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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