Information entropy of earthquake populations in northeastern Italy and western Slovenia

- We study eight seismicity populations with the Shannon entropy and fractal analysis.
- The seismicity sets are characterized by fluctuations of entropy and fractal dimension.
- Aftershock sequences are characterized by a marked decrease of the Shannon entropy.
- Entropy and fractal fluctuations are interpreted as variations of crustal strain energy.

The spatio-temporal evolution of eight seismicity populations, preceding and following moderate earthquake sequences occurred in NE-Italy and W-Slovenia, are investigated by means of the normalized Shannon entropy and the fractal dimension. Three phases are recognized in the temporal seismic series. The period preceding the mainshock is characterized by oscillations of the Shannon entropy around a nearly constant level and by fluctuations of the fractal dimension. The phase of mainshock and aftershock sequences is characterized by a significant decrease of the Shannon entropy. A simultaneous marked decrease of the fractal dimension is observed in five cases. After the sequence, the entropy recovers the nearly constant trend before the mainshock and the fractal dimension is characterized by fluctuations. We interpreted the fluctuations of the normalized Shannon entropy and the fractal dimension caused by the coupling between the stress field and the mechanical heterogeneities of the crust that results in spatial and temporal fluctuations of the strain energy.