The catastrophic 1883 earthquake at the island of Ischia (southern Italy): macroseismic data and the role of geological conditions

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.

The 21 August 2017 Ischia (Italy) Earthquake Source Model Inferred From Seismological, GPS, and DInSAR Measurements

“…The 21 August 2017 IE represents the largest seismic event affecting the Island ever observed with modern techniques. The level of damage and its distribution (EMERGEO Working Group, Nappi et al, 2017; Gruppo di Lavoro INGV sul terremoto dell'isola di Ischia, 2017), with respect to the magnitude, agree with the historical observations of low magnitude-high intensity occurrence and strong attenuation (Carlino et al, 2010). The most relevant damage, observed just in a narrow area of Casamicciola Terme, is related to the shallowness of the source.…”

“…In this case the magnitude (which ranges from 4.2 to 5.2) was inferred from the empirical relationships, for volcanic areas, that correlate this parameter to the maximum intensity, the extension of the source, and the involved area (Cubellis & Luongo, 1998;Okada, 1983;Shebalin, 1972). Macroseismic data also show that earthquakes are characterized by very shallow hypocenter depths (1-2 km), as demonstrated by high-intensity values recorded in the epicentral area and strong attenuation (Carlino et al, 2010). Following the 1883 earthquake, an aseismic period was observed in the Island, but low-magnitude earthquakes have been recorded in the last 40 years in the northern part of the Island ( Figure S2).…”

The structural setting of the Ischia Island (Phlegrean Volcanic District, Southern Italy): Inferences from geophysics and geochemistry

“…Except for the latter, occurred in the eastern sector of the island, the seismicity has been localized along the northern sector of the Mt. Epomeo resurgent block (Carlino et al, 2010 and references therein). The Casamicciola earthquake of 1883 AD, the documented most destructive earthquake occurred on the island, caused major damages on man-made structures and triggered several landslides in the north-western slopes of Mt.…”

“…This effort is mandatory for an active volcanic area, as Ischia is, characterized by a high volcanic hazard due to the explosiveness of its activity occurred in historical times (e.g., Lirer et al, 2010). Furthermore, the island has been hit by strong earthquakes and landslide events occurred in the past centuries (e.g., de Vita et al, 2006;Carlino et al, 2010). All this, and the high exposed value, increasing especially during summertime when the island is reached by thousands of tourists that add to the permanent local population of ca.…”

Coseismic displacements on Ischia island, real-time GPS positioning constraints on earthquake source location

“…The seismic activity is rather low and generally with low magnitude, however the historical record shows that Ischia island was struck by several earthquakes, a few of them causing severe damages and fatalities. The most outstanding in 1883 in which more than 2,000 people were killed and causing the total destruction of the village of Casamicciola [Carlino et al, 2010]. On August 21th, 2017, an earthquake (Md=4; Mw 3.9) occurred,damaging a limited portion of the island in the Casamicciola area, reaching a maximum intensity of 8 on the European Macroseismic Scale (EMS) and causing two fatalities [Azzaro et al, 2017].…”