NH4.1

Recent earthquake events have shown that besides the strong ground motions, the coseismic faulting often caused substantial ground deformation and destructions of near-fault structures. In Taiwan, many high-rise buildings with raft foundation are close to the active fault due to the dense population. The Shanchiao Fault, which is a famous active fault, is the potentially dangerous normal fault to the capital of Taiwan (Taipei). This study aims to use coupled FDM-DEM approach for parametrically analyzing the soil-raft foundation interaction subjected to normal faulting. The coupled FDM-DEM approach includes two numerical frameworks: the DEM-based model to capture the deformation behavior of overburden soil, and the FDM-based model to investigate the responses of raft foundation. The analytical approach was first verified by three  benchmark cases and theoretical solutions. After the verification, a series of small-scale sandbox model was used to validate the performance of the coupled FDM-DEM model in simulating deformation behaviors of overburden soil and structure elements. The full-scale numerical models were then built to understand the effects of relative location between the fault tip and foundation in the normal fault-soil-raft foundation behavior. Preliminary results show that the raft foundation located above the fault tip suffered to greater displacement, rotation, and inclination due to the intense deformation of the triangular shear zone in the overburden soil. The raft foundation also exhibited distortion during faulting. Based on the results, we suggest different adaptive strategies for the raft foundation located on foot wall and hanging wall if the buildings are necessary to be constructed within the active fault zone. It is the first time that the coupled FDM-DEM approach has been carefully validated and applied to study the normal fault-soil-raft foundation problems. The novel numerical framework is expected to contribute to design aids in future practical engineering.

Keywords: Coupled FDM-DEM approach; normal faulting; ground deformation; soil-foundation interaction; raft foundation.

How to cite: Ru-Ya, F., Cheng-Han, L., and Ming-Lang, L.: Validation of Coupled FDM-DEM Approach on the Soil-Raft Foundation Interaction Subjected to Normal Faulting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9783, https://doi.org/10.5194/egusphere-egu21-9783, 2021.

Liquefaction is a phenomenon where soil loses its strength. The phenomenon of liquefaction occurs on non-cohesive soils with medium to fine grains. The phenomenon of liquefaction occurs during an earthquake, the ground experiences shaking vibrations. Palu, Central Sulawesi, Indonesia is one of the areas affected by the liquefaction phenomenon which causes damage to infrastructure in the area. The Palu earthquake that occurred on September 28, 2018, at 18:02:44 WITA with a magnitude of M_w = 7.4, centered on 26 km north of Donggala, Central Sulawesi. One aspect of the assessment for soil susceptibility to potential liquefaction is laboratory tests. One common laboratory test that can be performed is the cyclic triaxial test. The factors affecting the liquefaction resistance of saturated sand are the relative density and cyclic stress ratio (CSR). The susceptibility of each relative density (30%, 50% and 70%) of the soil experiencing liquefaction and the cyclic stress ratio (0.15, 0.20 and 0.25) will be varied to see the amount of cyclic load needed until the soil experiences liquefaction, the load frequency to represent the earthquake load is 1 Hz with sinusoidal waves. This study will test the fine sands from Palu, Central Sulawesi, Indonesia, to determine their respective behavior when the soil is given a cyclic load.

Keywords: Cyclic Triaxial, Liquefaction, Cyclic Stress Ratio, Relative Density, Fine Sands.

How to cite: Andanawarih, M. F. and Prakoso, W. A.: The Effect of Cyclic Loading of Liquefaction on Palu Sands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11043, https://doi.org/10.5194/egusphere-egu21-11043, 2021.

