Local site effects estimation at Amatrice (Central Italy) through seismological methods

Abstract

We present the results of seismological and geophysical investigations performed by the "Istituto Nazionale di Geofisica e Vulcanologia" team operating in Amatrice village (Central Italy), in the emergency phases following the Mw 6.0 event of August 24th 2016, that caused severe damage in downtown and surrounding areas. Data from seven seismic stations equipped with both weak and strong motion sensors are analyzed in terms of standard spectral ratio to empirically define amplification function using a bedrock reference site. Ambient vibration spectral ratios between horizontal and vertical component of motion are also evaluated in a large number of sites, spread out in the investigated area, to recover the resonance frequency of the soft soil outcropping layers and to generalize the results obtained by earthquake data. Ambient noise vibration are also used for applying a 2D array approach based on surface waves techniques in order to define the near-surface velocity model and to verify its lateral variation. The results allows to better understand the amplification factors in the investigated area, showing spatial variation of site effects despite of the homogeneous shallow geological condition indicated by the microzonation studies available at moment of the described field campaign. The analysis reveals a diffuse amplification effect which reaches its maximum values in downtown area with a resonant frequency of about 2 Hz. The obtained results were used to integrate the microzonation studies and they can be used for urban planning and reconstruction activities.

Guest editorial: seismic microzonation of Central Italy following the 2016–2017 seismic sequence

“…In particular, the authors found that a significant ground-motion amplification may have characterized the area of the old Pescara del Tronto settlement. Milana et al (2019) illustrate the results of seismological and geophysical investigations performed in Amatrice village, in the emergency phases following the Mw 6.0 event of August 24th 2016. Data from weak-and strong-motions and from ambient vibrations have been used to better understand the distribution of amplification factors in the investigated area.…”

Seismological analyses of the seismic microzonation of 138 municipalities damaged by the 2016–2017 seismic sequence in Central Italy

“…The 3A network (http://doi.org/10.13127 /SD/ku7Xm 12Yy9 ) was deployed by INGV, ENEA, and CNR, and operated from September 19, 2016 through half of November, 2016 Milana et al 2019). It was composed of 55 stations ("Appendix" and Map of the Municipalities (thin grey lines) involved in the MS3 studies, divided by territorial grouping (six coloured areas).…”

Near-Source Simulation of Strong Ground Motion in Amatrice Downtown Including Site Effects

“…The village was built on an alluvial terrace 60-80 m higher than the surrounding valley (Amatrice basin) and elongated in NW-SE direction with a length of about 2000 m and a maximum width of about 600 m [25] (Figure 1). It is bounded both at NE and SW by two river valleys and bordered by a steep slope to the north and to the west, and by a gentler slope to the south.…”

“…The Amatrice basin is filled by about 60-m-thick Quaternary fluvial deposits, laying above the Messinian siliciclastic deposits of the Laga Formation, which play the role of seismic bedrock for the area [27,28]. They are made by conglomerates and sands alternations, few tens of meters thick, which can generate a seismic impedance contrast that, together with the topographic effects, is able to generate a strong amplification effect [25,29,30].…”

“…The analysis of earthquakes and ambient noise, recorded on the Amatrice terrace and around it for microzonation purposes, has revealed spatial variation of site effects [25,[31][32][33]: the diffuse amplification of ground motion reaches its maximum values in the downtown area, at the western limit of the terrace, with a resonant frequency of about 2.0-2.5 Hz. Here, the amplification is larger in the northern sector and varies from the central part to the edge, where [25] observed a clear directionality of the resonant peak (on the contrary, [34] concluded that maximum directions are linked to the presence of noise sources). These observations suggest a lateral variability of the geological conditions, due to the spatial variation of the impedance contrast, and the presence of topographic effects, due to the morphology of the area [25,29,30].…”