Seismic Precursors in Central Italy using Autocorrelation Measurements

The aim of this study is to identify precursors related to fluid pressure changes at depth to large earthquakes that occured in central Italy such as the 2009 Mw6.3 L'Aquila and the 2016 Mw6.2 Amatrice earthquake. To that end, we monitor the temporal evolution of the crust using a new method called Coherence of Correlated Waveforms [CCW] that uses seismic noise autocorrelation measurements. This allow us to look for changes in the medium with a high temporal resolution of 5 days. Our measurements of the CCW show that the L'Aquila Earthquake (2009-M6.3) is preceded by a 150-day oscillation whose amplitude and frequency progressively increases until the rupture. The high Vp/Vs ratios measured on the foreshocks of L’Aquila earthquake correspond to the CCW drop periods, suggesting the sensitivity of the measurement to crusty fluids.

Analysing 17 years of data, we found that this signal occurred only before the L'Aquila and the Amatrice earthquakes. This suggests the existence of a unique nucleation process.

Finally, for the 2016 Amatrice Earthquake, using an array of 25 seismic stations we are able to map the geographical extension of this precursory signal. This pattern, evolving over time, suggests diffusion phenomena in the upper crust.