Formation of box canyons by mass failure in limestone : a modelling study of the role of groundwater seepage

Groundwater seepage has been shown to unambiguously lead to channel formation inunconsolidated sand to gravel sized sediments. However, its role in the evolution of bedrocklandscapes remains controversial. In this study, we use the coastline of the Maltese Islands as a case study to establish if and how groundwater seepage can form box canyons in limestones.peer-reviewe

A seismic site response survey of the Maltese Islands

The Maltese Islands in the Central Mediterranean are composed of a simple 5-layer sedimentary sequence of Oligocene - Miocene age. The sequence is highly disturbed by syn-sedimentary to recent faulting, and the islands are characterised by a variety of outcropping rock types and underlying lithology. The seismic history of the Maltese islands includes a number of earthquakes in which serious building damage was experienced, the maximum intensity being of EMS98 VII - VIII in 1693. A correspondence is observed between building damage and areas underlain by soft clay layers, for example, but a seismic risk assessment of the islands that takes into account such site effects has never been attempted. Since the last damaging earthquake, the building density has increased dramatically, and the building footprint extended to regions of diverse lithologies, while general building practices have not undergone any upgrade in standards. In this study, an extensive ambient noise survey of the two major islands (Malta and Gozo), utilising the Nakamura method, has been undertaken, covering all outcrop types and major morphological features. Clear correlations between geology and response frequency / amplitude will be presented and their implications in the light of building vulnerabilty and seismic risk discussed.N/

Inversion of surface wave data for shear-wave velocity profiles : case studies of a thick buried low-velocity layer in Malta

The islands composing the Maltese archipelago (Central Mediterranean) are characterised by a four layer sequence of limestones and clays, with the Lower Coralline Limestone being the oldest exposed layer. The hard Globigerina Limestone (GL) overlies this layer and is found outcropping in the eastern part of Malta and western part of Gozo. The rest of the islands are characterised by Upper Coralline Limestone (UCL) plateaus and hillcaps covering a soft Blue Clay (BC) layer which can be up to 75 m thick. Thus the BC layer introduces a velocity inversion in the stratigraphy, and makes the Vs30 parameter not always suitable for seismic microzonation purposes. Such a layer may still produce amplification effects, however would not contribute to the numerical mean of Vs in the upper 30m. Moreover, buildings are being increasingly constructed on this type of geological foundation. Obtaining the shear wave (Vs) profiles of the different layers around the islands is the first step needed for a detailed study of local seismic site response. A survey of Vs in each type of lithology and around the islands has never been undertaken.N/

Shear wave velocity inversion and its influence on seismic site response : case studies from Malta and Catania (Italy)

The aim of this work is to study some particular geological sequences on the Maltese islands and in the city of Catania (Southern Italy). Geologically the two sites are quite different but several measurement points in both localities are characterised by sequences that represent a significant shear wave velocity inversion. The Maltese is- lands in the Central Mediterranean are composed of a simple 4-layer sedimentary sequence of Oligocene–Miocene limestones and clays. In particular, limestones overlying a clayey formation represent the shallower lithotypes that characterize the surficial geology in a large portion of the Maltese archipelago. On the contrary, the present geo- logic features of the Catania area are the result of tectonic uplift, sea level changes and lava flows originating from Etna eruptions. The area shows complex features with lateral heterogeneities at a local scale, due to the presence of volcanic and sedimentary units. In this study we make use of ambient noise recordings as well as moderate mag- nitude earthquakes in order to study the role of local geology on the site response in Catania. The recorded data were processed through standard and horizontal-to-vertical spectral ratios. Results of ambient noise and earthquake analysis, although showing significant differences in amplitude, are comparable in frequency. On the lava flows spectral ratios, significant amplification of the vertical component, that appear related to velocity inversion, were observed. Our findings appear linked to the complex wave-field generated by the lithologic heterogeneities existing in the area which seem to be related to alternating outcropping sediments and basaltic lavas. On the Maltese islands the main aim of this study was to identify any differences in site response, both in frequency and amplification, observed on different outcrops. All the investigated sites exhibit a clear and consistent peak in the frequency range of 1 Hz to 2 Hz. This is tentatively attributed to the presence of the buried clay formation layer everywhere below a variable thickness layer of limestone formation. The site effect induced by the clay layer has implications for the assessment of seismic risk, as well as for the validity of using Vs30 as a proxy for site class and site amplification.peer-reviewe

Nonlinear inversion of multifrequency GPR data in tomographic configurations

The accurate tomographic reconstruction of structures starting from Ground Penetrating Radar (GPR) data is useful in many real-world scenarios, ranging from the characterization of buried regions to the inspection of tree trunks. Unfortunately, the practical application of advanced inverse-scattering methods requires an accurate modeling of the GPR system, and in particular of the antenna and antenna-medium interactions [1]. In this work, the combination of an advanced antenna modelling technique with a nonlinear multifrequency inversion method is investigated from an experimental point of view. The GPR measurements, acquired with a lightweight radar system prototype in different configurations, are processed with a hybrid reconstruction approach that aims at combining the benefits of qualitative processing and quantitative inversion techniques [2]. The reconstruction of cylindrical targets buried in a sand box and in free space are considered, evaluating the influence of the antenna and its modelling on the inversion. Results are promising and allow to draw indications about the applicability of the proposed method to GPR configurations. [1] A. De Coster and S. Lambot, “Full-wave removal of internal antenna effects and antenna-medium interactions for improved ground-penetrating radar imaging,” IEEE Transactions on Geoscience and Remote Sensing, 2019. [2] F. Boero et al., “Microwave tomography for the inspection of wood materials: imaging system and experimental results,” IEEE Transactions on Microwave Theory and Techniques, vol. 66, no. 7, pp. 3497–3510, Jul. 2018

