The Pollino seismic sequence/swarm

104° Congresso Nazionale della Società Italiana di Fisica - Università della Calabria - dal 17 al 21 settembre 2018

In the years between 2010 and 2015 in the Apennines-Calabrian arc boundary, in the Pollino
massif, a long seismic sequence took place. The area is subject to Northeast- Southwest
extension, which results in a complex system of normal faults striking Northwest-Southeast,
nearly parallel to the Apenninic mountain range. The seismic sequence includes more than
6000 earthquakes in the Pollino region, the maximum magnitude recorded is Ml=5.0 and
it happened in October 25th 2012 after about two years of ongoing activity; the peculiar
temporal evolution of the seismic sequence allows us to catalogue it as a swarm. Here
we describe the main seismological characteristics of this seismic sequence and characterise
the fracture field of the region. We analyse thousands of seismograms, deriving accurate
locations crust velocity model and anisotropic parameters in the crust. These parameters
yield clues and insights that may help understanding the physical mechanisms behind the
seismic swarm. Since the late 60s-early 70s era seismologists started developing theories that
included variations of the elastic properties of the Earth crust and the state of stress and
its evolution prior to the occurrence of a large earthquake. Among the others the theory of
the dilatancy: when a rock is subject to stress, the rock grains are shifted generating microcracks,
thus the rock itself increases its volume. Inside the fractured rock, fluid saturation and
pore pressure play an important role in earthquake nucleation, by modulating the effective
stress. Thus, measuring the variations of wave speed and of anisotropic parameter in time
can be highly informative on how the stress leading to a major fault failure builds up.
We systematically look at seismic-wave propagation properties to possibly reveal short-term
variations in the elastic properties of the Earth crust. In active fault areas, tectonic stress
variation influences fracture field orientation and fluid migration processes, whose evolution
over time can be monitored through the measurement of the anisotropic parameters. We
analysed waveforms recorded at permanent and temporary stations hold by the Istituto
Nazionale di Geofisica e Vulcanologia.