The Island of Ischia (Southern Italy)
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Natural Hazards 33: 379–393, 2004. 379 © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Management of Historical Seismic Data Using GIS: The Island of Ischia (Southern Italy) E. CUBELLIS1, S. CARLINO1,2, R. IANNUZZI2, G. LUONGO2 and F. OBRIZZO1 1Osservatorio Vesuviano – INGV, Via Diocleziano 328, 80124 Naples, Italy; 2Dipartimento Geofisica e Vulcanologia, Universitá Federico II, Largo S. Marcellino 10, 80136 Naples, Italy (Received: 25 February 2003; accepted: 15 December 2003) Abstract. This paper presents the results of the use of Geographic Information Systems (GIS) in managing information on the effects of earthquakes in historical times on the island of Ischia. The unpublished sources on the Casamicciola earthquake of 28 July 1883 and the extensive bibliography documenting the island’s seismicity from 1228 showed the need to proceed towards a type of data storage that would also allow management of the 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 in thematic vectorial levels and rasters. The overlapping themes constitute a cartographic data 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 of collapsed 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 be integrated with further georeferenced data and with other databases. It is thus able to provide support both for in-depth analyses of the dynamic processes on the island and extend the assessment to other natural risks (volcanic, landslides, flooding, etc.). Key words: Historical seismicity, GIS, seismic hazard, island of Ischia. 1. Introduction The capacity of Geographic Information Systems (GIS) to store and process data and images makes the use of such systems very valuable in the field of seismic zoning, because the latter is chiefly based on the processing of data regarding the damage produced by historical and present-day earthquakes as well as its distri- bution in space. We chose to construct a GIS on the basis of Ischia’s historical seismicity which, in terms of quantity and quality of the known data, is a good test for the creation of an integrated information system which permits a compar- ison of the data from different sources. The information on the island’s seismicity covers almost 800 years, starting from 1228, by means of historical maps, images, Author for correspondence. E-mail: [email protected] 380 E. CUBELLIS ET AL. Table I. Phases of volcanic activity on the island of Ischia on the basis of geological and stratigraphic data (according to Vezzoli, 1988) Older Cycle Pre-Mt. Epomeo Green Phase 1 >150,000 y. B.P. Older great pyroclastic activity Tuff Activity Phase 2 150,000 – 75,000 y. B.P. Lava dome emplacement Younger Cycle Mt. Epomeo Green Tuff Phase 3 55,000 – 33,000 y. B.P. Younger great pyroclastic activity Citara Formation Phase 4 28,500 – 18,000 y. B.P. Explosive and effusive activity in the south-western and south-eastern sectors Phase 5 10,000 y. B.P. – 1302 A.D. Prehistoric and historical activity elaborations and models. A cross-analysis of this data allows us to assess the dam- age, define its geographical distribution, identify the areas repeatedly affected by seismic events, and quantify the level of seismicity in the area. The island of Ischia is situated in the Bay of Naples and has an extension of 46 km2. It consists entirely of volcanic rocks deriving from eruptive centres largely destroyed or covered by subsequent activity and only partly recognisable. The oldest outcrops date back to about 150,000 years B.P., while the most recent eruption occurred in 1301–1302 in the eastern sector of the island (Vezzoli, 1988; Civetta et al., 1991) (Table I). The most important structural element is Mt. Epomeo (787 m a.s.l.) in the central sector of the island, interpreted as a resurgent structure (Vezzoli, 1988; Fusi et al., 1990; Orsi et al., 1991; Luongo et al., 1995; Cubellis and Luongo, 1998; Acocella and Funiciello, 1999; Tibaldi and Vezzoli, 1998) whose edges are marked by a system of significant faults (Figure 1). These faults delimit the resurgent structure of Mt. Epomeo starting from about 30,000 years ago. It is believed that the magma that fed the volcanic activity in the island’s eastern sector over the last 10,000 years migrated through such discontinuities, and re- leased seismic energy in the northern sector (Table II). Seismic activity on the island shows the peculiarities of volcanic areas, such as low energy of earthquakes and high intensity which decreases quickly with the distance from the epicentral area. Therefore a reliable risk assessment needs the definition of the