Directional amplification and ground motion polarization in Casamicciola area (Ischia volcanic island) after the 21 August 2017 Md 4.0 earthquake

In this paper, we investigated ground motion directional amplification and
horizontal polarization using ambient noise measurements performed in the
northern sector of Ischia Island which suffered damage (VIII EMS) during the
21 August 2017, Md 4.0 earthquake. Over 70 temporary seismic stations were
installed by the INGV EMERSITO task force, whose aim is to monitor site effects
after damaging earthquakes in Italy. To investigate ground motion directional
amplification effects, we have applied three different techniques, testing their
performance: the HVSR calculation by rotating the two horizontal components,
the covariance matrix analysis, and time–frequency domain polarization
analysis. These techniques resulted in coherent outcomes, highlighting the
occurrence of directional amplification and polarization effects in two main
sectors of the investigated area. Our results suggest an interesting pattern for
ground motion polarization, that is mainly controlled by recent fault activity and
hydrothermal fluid circulation characterizing the northern sector of the Ischia
Island.