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  • 1
    ISSN: 1573-157X
    Keywords: Nocera Umbra ; site effects ; weak motions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract During the ML = 5.6 and 5.8 earthquakes occurredin central Italy on 26 September 1997 the historiccentre of Nocera Umbra, lying on top of a 120 m highhill, was diffusely damaged (VII-VIII degrees of MCSintensity). Some recently built houses in the modernpart of the town suffered an even higher level ofdamage. A temporary seismic array was deployed toinvestigate a possible correlation between localamplifications of ground motion in this area and theobserved pattern of damage. After a geologic andmacroseismic survey, eight sites were selected asrepresentative of different local conditions, such astopographic irregularities, sharp hard-to-softlithology transitions, alluvium-filled valleys, andboth undisturbed and deformed rocks.Horizontal-to-vertical spectral ratios for bothmicrotremor and earthquake recordings, as well asspectral ratios referred to undisturbed rock sites,were used to quantify local variations of groundmotion. In spite of the diffuse damage in the historiccentre of Nocera Umbra, a small amplification isobserved at the stations on the hill's top. Thissuggests that the higher vulnerability of the ancientbuildings mainly accounts for the diffuse damage inthat part of the town. In the frequency band ofengineering interest (1 to 10 Hz) the largestamplifications of ground motion are found at softsites: in the Topino river valley, where many episodesof severe structural damage occurred, spectralamplification is significant over a broad frequencyband ranging from 2 Hz to more than 20 Hz. Inparticular, in the central part of the valley highamplification (〉 4) is found from 3 to 10 Hz,reaching a maximum of 20 around 4 Hz. At the edge ofthe valley, close to the soil-to-rock transition,amplification is as large as 10 in a frequency bandranging from 4 to more than 20 Hz. A significantamplification (by a factor of 10 around 10 Hz) isobserved also at one of the rock sites, possibly dueto the presence of a cataclastic zone related to theactivity of a regional fault that altered themechanical properties of the rock.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-157X
    Keywords: fault zone ; ground motion ; Nocera Umbra ; site effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract During the two mainshocks of September 26, 1997 inthe Umbria-Marche border a strong-motion accelerographrecorded peak ground accelerations as large as 0.6 g,approximately, in the town of Nocera Umbra, atdistances of 10 to 15 km from the epicentres. Thisvalue is significantly larger than expected on thebasis of the usual regressions with magnitude anddistance. A broad-band amplification up to a factor of10 was consistently estimated in previous papers,using both weak and strong motion data recorded at theaccelerograph site during local moderate earthquakes.To study the cause of this amplification we deployedsix seismologic stations across the tectonic contactbetween the Ceno-Mesozoic limestone and the Mesozoicmarly sandstone where the accelerograph is installed.Seismograms of 21 shallow aftershocks in the magnituderange from 2.2 to 4.0 and a subcrustal Mw = 5.3event are analysed. Regardless of epicentre location,waveforms show a large complexity in an approximately200 m wide band adjacent to the tectonic contact. Thisis interpreted as the effect of trapped waves in thehighly fractured, lower velocity materials within thefault zone.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2017-04-04
    Description: During the ML = 5.6 and 5.8 earthquakes occurred in central Italy on 26 September 1997 the historic centre of Nocera Umbra, lying on top of a 120 m high hill, was diffusely damaged (VII-VIII degrees of MCS intensity). Some recently built houses in the modern part of the town suffered an even higher level of damage. A temporary seismic array was deployed to investigate a possible correlation between local amplifications of ground motion in this area and the observed pattern of damage. After a geologic and macroseismic survey, eight sites were selected as representative of different local conditions, such as topographic irregularities, sharp hard-to-soft lithology transitions, alluvium-filled valleys, and both undisturbed and deformed rocks. Horizontal-to-vertical spectral ratios for both microtremor and earthquake recordings, as well as spectral ratios referred to undisturbed rock sites, were used to quantify local variations of ground motion. In spite of the diffuse damage in the historic centre of Nocera Umbra, a small amplification is observed at the stations on the hill’s top. This suggests that the higher vulnerability of the ancient buildings mainly accounts for the diffuse damage in that part of the town. In the frequency band of engineering interest (1 to 10 Hz) the largest amplifications of ground motion are found at soft sites: in the Topino river valley, where many episodes of severe structural damage occurred, spectral amplification is significant over a broad frequency band ranging from 2 Hz to more than 20 Hz. In particular, in the central part of the valley high amplification (〉 4) is found from 3 to 10 Hz, reaching a maximum of 20 around 4 Hz. At the edge of the valley, close to the soil-to-rock transition, amplification is as large as 10 in a frequency band ranging from 4 to more than 20 Hz. A significant amplification (by a factor of 10 around 10 Hz) is observed also at one of the rock sites, possibly due to the presence of a cataclastic zone related to the activity of a regional fault that altered the mechanical properties of the rock.
    Description: Published
    Description: 555-565
    Description: JCR Journal
    Description: reserved
    Keywords: site effects ; Nocera Umbra ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 4
    Publication Date: 2017-04-04
    Description: Moderate-magnitude shallow earthquakes in the Atlantic Ocean, hundreds of kilometres southwest of Lisbon, can generate efficient suboceanic Rayleigh waves (SRW) that are well recorded in Portugal. Here we compare moderate-size earthquakes recorded by seismic stations in Portugal with the Tyrrhenian Sea earthquakes recorded in peninsular Italy where SRW were recently observed. In spite of a different behaviour of high frequencies due to the different tectonic setting of the two areas, similar results are found in the intermediate-period range, suggesting that this effect, if extrapolated to a magnitude larger than 8, could be devastating at regional distance in terms of ground motion amplitude and duration. Through 1D models, we explore the hypothesis that the high level of destruction and the long duration of shaking felt during the Great 1755 Lisbon earthquake were caused by SRW. In this preliminary study, we check the role of critical model parameters. We find that duration and amplitude are largest when the average thickness of the water layer is 2 km and shear-wave velocity of the ocean floor is close to the speed of sound in the water. Both conditions are realistic for a source in the Atlantic Ocean, few hundreds of kilometres southwest of Lisbon. Moreover, the propagation of SRW at regional distances accounts for durations of more than ten minutes as the effect of a single large earthquake.
    Description: Published
    Description: 283-295
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: open
    Keywords: ground motion ; surface waves ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: book chapter
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