The role of site effects on elevated seismic demands and corollary structural damage during the October 30, 2020, M7.0 Samos Island (Aegean Sea) Earthquake

Abstract

On October 30, 2020 14:51 (UTC), a moment magnitude (M) 7.0 (USGS, EMSC) earthquake occurred in the Aegean Sea. This paper presents the reconnaissance findings regarding the site effects on recorded strong ground motion intensities and duration, along with the resulting induced-structural damage in Izmir Bay and Samos Island, respectively. In all rock records, relatively high intensity long period rock spectral accelerations were observed in the mid to long period range of 0.5-1.5 s, which are attributed to the source, more specifically, to the slower rupture-mechanism of the event. These rich spectral intensities were further amplified by soil site effects and soil-superstructure resonance, leading to two to six times amplified overall responses and prolonged seismic shaking durations, more pronounced in Bayrakli and other Izmir Bay sites in Turkey. However, these amplified and prolonged excitations are still below design basis earthquake levels, which addresses the lack of proper structural design and construction deficiencies, as the underlying causes for the collapse to heavy damage performance of 795 buildings. On the other hand, although located only about 10 km from the rupture (22 km from the epicenter) and within the near fault zone, the town of Vathy on Samos Island (Greece) was rather lightly affected by the earthquake, with relatively few collapsed or heavily damaged buildings, partially attributed to the low height/low weight of structures in the area. However, a concentration of damage in low-rise buildings in Ano Vathy hill is considered indicative of a combination of coupled valley and topography effects on the strong motion. This event once again addressed the need to develop region-specific zonation and provisions, when more general code practices are proven to be inadequate to assess these extreme site effects.

Geotechnical reconnaissance findings of the October 30 2020, Mw7.0 Samos Island (Aegean Sea) earthquake

“…Discussions presented in this paper will focus on the documentation and the preliminary assessment of (i) performance of building foundations, (ii) seismic soil liquefaction and induced ground failures, (iii) performance of slopes and deep excavations, and (iv) performance of retaining structures and quay walls on both Samos and the Anatolian side for each case. As will be notable throughout the paper, with the exception of the Bayrakli region where significant and detrimental site effects (as discussed by Cetin et al 2021) were observed, no major geotechnical effects were observed on the Anatolian side in the form of foundation failures, surface manifestation of liquefaction and lateral soil spreading, rock falls/landslides, failures of deep excavations, retaining structures, quay walls and subway tunnels.…”

“…Nevertheless, the associated phenomena were not spectacular, which indicates that liquefaction was manifested but not in a pervasive manner. This is surprising given the intensity (PGAs > 0.2 g) and duration (≈ 9 s in records at Vathy; see Cetin et al 2021) of ground motion in combination with the nearshore alluvial deposits. The precise dimensions of the liquefied zones is hard to establish without detailed site investigations.…”

“…21), which caused a temporary closure of the road. The main geological formations in the above area are marbles with intercalations of schist (Cetin et al 2021). However, the road access was quickly recovered after a few days by temporary restraining measures with large concrete blocks placed at the slope toe (Lekkas et al 2020).…”

“…Investigations of various aspects of the earthquake are available in literature (e.g. Akinci et al 2021, Akkar et al 2021, Binici et al 2022, Cetin et al 20212022, Demir and Altiok 2021, Evelpidou et al 2021, Ganas et al 2021, Kalligeris et al 2021, Lentas et al 2022, Mavroulis et al 2022, Nuhoglu et al 2021, Onat et al 2022, Plicka et al 2021, Toprak et al 2022, Yakut et al 2021.…”

Use of a-priori information to minimize uncertainties in surface wave inversion and subsequent site response analysis

“…To carry out SRA, a highly critical input parameter is the shear wave velocity (Vs) profile of the site. The Vs profile of a site is also required in studies related to geophysical characterization, site effects and many others (Ciancimino et al 2020;Cetin et al 2021;Pasten et al 2021). Presently, surface wave methods have become the most common techniques to get the Vs profile of the soil, particularly the MASW method (Park et al 1999;Xia et al 1999).…”