Seismic amplification in a fractured rock site. The case study of San Gregorio (L'Aquila, Italy)

The village of San Gregorio (SG), eight kilometres away from L'Aquila (central Italy), was severely
damaged by the April 6, 2009 L'Aquila earthquake (MW 6.1). A coseismic fracture zone was mapped along
SW-dipping fault segments crossing SG, which is situated at the base of a carbonate relief bounded by the
Aterno river alluvial plain. An interdisciplinary approach was used to investigate the seismic response of
the area based on geological-structural, geophysical and seismic analyses. We integrated our data with
available information from the recent microzonation studies. SG is partly built on alluvial fan deposits
constituted by cemented gravel, and partly on jointed carbonate bedrock. An extensive survey of noise
measurements showed strong and polarized peaks in the horizontal-to-vertical spectral ratios (H/V),
both on soft and rock sites in the 2e10 Hz frequency band. Further, we checked the stability with time of
H/V ratios at three sites of SG. An analysis on local earthquakes confirmed the results of noise measurements.
To understand the influence of rock mass jointing condition on site effects, we performed
structural surveys on carbonate bedrock. We also evaluated the propagation velocities at rock sites using
seismic active and seismic dilatometer test (SDMT) surveys. Our analysis showed low values of
compressional (VP) and shear wave (VS) velocities of the outcropping rock, where we also observed
strong H/V spectral peak and high-density rock fracturig.