Directional Amplification at Rock Sites in Fault Damage Zones

Abstract

Featured Application: In the seismic design codes of many countries, site amplification effects are accounted for through the use of scaling factors due to the presence of superficial soft soil deposits. In this framework, rock sites are assumed to show no local amplification. However, even rock sites can show site amplification, and the presence of large-scale open cracks or microcracks can affect large areas, such as across fault zones and on landslides. The existence of unexpected site-effects at rock sites can have significant implications in seismic hazard assessment. The current-knowledge is limited to relative amplification between horizontal and vertical components, and further estimates are needed in order to evaluate the absolute amplitude and to understand to what extent this effect could be important for seismic hazard and engineering applications. Site effects refer to the modification of ground shaking caused by the local geological conditions that can result in the strong amplification of ground motion. The best-known cause for site effects is the presence of superficial soft soil deposits, which are considered in seismic design codes of many countries through the use of scaling factors. Rock sites are assumed to show no local site amplification. However, even at rock sites, seismic waves can be locally amplified at frequencies of engineering interest, with larger motion along one site-specific azimuth on the horizontal plane (the so called “directional site resonance or amplification”). These effects have been related to the presence of large-scale open cracks or microcracks in different geological environments (faults, landslides, volcanic areas) everywhere with a common signature: maximum amplification occurs transverse to the predominant fracture strike. In this paper, we summarize our main results obtained in the last decade with regard to several fault zones with different kinematics, where ground motion is polarized (and amplified) perpendicularly to the predominant fracture field as an effect of the stiffness anisotropy. In order to give a further constraint, we also show some cases where the directional amplification effects were compared with the S-wave splitting analysis method.