Claudia Romagnoli

Height and gravity variations by continuous GPS, gravity and environmental parameter observations in the southern Po Plain, near Bologna, Italy

Abstract During 1996, a Global Positioning System (GPS) receiver and a superconducting gravimeter (SG) were installed at Medicina, Italy, in order to monitor ground deformation and signals related to global/regional processes and local environmental effects. First results provided by this combined observing strategy are presented, by focusing on common patterns as well as on the differences observed in the two time series. A marked annual signal of increasing amplitude, present in both series, is becoming quite recognizable towards the end of 1997. Seasonal loading effects induced by air pressure, the ocean and surficial water table were estimated and modeled for both data sets. For the gravity series, 12-h data of balloon radio sounding launches have been used to estimate the attraction effect of the seasonal vertical density distribution of the air pressure column above the station. Mass effects due to the surficial water table and the ocean were also estimated and accounted for. The comparison between the observed and modeled series, both in the case of GPS heights and gravity, shows a remarkable agreement and provides a convincing explanation for the observed seasonal fluctuations. In the July–September 1997 time frame, a sudden gravity increase occurred. This anomaly is in the order of 3 μGal and is present in the data series after accounting for the seasonal fluctuation. This step-like feature is responsible for most of the estimated positive linear trend present in the data series. The anomalous increase in gravity has been interpreted as due to mass/density variations, likely associated with the uprising of deep-seated waters in relation with local stress field changes. During the second half of 1997, concurrent seismic activity occurred both in the southern Po Valley and in central Italy (Umbria seismic crisis).

Influence of soil consolidation and thermal expansion effects on height and gravity variations

Abstract The daily GPS height series of the Medicina station were analyzed for the period July 1996–September 2001. The station is located in the middle Po Plain on fine-grained alluvial deposits. A seasonal oscillation in the order of 18 mm (peak-to-peak amplitude) is present in the data. This crustal deformation has been modeled by including variations in the atmospheric, oceanic and hydrologic mass. The vertical positions can also be affected significantly by soil consolidation. Geotechnical parameters derived by in situ tests and laboratory analyses of the clayey soil collected at Medicina allowed the estimate of the soil settlement relevant to the seasonal oscillation of the surficial water table. Thermal expansion of the geodetic monument has to be taken into account in the case of high-precision vertical positioning. In this work models both for the soil consolidation and the thermal expansion effects are provided. The continuous gravity observations collected at Medicina by means of a superconducting gravimeter also exhibit a marked seasonal oscillation, which has been interpreted as the sum of loading and Newtonian attraction effects, as well as of the contribution due to soil consolidation. Especially the study concerning the soil consolidation effect has allowed a better insight on the seasonal vertical movements occurring at the Medicina station by providing quantitative information on soil behavior due to change of effective pressures. The results can be applied to those stations characterized by similar fine-grained soils and surficial hydrogeology.

The Landslide Sequence Induced by the 2002 Eruption at Stromboli Volcano

The complex sequence of large-scale tsunamogenic instability phenomena occurred on the subaerial and submarine NW flank of the Stromboli Volcano soon after the beginning of the December 2002 eruption is reconstructed and its relationship with volcanic activity is evidenced. After a brief description of slope morphology and stratigraphy, geometry and kinematics of the landslides are described. Finally, instability mechanisms that controlled the subaerial and submarine slope failures are proposed with reference to the different geotechnical, hydraulic, and loading/strain conditions that characterized the different stages of the slope evolution.

Chapter 3 Characteristics and morphological evolution of the Aeolian volcanoes from the study of submarine portions

Abstract The seven Aeolian Islands exist in much greater volumes below sea level than above sea level; study of the submarine portions is therefore fundamental for a better understanding of the development of volcanism in the area, tectonic control from the regional structural setting and the morphological evolution of volcanic edifices. The interplay between destructive processes (marine erosion, mass-wasting and flank instability) and constructive processes in the growth and evolution of the Aeolian volcanoes is discussed based on a study of their present-day morphology. In particular, the distribution and characteristics of insular shelves in the shallow-water sectors of volcanic edifices are compared and discussed, considering that variable geological/local factors combined with sea-level fluctuations controlled the development and morphology of these features. It is proposed that submerged shelves may furnish relative chronological constraints for the development of the Aeolian volcanoes, enhancing present knowledge which is mostly based on subaerial portions.

Chapter 4 Bathy-morphological setting of the Aeolian Islands

Abstract An updated bathy-morphological setting of the Aeolian Islands is presented, based on new detailed bathymetric maps of the western, central and eastern sectors of the archipelago. In recent years, the acquisition of multibeam swath bathymetry has greatly expanded knowledge of the submarine portions of the Aeolian volcanic edifices, revealing that their submarine extension is much wider than that of the islands. Indications given by the submarine setting are fundamental for better understanding of the evolution of volcanism and the control exerted by main structural lineaments, as well as to locate large-scale flank instability events and recent submarine eruptive activity. DVD: Bathymetric maps of the eastern, central and western sectors are included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic

Morphology of Lipari offshore (Southern Tyrrhenian Sea)

High-resolution multibeam bathymetry was recently collected around Lipari, the largest and most densely populated island of the Aeolian Archipelago (Southern Tyrrhenian Sea). The data were acquired within the context of marine geological studies performed in the area over the last 10 years. We present the first detailed morphological map of the Lipari offshore at 1:100,000 scale (Main Map). A rugged morphology characterizes the submarine portions of Lipari volcano, reflecting both volcanic and erosive-depositional processes. The volcanic features include cones, lava flows and bedrock outcrops. Erosive-depositional features include an insular shelf topped by submarine depositional terraces related to Late-Quaternary sea-level fluctuations, as well as landslide scars, channelized features, fan-shaped deposits and wavy bedforms. The different distribution of volcanic and erosive-depositional features on the various sectors of Lipari is mainly related to the older age of the western flank with respect to the eastern one. The map also provides insights for a first marine geohazard assessment of this active volcanic area.

