ALBERT

Description: The spectacular Lusi mud eruption started in northeast Java, Indonesia, the 29th of May 2006 following a M6.3 strike slip seismic event. After the earthquake several mud pools aligned along a NE-SW direction appeared in the Sidoarjo district. The most prominent eruption site was named Lusi. Lusi is located ∼10 km to the NE of the northernmost cone of the Arjuno-Welirang volcanic complex with which it is connected by the Watukosek Fault System. In this study, we applied the HVSR method, which is a common tool used for site effect investigations as well as to infer buried structures and reconstruct sub-surface geology. The method is based on the ratio of the horizontal to vertical components of ground motion and it generally exhibits a peak corresponding to the fundamental frequency of the site. Spectral ratio results highlight a fundamental frequency band between 0.4 and 1.0 Hz in the Lusi neighborhood. We interpret these peaks as related to the velocity lithological contrast at depth between alluvial deposits and bluish grey clay. Our analysis also highlights the presence of a “depocenter”, characterized by fundamental frequency up to 0.3 Hz, which is interpreted as the subsidence caused by withdrawal of mud and fluids from depth (as also shown by the comparison of the HVSR results with gravimetry data). Moreover, in the area of the Lusi vent a broad-band frequency range is related to the Lusi conduit. In this paper, we show that detailed microtremor surveys could be used as a preliminary and fast approach to locate mud conduits with sufficient precision.

Description: Mud volcanoes are characterized by emissions of fluids and fragmented sedimentary rocks that may create large structures with different morphologies. The development of these structures occurs due to the presence in the subsurface of a) clay-bearing strata that can be buoyant in the surrounding units and b) over-pressured fluids that facilitate the formation of diapirs through sedimentary rocks, as well as structural and/or lithologic discontinuities. Mud volcanoes have been observed and studied in different localities on the planet. In this framework, we tested ambient vibration methods, which are common tools used for site effect investigations aiming to assess the thickness of the sediments and the body wave velocities. The approach commonly used for this type of studies is based on the ratio of the horizontal to vertical components of ground motion (HVSR) and on passive array techniques. The HVSR generally enable to recognize peaks that point out to the fundamental frequency of the site, which usually fit quite well the theoretical resonance curves. The combination of HVSR and shear wave velocity, coming from passive array techniques, enables to collect valuable information about the subsurface structures. Here we present new data collected at mud volcano and sedimentary hosted hydrothermal system sites in order to investigate the depths of the main discontinuities and of the hypothesized hydrocarbon reservoirs. Our results indicate that the ambient vibrations study approach, represents a swift and simplified methods that provides quick information on the subsoil structure of the investigated areas. This methodology allowed us to delineate a preliminary sketch useful to plan more specific and detailed investigations settled to outline a comprehensive model of the explored targets.

Description: The Maltese Islands, located in the central Mediterranean Sea, are intersected by two normal fault systems associated with continental rifting to the south. Due to a lack of evidence for offshore displacement and insignificant historical seismicity, the systems are thought to be inactive and the rift-related deformation is believed to have ceased. In this study we integrate aerial, marine and onshore geological, geophysical and geochemical data from the Maltese Islands to demonstrate that the majority of faults offshore the archipelago underwent extensional to transtensional deformation during the last 20 ka. We also document an active fluid flow system responsible for degassing of CH4 and CO2. The gases migrate through carbonate bedrock and overlying sedimentary layers via focused pathways, such as faults and pipe structures, and possibly via diffuse pathways, such as fractures. Where the gases seep offshore, they form pockmarks and rise through the water column into the atmosphere. Gas migration and seepage implies that the onshore and offshore faults systems are permeable and that they were active recently and simultaneously. The latter can be explained by a transtensional system involving two right-stepping, right-lateral NW-SE trending faults, either binding a pull-apart basin between the islands of Malta and Gozo or associated with minor connecting antitethic structures. Such a configuration may be responsible for the generation or reactivation of faults onshore and offshore the Maltese Islands, and fits into the modern divergent strain-stress regime inferred from geodetic data.

