Paleoseismological off- fault analyses in eastern Sicily: a contribute to the characterization of seismic sources

Abstract

In this thesis a paleoseismological off-fault research, consisting in the analysis and dating of seismogeological effects triggered by both historical and paleo- earthquakes (seismites), was performed. Off-fault paleoseismology results particularly useful in areas, like Sicily, where the seismogenic sources are scantly defined and so they can't be directly investigated by onfault researches. Indeed, even if this study does not provide precise and direct information on the seismogenic fault and the earthquake parameters (magnitude, intensity, fault length and elapsed time), however, it can supply useful information on the epicentral distance of the site where the effects developed, the earthquake magnitude threshold and the intensity reached at the site. Moreover, the finding of structures dated before the historical records can be useful to extend the seismic catalogues back in time. Sicily was affected by strong earthquakes among the most disastrous of the seismic Italian history, with intensity Io up to XI (MCS) and equivalent moment magnitude Mw up to 7 (CPTI04, Working Group 2004). The northeastern sector was destroyed by the 1908 Messina Strait earthquake and also suffered for seismic events located in southern Calabria, such as the 1783 seismic sequence. The southeastern sector was hit by the 1169 and the January 11th 1693 earthquakes and by other minor, however damaging, events such as the 1542 one. Western Sicily suffered a destructive seismic sequence started on January 13th 1968. Nevertheless, the seismogenic sources of these earthquakes are not well constrained because they occurred in pre-instrumental time and without clear evidence of surface faulting. Recently, different seismogenic source models have been proposed on the basis of geological and geomorphological evidences, historical and instrumental seismicity data and macroseismic intensity analyses, but the debate is still opened. However, these strong earthquakes triggered several geological effects described by the historical accounts, such as landslides, liquefactions, ground deformations and fracturing, hydrological anomalies and tsunamis characterized by waves that damaged the cities along the eastern coast of Sicily. Then, given its critic seismicity and its millenary historical memory, Sicily is an optimal laboratory to test different paleoseimological off fault methods. This work was undertaken with the main aim on a side of providing new and useful data to better define the eastern Sicily seismicity, on the other side to test original multidisciplinary approaches in a region where this kind of investigations are scarce. Indeed, off-fault paleoseismology is a young and yet few tested discipline of Earthquake Geology. For this reason there is not a unique technique of investigation, instead methods need severe testing and systematization and every study case requires a specific approach related to the site conditions and to the seismite typology. The performed research follows two different main lines: first the examination of the regional seismicity and of the historical accounts has been performed; then a multi-theme research was carried out in field to investigate directly the seismites. The Italian historical bibliography reporting seismogeological effects in Sicily (original sources and previous seismic catalogues) has been analyzed. Descriptions of effects such as landslides, ground deformations, liquefactions, hydrological anomalies have been collected in a georeferenziated database embodying all the information about the causative event, as well. Interactive maps of effects distribution have been realized by the use of Gis software. These data have been also used to define empirical relationships between earthquake parameters (intensity and magnitude) and epicentral distance of the sites where the effects occurred. Then, upper bound-curves, at regional scale, have been realized. This step of the research highlighted that Sicily is a region highly prone to the seismogeological effect development, especially as it regards landslides and ground deformations, mainly clustered in the eastern flank of Mt. Etna and in the northern sector of the region characterized by critical geological and structural setting. Whereas, liquefactions and hydrological anomalies occurred more numerous in areas with specific geological and hydrological features (Belice Valley and Catania Plain). Upper bound-curve graphs also showed that seismic parameters of some events could be misinterpreted, such as the magnitude of the 1823 earthquake (M = 5.87) that could be underestimated, while the new value proposed in the literature (M = 6.7) seems to be more plausible. The same analysisindicated the happening of possible site amplifications and/or exceptional site response during some events as showed by effects occurred at unexpected long epicentral distance at Messina during the 1783 earthquake and at Calatafimi (Trapani) for the 1693 event. Off-fault paleoseimological field study was focused to the finding and examination of liquefaction- induced deformations and tsunami deposits, because their investigation in field results easier than other effects. Indeed, liquefaction structures and tsunamiites remain in the sedimentary sequence as marker of seismicity and tsunami inundations; they have well defined features and take place in areas with specific characteristics, easily recognizable after geological and geomorphological surveys. On the contrary, for instance, hydrological anomalies are transitory phenomena and seismic landslides are not well differentiable from no seismic ones. Hence, after a critical examination of the historical data indicating the localities where these effects occurred during past earthquakes, fluvial and coastal areas of eastern Sicily have been chosen. A further selection was performed using satellite imagines and aerial photos and by geological and geomorphological field surveys, aimed to define the most prone areas. In three sites (Minissale, Agnone and Vendicari) field study allowed to investigate deformational patterns linked to liquefaction mechanism. In other sites (Augusta, Pantano Morghella, Capo Campolato, Vendicari and San Lorenzo) probable tsunami deposits, both sand and boulder accumulations, were found. As it regards the examination of