Sheared sheet intrusions as mechanism for lateral flank displacement on basaltic volcanoes: Applications to Réunion Island volcanoes

Field work carried out on the Piton des Neiges volcano (Reunion Island) suggests that the injection of magma along detachments could trigger flank failure by conjugate opening and shear displacement. We use 3-D numerical models to compare the ability of purely opened sheet intrusions, sheared sheet intrusions, and normal faults to induce flank displacement on basaltic volcanoes. We assume that shear stress change on fractures results from stress anisotropy of the host rock under gravity. Exploring a large range of stress anisotropies, fracture dips, and fracture depth over length ratios, we determine that the amount of shear displacement is independent of the proximity to the ground surface. Sheared sheet intrusions are the most efficient slip medium on volcanoes. Consequently, the largest flank displacement is induced by the longest, deepest sheared intrusion dipping closest to 45° in a host rock with the highest stress anisotropy. Using our model in a forward way, we provide shear and normal displacements for buried fractures. Applying the model to a pile of sills at the Piton des Neiges volcano, we determine that the mean shear displacement caused by each intrusion was 3.7 m, leading to a total of a 180–260 m of lateral displacement for the 50 m high pile of sills. Using our model in an inverse way, we formulate a decision tree to determine some fracture characteristics and the host rock stress anisotropy from ratios of maximum surface displacements. This procedure provides a priori models, which can be used to bound the parameter space before it is explored through a formal inversion. Applying the decision tree to the 1.4 m coeruptive flank displacement recorded at Piton de la Fournaise in 2007, we find that it probably originated from a shallow eastward dipping subhorizontal normal fault.

Historical Shoreline Changes at an Active Island Volcano: Stromboli, Italy

Abstract Rapid shoreline changes at Stromboli Island have been documented from maps and aerial photographs dating from 1938 to 2003. Shorelines digitized from aerial photographs, taken at decadal intervals on average, show that while cliffed tracts were relatively stable, beaches on the N and NE side of the island experienced alternating accretion and erosional stages linked, respectively, with the contemporary eruptions and with sediment redistribution and/or the washing out and loss of sediment to deeper waters. Sciara del Fuoco, on the central-western side of the island, is where most of the volcanic products have accumulated during the practically continuous activity of the last two thousands years; from there, sands and gravel drift alongshore and are added to beaches on the N and NE side of the island. A simple model of clockwise longshore drift along the W, N, and NE coasts of the island is presented, taking into account the influences of manmade structures on shoreline changes.

Slope Failures Induced by the December 2002 Eruption at Stromboli Volcano

We reconstruct the sequence of landslides that occurred soon after the beginning of the December 2002 eruption on the NW flank of Stromboli volcano. Landslides involved the northeastern part of the Sciara del Fuoco (SdF) slope, an old collapse scar filled by products of volcanic activity, producing tsunami waves that severely damaged the coast of the island of Stromboli. Volumes of the mass detached from the subaerial and submarine slope were quantified by comparing preslide and postslide slope surfaces obtained by aerophotogrammetric and bathymetric data, which also allowed, in conjunction with field observations and helicopter surveys, the reconstruction of geometry and kinematics of landslides. According to the reconstructed sequence, 2 days after the beginning of the eruption, the upper part of the NE sector of the SdF slope experienced major displacements (few tens of meters). Movements propagated downslope and affected the nearshore portion of the submerged slope without a rapid sliding of the displaced mass into the sea. The following hours were characterized by a progressive increase of deformations, localized along shear zones extending over two thirds of the subaerial slope. This phase proceeded until a submarine slide about 6 x 10 6 m 3 in volume occurred, causing a first tsunami wave. The subaerial mass delimited by the shear zones and unbuttressed at its foot, then slipped into the sea producing a second tsunami wave. The main landslide event (and the minor slumps which followed) removed a volume of about 10 x 10 6 m 3 of the infilling deposit, to a thickness of at least 65 m. Hypotheses were formulated on the mechanisms that controlled the different phases of the instability sequence. Since hydraulic and stress/strain conditions progressively changed during the slope evolution, the formulated mechanisms are also based on geotechnical analyses and considerations on the mechanical behavior of volcaniclastic materials. The process that led to the landslide events.

Marine geological and archaeological evidence of a possible pre-Neolithic site in Pantelleria Island, Central Mediterranean Sea

Abstract Recent underwater archaeological surveys recovered hundreds of flint artefacts between depths of 18 and 21 m at Cala Tramontana, a small bay located in the eastern part of Pantelleria Island. Most of the flint artefacts indicate debitage, and are characterized by cores and flakes without any specific morphology. Different lithic tools were also identified, such as fragments of blades, truncations, end-scrapers, points and crested blades. An initial hypothesis is that this lithic industry represents the oldest traces of human visitation to the island, possibly related to the exploitation of the nearby obsidian source, and favoured because of the sheltered coastal configuration of Cala Tramontana and Cala Levante with respect to the dominant winds and related storms. However, the present-day coastal setting in the bay is rather inhospitable, with high cliffs and difficult marine access. In contrast, palaeo-landscape reconstructions by means of high-resolution multibeam bathymetry reveal the possible presence of a small palaeo-beach in the inner part of the bay when the sea level was 15 m lower than at present. By comparing this palaeo-sea level with the eustatic curve (and by excluding possible vertical movements), we roughly estimate an age of the lithic industry of 9.6–7.7 cal ka BP.