Description: Amplitudes and frequency content of the seismic ground motion generated by an earthquake and recorded at a specific location depends on the characteristics of the source, the path from the source to the site and the local geologic conditions. The local site seismic response is produced by multiple physical phenomena (i.e., reflection, diffraction, focusing, resonance effects, non-linear soil behavior) that can amplify or decrease amplitudes of seismic waves near to the surface causing high variability in the observed ground motions. In particular, vertical discontinuities and abrupt changes in the velocity profile, or lateral heterogeneities such as faults and/or stratigraphic contacts can have a strong impact. A correct and quantitative assessment of site effects is required for both the interpretation of observed waveforms and the reliable prediction of resultant ground motions (e.g., computation of specific earthquake scenarios). In addition, the extent and distribution of building damage due to moderate and large earthquakes in densely populated areas are a result of the combined effect of local site response and the dynamic properties of man-made structures. The quantification of ground motion amplification are therefore of primary interest for seismologists and engineers in order to reduce associated risks. Recent advances in engineering seismology research resulted in improvements in the study of seismic site response both from the theoretical and experimental points of view. For example, new numerical modelling techniques have become available, which account for non-linear soil behavior. Growing seismic networks allow for the more advanced site response estimates compared to the past as well (e.g. the development of more reliable ground motion prediction equations). This thematic issue focuses on local seismic site effects and represents a collection of research papers and case studies on the effect of subsurface structure on ground motion from new observations, numerical modelling, as well as geophysical imaging. This volume also includes contributions related to the Earth system response to earthquake processes. In the first paper of this volume, Germoso et al. (2017) analyse the effects of fractional derivatives in visco-elastic dynamics for site response analysis. They prove that the use of fractional derivatives for representing the viscous terms offers a larger flexibility in the resulting models (compared to standard methods), and allow them to better quantify the degree of dissipation as well as the magnitude of deformation and phase angle. Poggi et al. (2017) present three different soil amplification models for 5% damped pseudo-spectral acceleration response spectra using recordings of 88 selected stations of the Japanese KiKNet strong-motion network. While they do not provide a ranking of the applied methods, they evaluate the strengths and weaknesses of the each tested technique. Michel et al. (2017) present a site amplification study for the city of Basel (Switzerland), by combining data achieved using geophysical site characterization and site response modelling. They obtain amplification maps of the response spectrum at different periods for earthquake engineering and maps for implementation in ShakeMap. Pischiutta et al. (2017) perform geophysical investigations in the northwestern sector of the island of Malta to reconstruct velocity-depth models by using active and passive methods. They observe ground motion amplification at rock sites, highlighting the importance of performing velocity measurements even for such sites. In fact, using only a lithological criterion and following the EuroCode EC8, rock sites would be associated to class “A” where no amplification is expected. Hayashi and Craig (2017) measure S-wave velocity profiles at eleven sites in the Eastern San Francisco Bay area using surface wave methods. A S-wave velocity cross section which runs perpendicular to the Hayward fault is derived and the theoretical site amplification is calculated using a viscoelastic finite-difference method. Their results show that ground motion is amplified on the west side of the Hayward fault as an effect of the lateral variations of the S-wave velocity. Panzera et al. (2017) investigate the characteristics of the local seismic response in Lampedusa (Italy), a carbonate shelf belonging to the foreland domain at the northern edge of the African plate. Ninety-two ambient noise recordings were collected and processed through spectral ratio techniques. Their results point out the importance of seismic site effects by the presence of both morphologic and tectonic structures. Moisidi (2017) examine the potential soil-building resonance at selected buildings in a complex geological setting of the small scale Paleohora Basin (southwest Crete). This study highlights the necessity of incorporating the determination of potential coupling effects between site and buildings into urban planning for risk mitigation studies. Di Naccio et al. (2017) present an interdisciplinary approach to investigate the seismic response of the San Gregorio (L'Aquila, Italy), a rock site severely damaged by L'Aquila 2009 earthquake. Based on geological-structural, geophysical and seismic analyses, their results highlight the role of rock mass fracturing on seismic amplification, that generates lateral variations in seismic velocity. Bonilla et al. (2017) apply seismic interferometry to compute the in-situ shear wave velocity to evaluate the seismic response of sediments. They conclude that their approach is a robust method to extract shear wave velocity profiles and evaluate non-linear soil response. A seismic characterization of the flat Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce Physics and Chemistry of the Earth 98 (2017) 1e2 http://dx.doi.org/10.1016/j.pce.2017.04.005 1474-7065/© 2017 Published by Elsevier Ltd. top area of Monteluco (Italy) carbonate mountain using a multidisciplinary approach was performed by Durante et al. (2017). They hypothesize that local seismic amplification is related to topography and to an intensely fractured shallow-seated formation with relatively low shear wave velocity. Pazzi et al. (2017) investigated the Castagnola (La Spezia, Italy) and Roccalbegna (Grosseto, Italy) landslides through ambient vibrations. They estimated horizontal to vertical spectral ratio on a dense grid of points and obtained useful information on the main impedance contrast depths for large areas. The interpolation of the obtained fundamental frequencies enables the detection and reconstruction of the landslides' slip surfaces. The thematic issue is closed by the papers of Bogdanov et al. (2017) and Pierotti et al. (2017) that present the physical and chemical anomalies in the local environment before and after an earthquake.