deformational pattern, the detailed investigation of their features and a paleo-environmental reconstruction have been performed to exclude other possible causative mechanisms different from the seismic one. In general, the method for distinguishing subsequent events is based on stratigraphic criteria and cross-cutting relationships. When possible radiocarbon dating has been carried out, on charcoals and bulks, to constrain the age of the structures and to associate them with historical or paleoearthquakes. At Minissale (eastern flank of Mt. Etna, central eastern Sicily) and Agnone (Catania Plain, central eastern Sicily) liquefaction structures have been detected on two artificial trench walls. In these sites a preliminary hand-auger coring campaign was also performedto characterize the stratigraphic sequence and to qualitatively evaluate the terrain liquefaction susceptibility. Then, the deformational patterns, consisting of lateral spreading, dikes, faults, drag folds, recumbent folds, sheet slumps, warped top levels and boudinage, have been studied by the square division method. Terrain samples have been collected for sedimentological and micro-paleontological investigations. Paleo-environmental reconstruction allowed to exclude other causative mechanisms and to associate these deformations to seismic shaking. Radiocarbon dating, combined with the upper bound curves, allowed to associate the seismites detected at Minissale site with the 1169 and 1693 earthquakes and those of the Agnone site with the 1542 and 1693 earthquakes. At Vendicari (southeastern Sicily) a singular association of structures, affecting terrains since Pliocene up to Quaternary age, has been detected. Besides soft sediment deformations (autoclastic breccias, diapyr-like injections and thyxotropic wedges), probably linked to liquefaction mechanisms, brittle deformations, consisting of fractures generally opened and filled by sediments (sedimentary dykes) have been found. Fractures have been examined by a mesostructural investigation and the detailed observation under the microscope of filling material thin sections, as well, to highlight possible relationships with the regional stress field. After a critical analysis of the forms and the paleo-environmental reconstruction, seismic shaking was proposed as the most probable cause of the deformation development. Fractures could be also linked to the regional tectonics characterized by an almost NW-SE trending' 1. The overall investigation of seismites at Vendicari highlighted at least four triggering seismic events, whose age is not precisely constrained given the lack of datable material. However, their finding mark that these events had magnitude greater than 5.0 and intensity greater than IX, that are the threshold values for which this pattern can trigger in the epicentral area. The tsunamiite study was undertaken both searching anomalous sandy deposits and examining boulder accumulations along the southeastern coast of Sicily, inland and offshore. This research required a multi-theme approach combining geological, geomorphological, paleontological, X-ray, petro-chemical, morphoscopic and magnetic examinations. Geophysical applications, analysing sonar chirp profiles, were a preciousinstrument to find deposits off-shore. In some cases wave transport equations were used jointly with statistical analysis in order to determine the extreme events' geological or meteorological' responsible for the deposition. At Augusta and Pantano Morghella (southeastern Sicily) anomalous sandy layers, whose analysis highlighted a tsunamigenic origin, were found into a fine sedimentary sequence. Augusta site probably recorded tree events inland but, given their old interval age, no correlation can be made with the historical record. Only the more recent level could be tentatively related to the 365 AD Crete tsunami. Eleven anomalous layers have been also found off-shore thanks to the geophysical investigation of sonar chirp profiles. The age of some of these layers well matches with some disastrous tsunamis that hit eastern Sicily in historical time (such as the 1169, the 1693 and the 1908 events) and with that coming from Eastern Mediterranean such as the 365 AD Crete tsunami and the event of Santorini (about 3600 BP). At Pantano Morghella three anomalous levels were found. Deep investigations on one of the layers highlighted that it can be ascribed to a tsunami and in particular to the 365 AD Crete one. Further investigations and dating are in progress to understand the tsunamigenic origin and to constrain the age of the further two levels. Boulder accumulations at Capo Campolato, Vendicari and San Lorenzo, were studied with the aim to distinguish if they were deposited by storm waves or tsunamis. This analysis showed that strong storms occurring in the Ionian Sea are capable to emplace large boulders on the coast but up to a given distance from the shoreline. Indeed, boulders very far from the shore seem to require more energetic waves, with periods longer than that of known storms, to be deposited. These waves could be extraordinary unknown storm waves or tsunamis. Dating on some very far boulders highlighted the occurrence of at least two different probable tsunami inundations. The first event could correspond to the 1169, the 1542 or the 1693 tsunami, the second inundation can be ascribed to the 1693 or the 1908 tsunami. The following research allowed to find evidences of both historical and paleoearthquakes and tsunamis in field. Results confirm the potentiality and usefulness of thepaleoseismological off- fault methods and their integration with further information, provided by ulterior studies, should help to better define the seismicity of Sicily. This thesis is divided in seven chapters. First a general definition of the geologic and tectonic setting of Sicily and of the seismic source models, proposed for the strongest earthquakes, are exposed (Chapter 1). Then, a brief treatise on paleoseismology and the investigation methods is provided (Chapters 2 and 3). Performed researches are discussed separately for each different approach, exposing methods and results in the Chapters 4, 5 and 6. Finally, in the Chapter 7 a summary and a discussion on the main matters of this thesis, the applicability and usefulness of this kind of researches are exposed, arguing on how they can contribute to the improving of the knowledge of the eastern Sicily seismicity.