seismogenetic zone with the greatest accuracy by joint analysis of historical seismicity, geology, volcanic activity and the geodynamic processes which have affected the island. Analysis of the effects of earthquakes and the geological structure enables the stress fields generating the earthquake to be traced. In volcanic areas the interpretation of the stress field generating the seismicity is made even more complex because in addition to the regional stress field, there is that generated by the migration of magmatic masses towards the surface, as well as the rheological properties of the rocks conditioned by the high geothermal gradient. The structural and dynamic conditions of Ischia and its high housing density and MANAGEMENT OF HISTORICAL SEISMIC DATA USING GIS 381 Figure 1. Principal deformation structures in the island of Ischia. The resurgent area is confined by a NE–SW and NW–SE fault system (according to Fusi et al., 1990). high economic value exposed in the summertime make the island of considerable interest for mitigating the seismic risk. In this paper we present a test on the use of a GIS for processing, managing and representing the area data relating to the 1883 Casamicciola earthquake. 2. The Earthquake of 28 July 1883 The Casamicciola earthquake of 28 July 1883 (Imax = XI degree MCS), was the first great catastrophe after Italy’s unification in 1860. After this earthquake the first seismic safety act in Italy was approved with the indication of lower risk zones for new settlements (Figure 2). The decision to approve the law was also taken in light of the many earthquakes that struck the island (Table II). As early as 1228, historical sources record a severe earthquake with an intensity of IX–X degree MCS, which caused approximately 700 deaths. In the last eight centuries mention is made of the many events with epicentral locations in the northern part of the island and intensity higher than VII degree MCS (Figure 3). The July 1883 earthquake represents the strongest earthquake that occurred at Ischia in historical times (Cubellis, 1985; Luongo et al., 1987; Cubellis and Luongo, 1998). The ef- 382 E. CUBELLIS ET AL. Table II. Historical seismicity: major earthquakes since 1228 Year Locality Maximum intensity Damage and victims (MCS Scale) 1228 Casamicciola IX–X 700 deaths, large landslide from Mt. Epomeo 1302 Eastern area of the island VIII Many buildings collapse 1557 Campagnano VII–VIII Collapse of the Parish Church 1762 Casamicciola VII Damage to houses in Casamicciola 1767 Eastern area of the island VII–VIII Collapse of Rotaro’s Church 1796 Casamicciola VIII 7 deaths, serious damage in the upper part of Casamicciola 1828 Casamicciola VIII–IX 28 deaths, 50 injured, serious damage and collapses in the upper part of Casamicciola 1841 Casamicciola VII Cracks in the buildings 1863 Casamicciola VII Collapse of dry walls, small landslides from Mt. Epomeo 1867 Casamicciola VI–VII Buildings damaged at Casamicciola 1881 Casamicciola IX 129 deaths, many injured, many collapsed buildings at Casamicciola and Lacco Ameno 1883 Casamicciola XI 2333 deaths, 762 injured, many collapsed buildings at Casamicciola, Lacco Ameno and Forio fects were disastrous: the town of Casamicciola was almost razed to the ground; severe damage affected Lacco Ameno and Forio, relatively less intense at Serrara Fontana and Barano. The lowest effects were observed at Ischia town. The death toll, according to different sources, was between 2313 and 2333, of whom at least 600 were guests on the island, while 726 were injured. After the earthquake there was a period of seismic quiescence except for some isolated events felt at the beginning of the last century and the very occasional micro-earthquakes recorded in the last 20 years in the northern part of the island (Parascandola, 1937; Luongo et al., 1987; De Natale et al., 1998). We cannot rule out a renewal of seismic activity in the area connected both to tectonic and volcanic activity. This once again raises the problem of mitigation of seismic risk. It is therefore necessary to conduct a more in-depth study of the areas that have repeatedly experienced seismic events, together with an analysis of the island’s geological structure, so as to define the seismogenetic sources and mechanisms that govern seismic energy release processes and identify the areas MANAGEMENT OF HISTORICAL SEISMIC DATA USING GIS 383 Figure 2. Historical map of the hazardous zones drawn up after the earthquake (Building Regulation 29 August 1884). It shows the areas where the greatest damage is expected in the event of a future earthquake in Casamicciola, Lacco Ameno and Forio.