Description: Published

Description: 1-2

Description: 4T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: This article has been accepted for publication in Geophysical Journal International ©: The Authors 2018. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.

Description: In this paper we characterize the high-frequency (1.0–10 Hz) seismic wave crustal attenuation and the source excitation in the Sicily Channel and surrounding regions using background seismicity from weak-motion database. The data set includes 15 995 waveforms related to earthquakes having local magnitude ranging from 2.0 to 4.5 recorded between 2006 and 2012. The observed and predicted ground motions form the weak-motion data are evaluated in several narrow frequency bands from 0.25 to 20.0 Hz. The filtered observed peaks are regressed to specify a proper functional form for the regional attenuation, excitation and site specific term separately. The results are then used to calibrate effective theoretical attenuation and source excitation models using the random vibration theory. In the log–log domain, the regional seismic wave attenuation and the geometrical spreading coefficient are modelled together. The geometrical spreading coefficient, g(r), modelled with a bilinear piecewise functional form and given as g(r) ∝ r−1.0 for the short distances (r 〈 50 km) and as g(r) ∝ r−0.8 for the larger distances (r 〈 50 km). A frequency-dependent quality factor, inverse of the seismic attenuation parameter, Q(f) = 160f/fref0. 35 (where fref = 1.0 Hz), is combined to the geometrical spreading. The source excitation terms are defined at a selected reference distance with a magnitude- independent roll-off spectral parameter, κ 0.04 s and with a Brune stress drop parameter increasing with moment magnitude, from σ = 2 MPa for Mw = 2.0 to σ = 13 MPa for Mw = 4.5. For events M ≤ 4.5 (being Mwmax = 4.5 available in the data set) the stress parameters are obtained by correlating the empirical/excitation source spectra with the Brune spectral model as function of magnitude. For the larger magnitudes (Mw〉4.5) outside the range available in the calibration data set where we do not have recorded data, we extrapolate our results through the calibration of the stress parameters of the Brune source spectrum over the Bindi et al. ground-motion prediction equation selected as a reference model (hereafter also ITA10). Finally, the weak-motion-based model parameters are used through a stochastic approach in order to predict a set of region specific spectral ground-motion parameters (peak ground acceleration, peak ground velocity, and 0.3 and 1.0 Hz spectral acceleration) relative to the generic rock site as a function of distance between 10 and 250 km and magnitude between M 2.0 and M 7.0.

Description: Published

Description: 148–163

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: Pyroclastic cones are a typical feature on volcanoes characterized by flank activity. Their distribution and orientation are important markers to obtain information on the maximum horizontal compressional stress acting on a volcano. A geophysical survey was performed on the pyroclastic cone of Mt. Vetore (Mt. Etna volcano, Southern Italy) to obtain information on its internal structural setting and to support the standard morphometric analysis. Results highlighted evident frequency peaks at 1.0 Hz inside the cone, which are attenuated away from it. The random decrement method was applied to this peak to compute damping and then to exclude links with anthropogenic sources. Moreover, time-frequency polarization analysis revealed that ambient vibrations are strongly polarized in a narrow frequency band, centered at a frequency of 1.0 Hz, with a preferred oscillation azimuth of 70–90° N. Array measurement of ambient vibrations was also used to obtain a shear wave velocity profile and then to retrieve the main interfaces with high seismic impedance. Results suggest a cone structure having a feeder dike consisting of fractured rocks with thickness of about 50 m surrounded by pyroclastic material lying on a high-velocity substrate. Finally, a 3D model of Mt. Vetore cone was built employing the finite element method to reproduce an experimental modal frequency of the cone itself. The numerical results successfully reproduced the experimental ones collected by the geophysical survey.

