The use of Rayleigh-wave ellipticity for site-specific hazard assessment and microzonation: application to the city of Lucerne, Switzerland

The sediments underlying the city of Lucerne (Switzerland) consisting of fluvio-lacustrine deposits of Quaternary age have the potential to produce strong amplification of the seismic wavefield. To obtain a reliable estimation of the deep soil structure, we combine different methodologies based on ambient noise recordings, such as single station horizontal to vertical ratios and three-component array analysis. Two novel techniques to estimate Rayleigh-wave ellipticity from ambient noise recordings are tested. These are based on a single- and a multistation approach, respectively. The first utilizes the continuous wavelet transform to perform a decomposition of the noise wavefield and to isolate and extract the Rayleigh-wave contribution. The second, conversely, relies on a high-resolution f-k method to achieve the same result. We compare the results from the two techniques to provide an evaluation of their capabilities and limitations. A two-step inversion scheme is then presented to improve resolution on the bedrock depth. In particular, the surface wave dispersion information is initially used to constrain the soft sediment part, while the Rayleigh-wave ellipticity peak is subsequently used for constraining the bedrock depth. It is shown that such an approach is beneficial to map the bedrock geometry over dense urban areas. The output velocity model is then used to compute the local seismic amplification by means of gridded 1-D approximatio

Advance in seismic site response: Usual practices and innovative methods

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.Published1-24T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Monitoring the Preonzo Rock Slope Instability Using Resonance Mode Analysis

Reliable monitoring of unstable rock slopes is a prerequisite for successful mitigation of landslide hazards. However, most state‐of‐the art techniques rely on measuring the local surface displacement in the potential release area. In contrast, recording ambient vibration data allows for analyzing structural dynamic parameters of the unstable slope, such as resonance frequency, polarization of vibration, and energy dissipation. These parameters can be linked to properties of the instability, for example, to rock stiffness and fracture network orientation. We developed a processing method for continuous seismic data based on enhanced frequency domain decomposition modal analysis and applied it to the unstable rock slope Preonzo in Switzerland (∼140,000 m3). Four years of ambient vibration data recorded at two permanent seismometers on the instability were analyzed, providing the resonance frequency, damping ratio, and normal mode shapes of the fundamental (∼3.5 Hz) and the first higher (∼4.2 Hz) vibrational mode. We found that modal analysis can be reliably used to monitor the dynamic response of an unstable rock slope. We observed annual changes of all parameters with a damping ratio varying between 6.0% and 9.7% for the fundamental mode. The dynamic parameters appear to be primarily driven by temperature and only secondarily by opening and closing of fractures. No large slope failure was registered during the observation period. However, the data provide a baseline model for ongoing slope monitoring to recognize structural changes before a future collapse. The setup proposed builds a complementary monitoring system to displacement‐based surveying.ISSN:0148-0227ISSN:2169-9003ISSN:2169-901

Ambient vibration characterization and monitoring of a rock slope close to collapse

We analyse the ambient vibration response of Alpe di Roscioro (AdR), an incipient rock slope failure located above the village Preonzo in southern Switzerland. Following a major failure in May 2012 (volume ∼210 000 m3), the remaining unstable rock mass (∼140 000 m3) remains highly fractured and disrupted, and has been the subject of intensive monitoring. We deployed a small-aperture seismic array at the site shortly after the 2012 failure. The measured seismic response exhibited strong directional amplification (factors up to 35 at 3.5 Hz), higher than previously recorded on rock slopes. The dominant direction of ground motion was found to be parallel to the predominant direction of deformation and perpendicular to open fractures, reflecting subsurface structure of the slope. We then equipped the site with two semi-permanent seismic stations to monitor the seismic response with the goal of identifying changes caused by internal damage that may precede subsequent failure. Although failure has not yet occurred, our data reveal important variations in the seismic response. Amplification factors and resonant frequencies exhibit seasonal trends related (both directly and inversely) to temperature changes and are sensitive to freezing periods (resonant frequencies increase with temperature and during freezing). We attribute these effects to thermal expansion driving microcrack closure, in addition to ice formation, which increase fracture and bulk rock stiffness. We find the site response at AdR is linear over the measured range of weak input motions spanning two orders of magnitude. Our results further develop and refine ambient vibration methods used in rock slope hazard assessment.ISSN:0956-540XISSN:1365-246

JRA1: final version (4.9.2014)

EU project NERAPublished4T. Sismologia, geofisica e geologia per l'ingegneria sismic

Coupled seismogenic geohazards in Alpine regions

COupled seismogenic GEohazards in Alpine Regions (COGEAR) is an interdisciplinary natural hazard project investigating the hazard chain induced by earthquakes. It addresses tectonic processes and the related variability of seismicity in space and time, earthquake forecasting and short-term precursors, and strong ground motion as a result of source and complex path effects. We study non-linear wave propagation phenomena, liquefaction and triggering of landslides in soil and rock, as well as earthquake-induced snow avalanches. The Valais, and in particular parts of the Rhone, Visper, and Matter valleys have been selected as study areas. Tasks include detailed field investigations, development and application of numerical modeling techniques, assessment of the susceptibility to seismically induced effects, and installation of different monitoring systems to test and validate our models. These systems are for long-term operation and include a continuous GPS and seismic networks, a test installation for observing earthquake precursors, and a system to study site-effects and non-linear phenomena in two test areas (Visp, St. Niklaus / Randa). Risk-related aspects relevant for buildings and lifelines are also considered