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

Abstract

Abstract-The strong earthquakes that occurred in Italy between 2009 and 2016 represent an abrupt acceleration of seismicity in respect of the previous 30 years. Such behavior seems to agree with the periodic rate change I observed in a previous paper. The present work improves that study by extending the data set up to the end of 2016, adopting the latest version of the historical seismic catalog of Italy, and introducing Schuster spectrum analysis for the detection of the oscillatory period and the assessment of its statistical significance. Applied to the declustered catalog of M w C 6 earthquakes that occurred between 1600 and 2016, the analysis individuates a marked periodicity of 46 years, which is recognized above the 95% confidence level. Monte Carlo simulation shows that the oscillatory behavior is stable in respect of random errors on magnitude estimation. A parametric oscillatory model for the annual rate of seismicity is estimated by likelihood maximization under the hypothesis of inhomogeneous Poisson point process. According to the Akaike Information Criterion, such model outperforms the simpler homogeneous one with constant annual rate. A further element emerges form the analysis: so far, despite recent earthquakes, the Italian seismicity is still within a long-term decreasing trend established since the first half of the twentieth century.

Correlation between Seismic and Volcanic Activity at a Large Spatial Scale in Italy: Examples from the Neapolitan Volcanic District (Vesuvius Volcano, Southern Italy)

“…The earthquake catalog herein adopted is the same of [13]. Its main characteristics and the processing steps are summarized as it follows.…”

“…The characteristics of such oscillations were explored graphically in [6]. A more formal test based on Schuster spectrum analysis [26] has been adopted in [13] for the oscillations of seismicity. It demonstrates the statistical significance of the oscillations and refines the estimation of their time period to 46 years.…”

“…Perhaps, more general mechanisms should act. Previous works [6,12,13] noted the correspondence between the seismic transient that took place between 1600 and 1900 (three time the annual rate of destructive earthquakes in respect to the previous period) and the occurrence of the Little Ice Age (LIA). Furthermore, the current phase of global warming is synchronous with the gradual reduction of seismic activity through the last century (Figure 4).…”

“…Recent works [6,12,13] have pointed out the correspondence between variations in the rate of seismicity throughout Italy and the climatic changes of the last millennium. In particular, the seismic activity accelerated during the most severe period of the Little Ice Age (between 1600 and 1900), while it seems to decrease in the current phase of global warming.…”

Time-invariance and geographical divide of earthquake mortality in Italy since 1800

“…In particular, Figure 3c shows three major steps: at the beginning of the 20 th century, in the time interval 1960-1980, and more recently, with a further increasing phase opened by the L'Aquila earthquake of 2009 and continued with the Emilia earthquake of 2012 and the Central Italy earthquake of 2016. The alternate behavior of Figure 2 and Figure 3, with the time clustering of earthquakes at the national level, has been analyzed by Bragato [2017b], who recognized a significant periodicity with cycles of about 46 years. Furthermore, the decrease of the last century [Bragato and Sugan, 2014] seems to indicate the conclusion of a seismic transient that struck Italy starting in the 17 th century [Bragato, 2017a].…”