Description: Published

Description: id 74

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: The Maltese Islands, located in the central Mediterranean Sea, are intersected by two normal fault systems associated with continental rifting to the south. Due to a lack of evidence for offshore displacement and insignificant historical seismicity, the systems are thought to be inactive and the rift-related deformation is believed to have ceased. In this study we integrate aerial, marine and onshore geological, geophysical and geochemical data from the Maltese Islands to demonstrate that the majority of faults offshore the archipelago underwent extensional to transtensional deformation during the last 20 ka. We also document an active fluid flow system responsible for degassing of CH4 and CO2. The gases migrate through carbonate bedrock and overlying sedimentary layers via focused pathways, such as faults and pipe structures, and possibly via diffuse pathways, such as fractures. Where the gases seep offshore, they form pockmarks and rise through the water column into the atmosphere. Gas migration and seepage implies that the onshore and offshore faults systems are permeable and that they were active recently and simultaneously. The latter can be explained by a transtensional system involving two right-stepping, right-lateral NW-SE trending faults, either binding a pull-apart basin between the islands of Malta and Gozo or associated with minor connecting antitethic structures. Such a configuration may be responsible for the generation or reactivation of faults onshore and offshore the Maltese Islands, and fits into the modern divergent strain-stress regime inferred from geodetic data.

Description: Published

Description: 361-374

Description: 6A. Geochimica per l'ambiente e geologia medica

Description: JCR Journal

Description: Within an E.U.-funded project, BESS (Pocket BeachManagement and Remote Surveillance System), the notion of a geographic information system is an indispensable tool for managing the dynamics of georeferenced data and information for any form of territorial planning. This notion was further explored with the creation of aWebGIS portal that will allow local and regional stakeholders/authorities obtain an easy remote access tool tomonitor the status of pocket beaches (PB) in theMaltese Archipelago and Sicily. In this paper, we provide a methodological approach for the implementation of aWebGIS necessary for very detailed dynamic mapping and visualization of geospatial coastal data; the description of the dataset necessary for the monitoring of coastal areas, especially the PBs; and a demonstration of a case study for the PBs of Sicily andMalta by using the methodology and the dataset used during the BESS project. Detailed steps involved in the creation of theWebGIS are presented. These include data preparation, data storage, and data publication and transformation into geo-services. With the help of different Open Geospatial Consortium protocols, theWebGIS displays different layers of information for 134 PBs including orthophotos, sedimentological/geomorphological beach characteristics, shoreline evolution, geometric and morphological parameters, shallow water bathymetry, and photographs of pocket beaches. TheWebGIS allows not only for identifying, evaluating, and directing potential solutions to present and arising issues, but also enables public access and involvement. It reflects a platform for future local and regional coastal zone monitoring and management, by promoting public/private involvement in addressing coastal issues and providing local public administrations with an improved technology to monitor coastal changes and help better plan suitable interventions.

Description: Published

Description: 8233

Description: 7SR AMBIENTE – Servizi e ricerca per la società

Description: JCR Journal

Description: The Victoria Lines Fault (island of Malta) is a 〉15 km-long and N260°-striking segmented normal fault-system, which is probably inactive since the late Pliocene. In the westernmost part, the Fomm Ir-Rih segment displays comparable geologic throw and escarpment height (~150–170 m), moreover its hangingwall hosts thin patches of Middle Pleistocene clastic continental deposits (red beds), which are poorly preserved elsewhere. We acquired two seismic transects, by collecting ambient vibration recordings, processed by using horizontal-to-vertical spectral ratios, complemented by one high-resolution 2-D refraction tomography survey crossing this fault where it is locally covered by red beds and recent colluvial deposits. We found a resonance peak at ~1.0 Hz in the hangingwall block, whereas clear peaks in the range ~5.0–10.0 Hz appear when approaching the subsurface fault, and we relate them to the fractured bedrock within the fault zone. The best-fit tomographic model shows a relatively high-Vp shallow body (Vp 2200–2400 m/s) that we relate to the weathered top of the Miocene Upper Coralline Limestone Fm., bounded on both sides by low-Vp regions (〈1400 m/s). The latter are the smeared images of steep fault zones. Tomography further reveals a thick (~15–20 m) low-Vp (〈1000 m/s) zone, which could be a syn-tectonic wedge of colluvial deposits developed in the downthrown block. Surface waves analysis indicates lateral changes of the average shallow shear wave velocity, with Vs ~130 m/s within the inferred fault zone, and Vs 〉230 m/s above the weathered top-bedrock. Our results depict a clear seismic signature of the Victoria Lines Fault, characterized by low seismic velocity and high amplification of ground motion. We hypothesize that, during the Middle Pleistocene, faulting may have affected the basal part of the red beds, so that this part of the investigated complex fault-system may be considered inactive since ~0.6 Myr ago.

Description: Published

Description: 220-233

Description: 1T. Struttura della Terra

Description: JCR Journal