The catastrophic 1883 earthquake at the island of Ischia (southern Italy): macroseismic data and the role of geological conditions |
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Authors: | S Carlino E Cubellis A Marturano |
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Institution: | (1) Istituto Nazionale di Geofisica e Vulcanologia, INGV Sezione di Napoli Osservatorio Vesuviano, Via Diocleziano 328, 80124 Naples, Italy |
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Abstract: | This article presents the results of a detailed study of the effects of the 1883 earthquake, which occurred at the island
of Ischia (Gulf of Naples) and produced the total destruction of buildings in the epicentral area (Casamicciola town). Despite
the moderate magnitude, this event was characterised by very high intensities (I
max = XI degree MCS) mainly due to the shallow depth of the source. The study of the earthquake shows that the intensities, which
decreased rapidly with distance, were affected by source directivity, according to the causative fault geometry and tectonic
structures, while local amplification of damage was observed where soft soils outcrop. The attenuation of seismic intensity
with distance was evaluated using the well-known relation of intensity versus epicentral distance (Blake’s method). The diverse
gradients of attenuation, observed in different directions, were ascribed to the various geological features of the shallow
crust of the island. In order to evaluate the role of geology in the damage level, we computed different attenuation models
for stiff and soft soils outcropping on the island. A systematic local amplification of about 1 MCS degree associated to the
presence of reworked tuffs was obtained. This study also shows the influence of geological conditions on the evaluation of
macroseismic data and supplies useful elements to derive a predictive map of potential site effects. |
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