Ground motion amplification at rock sites: the competing role of topography and fractured rocks in the San Giovanni fault, central Italy.

Abstract

Amplification at rock sites in areas of high topographic relief has been increasingly observed in the last years, with unexpected level of damage after strong earthquakes. In regions affected by recent tectonic activity, topographic irregularities can include fault damage zones. In such conditions, seismic waves can be locally amplified as a double effect of wave focusing along the topography and /or the presence of fractures/joints or locally weakened rocks. The role of topography vs. geological complexities in controlling the ground motion amplification at rock sites is a newly debated issue in the seismological community. The most crucial questions regard what is the real contribution of the topography shape and fracturing, and how to parameterize such effects for their inclusion in the seismic design codes. In this framework, the EMERSITO INGV task force installed 7 seismic stations across the San Giovanni fault, after the Amatrice mainshock of the 2016 sequence in Central Italy. This active normal fault is located in the area of the Montereale intermountain basin (Abruzzi region, Italy) and bounds the southwestern slope of Mt. Mozzano, a roughly 2D-shaped, up to 1450 m high pronounced topography. Moreover, this fault has been recently studied by several authors who performed detailed geological and geophysical surveys. Our stations recorded more than 100 earthquakes with magnitude ranging from 2.5 to 3.9 as well as a 4.4 M earthquake with hypocenter in Capitignano district, few kilometres far. We have analyzed in detail the recorded signals calculating the traditional spectral ratios at single station (HVSRs) and using the reference site (SSRs) using both ambient noise and earthquakes. In order to obtain a robust estimate of the site amplification effect at each station, we have investigated the influence of backazimuth and epicentral distance. We have also applied the time-domain covariance matrix analysis and the frequency domain polarization analysis. We have found that, in spite of the complexity of the seismic data, the observed polarization pattern is generally oriented orthogonal to the ridge elongation, as well as to the fault strike, suggesting the existence of a high angle relation between ground motion polarization and fracture systems