Site effects estimation and their effects on strong ground motion at
Amatrice village (Central Italy)
Abstract
We present a summary of seismological and geophysical investigations at
Amatrice (Central Italy), a village seated on an alluvial terrace and
severely stroke by the Mw 6.0 event of August 24th 2016. The high
vulnerability alone could not explain the heavy damage (X-XI MCS),
whereas the vicinity of the seismic source and the peculiar site effects
should be claimed to understand the ground motion variability. After the
first mainshock, we investigated the Amatrice terrace for microzonation
purposes together with several Italian institutions (Priolo et al.,
Bull. Earthquake Eng. 2019). In particular: (i) we installed 7 seismic
stations as a part of the 3A network (DOI: 10.13127/SD/ku7Xm12Yy9; Cara
et al., Sci. Data 2019); we performed (ii) an extensive campaign of 60
single-station ambient noise measurements (downtown stations recorded
also few earthquakes), and (iii) several 2D passive seismic arrays aimed
at obtaining Vs profiles down to a depth of few tens of meters (Milana
et al., Bull. Earthquake Eng. 2019). Earthquake recordings were used to
empirically evaluate ground motion amplification effects through
spectral ratio approaches, and noise data were collected for defining
the spatial distribution of the resonance frequencies. Data analysis
reveals a diffuse amplification effect that reaches its maximum values
in downtown area with a resonant frequency (f0) of about 2 Hz. Seismic
amplification is also characterized by spatial variation and directional
amplification, mainly in downtown to the west side of the alluvial
terrace, and related to both stratigraphic and topographic effects. This
effect tends to decrease and almost vanishes in the central part of the
terrace, and it increases again moving towards its eastern edge with a
clear shift of f0 towards higher frequencies. Empirical transfer
functions were then used to recover the ground motion that could have
hit the historical center of Amatrice during the August 24th mainshock,
through the convolution with the only record in the vicinity (IT.AMT
station experienced a PGA of 0.87 g). The reconstructed peak values are
much greater than expected from ground motion models, showing that
detailed studies on local site response can largely modify the seismic
hazard assessment.