 years. “It can therefore be said that the previous events anticipated the occurrence of the earthquake of October 30 by over a century, but at the same time limited its magnitude, probably halving the available energy, which corresponds to a decrease in magnitude equal to 0.2”, conclude the researchers.
The results of the study Clock advance and magnitude limitation through fault interaction: the case of the 2016 central Italy earthquake sequence they have just been published in the magazine Scientific Reports.
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picture 1- Map of the seismic sequence of Central Italy in 2016. The figure shows the events from the beginning of the sequence (August 24) to October 30, the day in which the strongest earthquake occurred (magnitude 6.5). The color and size of the symbols change according to the time of occurrence and the magnitude (except for events with a magnitude lower than 2, which are shown in white). The focal mechanisms of the 4 major earthquakes and the surface projection of the fault planes associated with these events are also indicated. The thicker lines indicate the intersection of the planes with the surface (figure taken from Pino et al., 2019).
picture 2 - Variation of stress (Coulomb stress) on the fault plane of the Norcia earthquake, caused by the 3 strongest events in the sequence, prior to October 30th. Each panel shows the variation caused by the earthquake that occurred at the time indicated above (in b and c the variation is cumulative with those produced by previous earthquakes). The locations of the earthquakes that occurred within 350 m of the fault plane are also reported and their occurrence time is indicated according to the color scale shown in panel a. The star indicates the point where the rupture of October 30 started (figure taken from Pino et al., 2019).
picture 3 - Map of the dislocation associated with the rupture of the October 30 earthquake, obtained from the results of the seismological and geodetic analyzes (Chiaraluce et al., 2017; Cheloni et al., 2017). The locations of the earthquakes occurring within 350 m of the fault plane are also reported and their occurrence time is indicated according to the color scale shown in figure 2a. The star and the arrow respectively indicate the point where the rupture of October 30 started and the dominant direction of its propagation (figure taken from Pino et al., 2019).