Soil liquefaction in seabed not only drives vertical sediment transportation but also weakens coastal infrastructures such as pipeline or underwater foundation. The damage of seabed liquefaction events had been documented in literature. Due to the threat to lives and environment, it is important to have an exhaustive analysis on the risk assessment of seabed liquefaction subjected to ocean waves. The objective of this study is to assess the critical wave condition triggering seabed liquefaction in oceanic environment through theoretical modelling. Our investigation focused on the wave condition of momentary liquefaction induced by periodic wave loading. The scenario considers a permeable seabed on which a wide range of ocean waves propagates, and the critical wave parameters of wave height and wavelength causing liquefaction are examined. A two-dimensional analytical solution of seabed response based on Biot’s consolidation theory is applied with nonlinear water wave theory to predict the soil response and the consequence of liquefaction.  In contrast to the previous studies of seabed response applying the analytical solutions which only valid for a restricted wave range, we use a numerical approach of finite-amplitude wave to reflect the nonlinearity effect in a wide range of water wave from deep water to shallow water.  The present assessment of liquefaction is compared with the extant solution of seabed response under Stokes wave and cnoidal wave for validation. In additional, the potential of liquefaction instability is performed by a critical curve of wave condition covering the range of ocean waves from deep water to shallow water. Our study provides advanced theoretical framework and robust mathematical model for the assessment of wave-induced seabed instability under water waves, and the detailed analysis sheds insight into the impact of ocean waves on the seabed liquefaction.

How to cite: Hsu, C.-J. and Hung, C.: Critical trigger condition of seabed liquefaction under ocean waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4900, https://doi.org/10.5194/egusphere-egu21-4900, 2021.

The Seismic Microzonation studies (SMs), promoted all over the Italian territory by the Department of Civil Protection, provide fundamental knowledge of the subsoil response in seismic conditions at the urban scale. Amplification phenomena related to lithostratigraphic and morphological characteristics, instabilities and permanent deformations activated by the earthquake, are highlighted in hazard maps produced at increasing reliability levels (level 1 to 3 of SM). In particular, zones prone to liquefaction instability are firstly identified following the predisposing factors, such as geological and geotechnical characteristics and seismicity. The robustness of the definition of these areas is strongly correlated to the availability and the spatial distribution of surveys. Moreover, the typology and quality of the investigations considerably influence the method of analysis and the degree of uncertainty of the results.

This work aims to establish an updated procedure of the actual SM guidelines and integrates recent research activities at different levels of SMs, to improve the hazard maps accuracy in terms of liquefaction susceptibility. For the scope, the case of the Calabria region in the south of Italy, well known for the high level of seismicity, was studied. At a regional scale, the base-level analysis was implemented for a preliminary assessment of the Attention Zones (AZ), potentially susceptible to liquefaction. The predisposing factors were implemented at a large scale, taking advantage of geostatistical tools to quantify uncertainties and filter inconsistent data. The regional-scale analysis allowed to highlight areas prone to liquefaction and effectively addressed the subsequent level of analysis. At a local scale, the quantitative evaluation of the liquefaction potential was assessed using simplified methods, integrating data from different survey types (CPT, SPT, Down-Hole, Cross-Hole, MASW) available in SM database. The definition of Susceptibility Zones (SZ) was provided considering additional indexes, combining the results obtained from different surveys typologies and quantifying the uncertainty due to the limited data availability with geostatistical methods. The analyses at the regional and municipality scale were matched with seismic liquefaction evidence, well documented in past seismic events. This multi-scale process optimises resource allocation to reduce the level of uncertainty for subsequent levels of analysis, providing useful information for land management and emergency planning.

How to cite: Spacagna, R. L., Cesarano, M., Fabozzi, S., Peronace, E., Porchia, A., Romagnoli, G., and Moscatelli, M.: Geostatistical approach for multi-scale seismic liquefaction risk assessment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12789, https://doi.org/10.5194/egusphere-egu21-12789, 2021.

