Management of Historical Seismic Data Using GIS: The Island of Ischia (Southern Italy)

This paper presents the results of the use of Geographic Information Systems (GIS) inmanaging information on the effects of earthquakes in historical times on the island ofIschia. The unpublished sources on the Casamicciola earthquake of 28 July 1883 andthe extensive bibliography documenting the island's seismicity from 1228 showed theneed to proceed towards a type of data storage that would also allow management ofthe same data. Application of GIS techniques allowed us to insert, extract, handle,manage and analyse the data for the zoning of seismic damage on the island of Ischia.The end-product consists of information layers, such as maps of isoseismals, the damage, and hazard involved, as well as numerical tables associated to maps.The study was developed using GIS Arc-View 3.2 software (ESRI) and is structured inthematic vectorial levels and rasters. The overlapping themes constitute a cartographicdata base of the island. The damaged sites are located on a map at a scale of 1:10,000,with all the information on the 1883 earthquake (total number of houses, number ofcollapsed houses, collapsed or damaged rooms, photographs, plans of buildings, etc.)being associated to each site. The GIS is structured in such a way as to be able to beintegrated with further georeferenced data and with other databases. It is thus able toprovide support both for in-depth analyses of the dynamic processes on the island andextend the assessment to other natural risks (volcanic, landslides, flooding, etc.).     

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

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.     

Power Laws Governing Historical Earthquakes in the Apennine Chain (Southern Italy)

Rank-ordering analysis is applied to the intertimes between seismic events recorded in the Apennine belt between 40–42° N and 14–16° E from the 15th century onwards. It shows a power law capable of governing the intertimes between 1529 and 368 months and another power law which approximates a random simulation, for the intertimes shorter than 368 months. Only the first power law allows the computation of the return period of major events. Earthquakes with the same energy that are aligned according to different power laws imply the presence of two different populations, indicating, in turn, that the physics of seismic phenomena in the region examined is not straightforward, that the stress is probably not unidirectional and that it acts on a non-isotropic medium. The most probable estimated intertime value for the next event is found to be equal to 60 ± 20 years.     

The roots of Mt. Vesuvius deduced from gravity anomalies

Summary The structures of the Somma-Vesuvius volcanic complex are modelled on the basis of the interpretation of gravity anomalies obtained from data available in the literature and acquired along a new profile along the coastline from Naples to Castellammare di Stabia. In order to highlight the contribution of shallow crustal structures, the residual anomalies were considered. A marked gravity low was recognised in the eastern sector of Vesuvius. Furthermore data interpretation was carried out along two profiles centred on the low gravity region in question: a first profile crossing the Vesuvius crater in direction WNW-ESE, and a second one in NNE-SSW direction. The 2 ? D model obtained reveals a crustal structure characterised by sediments of 2.3 Mg/m³ density, overlying bedrock with a density of 2.6 Mg/m³. Near the volcanic system the model becomes more complex due to the presence of light sediments with a density of 2.1 Mg/m³ overlying a body with a density of 2.4 Mg/m³ which extends into depth. The latter is thought to be closely related to the zone of magma ascent developed along the volcanic axis. Along the coast the volcanic component is reduced and the model shows that the layer with a density of 2.3 Mg/m³ ranges in thickness from 0 to about 3500 m. An additional body between 1500 and 3000 m with a density of 2.4 Mg/m³ was considered in order to account for the slight rise in the residual anomaly in the area in the vicinity of Mt. Vesuvius. The analysis of the gravity anomaly pattern coincides with the complex system of faults and fractures intersecting the carbonate basement and the volcanic area in question, which developed as a consequence of extensional processes at the continental edge of the Italian Peninsula due to the opening of the Tyrrhenian basin. This extensional tectonics has created favourable conditions for the collapse of the south-western slope of Mt. Vesuvius and the development of eruptive vents and cracks on its flanks. Received May 18, 2000; revised version accepted March 6, 2001     

Contribution to knowledge regarding the sources of earthquakes on the island of Ischia (Southern Italy)

Natural Hazards - This paper analyses data regarding the seismicity and ground deformations of the island of Ischia. The goal is to describe these phenomena as a space–time process, exploring...