Italian seismicity and Vesuvius' eruptions synchronize on a quasi 60 year oscillation

Abstract

The analysis of the time distribution of the major earthquakes that occurred in Italy since 1600 reveals that seismicity alternates periods of higher and lower activities over cycles of about 60 years. The same rhythm is present in the eruptive history of the Vesuvius. The large spatial scale of synchronization leads to suppose an external forcing. A hint on its possible origin comes from climatology, where a quasi 60 year cycle is found for various parameters and processes. A hypothesis, also supported by other studies, is that seismicity and volcanism are controlled by cycles of sea level rise. The mechanism could justify similar regularities observed for seismicity in other areas (e.g., cycles of earthquakes in the southern San Andreas Fault, in California) or for strong earthquakes at the global level. The increasing oceanic load of the last century could be coresponsible of the current long-lasting rest of the Vesuvius.

Periodicity of Strong Seismicity in Italy: Schuster Spectrum Analysis Extended to the Destructive Earthquakes of 2016

“…After 1600, the magnitude of completeness ranges between 5.91 and 6.14. I performed the analysis for the intermediated threshold M w C 6 [the same value used in (Bragato 2015)] and then confirmed the results for M w C 5.8 and M w C 6.2.…”

“…The occurrence of such events represents a sudden acceleration of seismic activity over the last 30 years: in fact, after two strong earthquakes on 1976 (Friuli earthquake, M w 6.5) and 1980 (Irpinia earthquake, M w 6.8), a magnitude near to 6 was reached just one time, for the Umbria-Marche sequence of September 1997. In a previous study (Bragato 2015), I found that similar accelerations occur cyclically in the seismic history of Italy, more clearly since the seventeenth century. The strong social and economical impact of the earthquakes of 2016 in Central Italy led me to update that analysis and assess the strength of the periodicity.…”

“…The period of the oscillation estimated in the present work differs from that of Figure 7 Smoothed rate function (h = 7 years) of M w C 6.0 earthquakes that occurred in Italy since 1600 and selected according to different types of declustering (GK, overdeclustering and no declustering) Figure 8 Smoothed rate function (h = 7 years) of the mainshocks that occurred in Italy since 1600 and selected according to different values of the minimum magnitude. For a better comparison, the rate is normalized to the total number of earthquakes in each catalog (100kðtÞ=N eq ) (Bragato 2015), reducing from 55 to 46 years. The previous assessment converged to a solution maximizing the fit to the cluster centered on 1855.…”

“…A number of alternative hypotheses are discussed in (Bragato 2015) with reference to the available literature. They include the mutual triggering of earthquakes through different mechanisms of stress transfer, the existence of tectonic pulses as well as the control of seismicity by processes that are external to the geosphere.…”

A Statistical Investigation on a Seismic Transient Occurred in Italy Between the 17th and 20th Centuries

“…The robustness and stability of the result are also checked in various ways: considering the uncertainty associated with the magnitude of each earthquake (i.e., introducing random changes to magnitude); using increasing values of the minimum magnitude; and partitioning the national territory in sub-areas with different seismic characteristics. The second study (Bragato 2015) starts from the same seismic catalog of the present work, furnishes an estimate the overall time density of events since 1100, and concentrates on its oscillatory component since 1600, characterized by a period of about 55 years. The present work neglects the oscillatory component and focalizes on the long-term behavior, with special attention on the pre-1600 trend, with the aim to emphasize and confirm the strong acceleration of the 18th century.…”