Summary:
The unrest period in Santorini's caldera during 2011-2012, led several studies to raise
the important question of whether seismicity is associated with an impending and
potential volcanic eruption or if it solely relieves the accumulated tectonic energy. In the
present work, we study seismic coda waves, generated by local earthquake events prior,
during and after the Santorini’s unrest period (volcanic and seismic crisis) that occurred
within the caldera area. Coda waves are interpreted as scattered seismic waves
generated by heterogeneities within the Earth, i.e. by faults, fractures, fissure
structures, microcracks, velocity and/or density boundaries/anomalies, etc. In particular,
we utilize all three components of the seismograms recorded by three seismological
stations on the Santorini island and estimate the duration of the coda waves by
implementing a five-step procedure that includes the signal-to-noise ratio, the STA/LTA
like method, in time domain and upon analytic signal obtained after implementing
Hilbert transformation, and the short time Fourier transform, using gaussian windowing
as a window function. The final estimation was verified or re-estimated manually due to
the existent ambient seismic noise. Because of the nature and the path complexity of
the coda waves and towards achieving a unified framework for the study of the
immerse geo-structural seismotectonic complexity of the Santorini volcanic complex, we
use Non-Extensive Statistical Physics (NESP) to study the probability density
functions (PDFs) of the increments of seismic coda waves. NESP, forms a generalization
of the Boltzmann-Gibbs statistical mechanics that has been extensively used for the
analysis of semi-chaotic systems that exhibit long-range interactions, memory effects
and multi-fractality. The analysis and results demonstrate that the seismic coda waves
increments, deviate from the Gaussian shape and their respective pdfs could adequately
be described and processed by the q-Gaussian distribution. Furthermore and in order to
investigate the dynamical evolution of the volcanic-tectonic activity, we estimate the qs indices derived from the PDFs of the coda wave time series increments, and present
their variations as a function of time, of space (spaciotemporal variation), of local
magnitude (ML) and epicentral distance (measured in km) prior, during and after the
caldera unrest period from 2009 to 2014.
Keywords:
nonextensive statistical mechanics, Santorini unrest period, coda waves, Tsallis entropy, earthquakes, seismology