Fluid escape structures in the Graham Bank region (Sicily Channel, Central Mediterranean) revealing volcanic and neotectonic activity

In the Sicily Channel, (Central Mediterranean), two geodynamic processes overlap each other, the Maghrebides- Apennines accretionary prism and the Sicily Channel rift. Moreover, the northwestern sector (Banks sector) is characterised by an irregular seafloor morphology linked to the recent volcanic and tectonic activity.In order to discriminate the role exerted by both the processes in the morphostructural setting of the area we used a dataset of both high and very high resolution single-channel and multi-channel profiles, acquired in the frame of the RITMARE project respectively with CHIRP and sparker, and airgun sources, and high resolution (5 m cell) morpho-bathymetric data. The data allowed us to identify and characterise two areas where different geological features (sedimentary and volcanic) are prevailing. They present fluid escaping evidence, which often appears to be active and generating different types of morphologies (both positive and negative). In the western sector we recognised pockmarks at water depths of 195 to 317 m, with diameters from 25 to 580 m, depths from 1.3 to 15 m, and slope up to 23◦ . They show sub-circular shape in plan-view and reflectors with upward concavity in cross section, and are oriented along a NW-SE trend.The CHIRP and multichannel profiles highlight fluids that affect the Plio-Quaternary succession, especially in areas where the top surface of the Messinian succession is shallower. Conversely, wipe-out acoustic facies were recognised in proximity of: i) extensional faults of Mesozoic age with NW-SE trend; ii) dip/strike slip faults of Cenozoic age with NW-SE, N-S and about NNE-SSW trends, and iii) extensional neo-tectonic faults with NW-SE and NNW-SSE trends. We cannot exclude that they could feed the shallower reservoir producing a mixing between the two. In the eastern sector we recognised a cluster of volcanoes composed of seven cone-shaped structures (SCV1-7), pertaining to a wide area known as Graham Bank. A detailed morphometric analysis of these volcanoes has been conducted: they are up to about 115-160 m high and 500-1500 m wide. Most of them show very strongly inclined flanks with 30◦ of average slope. The SCV2 and SCV3 form the Graham Bank, 3.5X2.8 km wide, elongated in the NW-SE direction. At the top of SCV2 focused seepage plumes were observed in the entire water column, through the CHIRP data, where we calculated that they release, a volume of about 10950 m3 and 43960 m3of gases, respectively. In this work, we present the first results of a data collection that have got as main result the identification and mapping of the fluid escape structures revealing the relationship between the active tectonic with migration of fluids, to be used to assess the Submarine Geo-Hazard in the Sicily Channel. We identified two fluid escape fields whose genesis and evolution appear linked to the neotectonic and volcanic activities respectively, that represent the main controlling factors for the migration of fluid; considering the good correlation between pockmarks and the main identified fault systems. In conclusion, our results suggest that the degassing of fluids in this region is rooted at depth, and is mainly aligned with the NW-SE dip/strike slip fault systems, repeatedly reactivated, and linked to the volcanic activity.peer-reviewe

COST Action TU1208 – Working Group 1 – Design and realisation of Ground Penetrating Radar equipment for civil engineering applications

This work aims at presenting the main results achieved by Working Group (WG) 1 "Novel Ground Penetrating Radar instrumentation" of the COST (European COoperation in Science and Technology) Action TU1208 "Civil Engineering Applications of Ground Penetrating Radar" (www.cost.eu, www.GPRadar.eu). The principal goal of the Action, which started in April 2013 and is ending in October 2017, is to exchange and increase scientific-technical knowledge and experience of Ground Penetrating Radar techniques in civil engineering, whilst promoting throughout Europe the effective use of this safe non-destructive technique. The Action involves more than 300 Members from 28 COST Countries, a Cooperating State, 6 Near Neighbour Countries and 6 International Partner Countries

Failure surface development due to shallow gas : a case study from the Hikurangi Margin, New Zealand

The characteristic morphology of spreading, in the form of a recurring and parallel pattern of ridges and troughs, has been observed in numerous submarine landslides around the world. Limit equilibrium modelling of slope failure processes in the Storegga Slide had indicated that an increase in pore pressure is likely an important cause of spreading. In this study we explore the hypothesis that pore pressure generation in sub-seafloor sediments by shallow gas can promote the development of a weak layer above which submarine spreading can occur. We do this by analysing multibeam echosounder, sub-bottom and 2D multichannel seismic data acquired offshore the east coast of the North Island, New Zealand.peer-reviewe

Forensic geosciences investigations on experimental fields

The search for homicide graves is a very strenuous activity that may lead to the identification of the burial site if it is planned based on articulated scientific approaches considering several aspects of forensic sciences. Moreover, another difficult task in such criminal cases may be the estimation of the Post-Mortem Interval (PMI) of the victim. Discrepancies between PMI estimation through entomological studies and other evaluations may be. This inconsistency is at the base of the necessity to examine and well understand the human decay process of human beings and the decay consequences in the surrounding environmental context. It is noteworthy that several processes may occur on the surrounding site the burial.peer-reviewe

Satellite investigation to study pocket beach dynamics in Malta. The SIPOBED project

Pocket Beaches are small beaches limited by natural headlands, strongly jutting into the sea, free from direct sedimentary contributions that are not eroded from back-shore cliffs. Malta’s pocket beaches are one of the most significant geomorphologic features of the archipelago. They play an important role for a variety of ecological and economic reasons. In this sense, sediment (mostly sand) dynamics is the most relevant factor to consider in the beach system. Sediment movement can be driven by a variety of factors, including wave action, currents, wind and direct and indirect anthropic action, leading to extreme morphological modifications in some cases.peer-reviewe