The large majority of existing studies of seismic liquefaction effects on structures considers them isolated, i.e., far from other structures. The same holds for design methods for liquefaction mitigation techniques. Hence, there is very little consideration for structure-soil-structure-interaction (SSSI) effects that appear unavoidable in urban environments. This paper explores numerically these SSSI effects for structures on surface foundations by performing fully coupled non-linear dynamic analyses with the finite difference method (FLAC) and a state-of-the-art constitutive model (NTUA-SAND) for the liquefaction response of loose, saturated granular soils. It shows that SSSI effects may prove both beneficial (e.g., settlement reduction) and detrimental (e.g., tilt apparition) depending on the dimensions, distance and static loading of the neighboring structures. These SSSI effects become more complex when ground improvement methods are used in one of the structures, but not its neighbors. By considering three alternative types of ground improvement that have a completely different rationale (perimetric walls, colloidal silica grouting, gravel columns), this paper also shows numerically that, regardless of its type, ground improvement in one structure may potentially prove detrimental to its neighboring unimproved structures (e.g., increase of tilt).

How to cite: Papadimitriou, A., Tsepelidou, N., Sideris, A., Pavlopoulou, A., and Katsoularis, V.: Seismic soil liquefaction under shallow founded structures and its mitigation in urban environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4620, https://doi.org/10.5194/egusphere-egu21-4620, 2021.

In May and June 2012, Emilia region (Italy) was struck by a seismic crisis characterized by more than 2000 earthquakes with two main shocks (20 May and 29 May events with ML 5.9 and 5.8, respectively) and several earthquake-induced effects. Relevant liquefaction events were observed all over the area showing a maximum intensity at San Carlo and Mirabello, two main hamlets in the Terre del Reno Municipality. In this work, a methodology is proposed for assessing liquefaction susceptibility in wide areas characterized by complex geo-stratigraphic conditions through a multi-level approach based on simplified models. To this aim, extensive geological studies and more than one thousand geophysical and geotechnical surveys available from previous studies have been collected in a dedicated geographical information system. The database is structured to guarantee data and metadata harmonization and standardization, useful for the realization of an integrated and interoperable system progressively supplemented with new information. Preliminary 2D and 3D high resolution geological and geotechnical models are elaborated to reconstruct the complex subsoil setting of Terre del Reno area.  This study forms the base for the 2D numerical modelling carried out with a finite difference code (FLAC) to identify the mechanism of pore pressure increase and of liquefaction triggering. The rationale behind this study concerns the definition of a simplified approach based on synthetic indicators. Specifically, starting from parametric analyses, the role of different variables on the triggering process is evaluated together with the definition of set of thresholds able to model the occurrence of liquefaction effects. The spatial variability of the soil properties, layering and mechanical characteristics is considered with a geo-statistical approach. A comparison between the liquefaction effects observed in 2012 and the results obtained from calculations is performed for demonstrating the reliability of the proposed approach in extensively simulating a liquefaction occurrence.

How to cite: Varone, C., Baris, A., Caciolli, M. C., Fabozzi, S., Gaudiosi, I., Marcini, M., Martelli, L., Modoni, G., Moscatelli, M., Paolella, L., Simionato, M., Spacagna, R. L., and Razzano, R.: PERL: A multilevel strategy for liquefaction hazard assessment in complex stratigraphic successions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9771, https://doi.org/10.5194/egusphere-egu21-9771, 2021.

At the early stages of seismology, seismic stations were installed directly on rock to minimize the effects of the fine sediments/weathering on the recorded seismic waves. The bulky size of permanent installation seismometers, their need for external batteries, cables and levelling, led to place seismic stations on artificial ground, such as ad hoc concrete platforms. In addition, to ensure protection from environmental conditions, vandalism and to facilitate maintenance, many seismic stations were placed inside structures. A common installation in Italy, as an example, is at the base of the (5-8 m tall) towers of the electrical national service.

The presence of a structure around the instrument perturbs the recorded motion. This phenomenon, generally referred to as soil-structure interaction, can be summarized into three main effects. The first one is the transmission of the structure own motion to the surrounding ground. When seismic waves hit a building, the building enters forced oscillation and this vibration is re-transmitted to the ground. Sensors placed inside the building record, therefore, a composite signal, made of seismic waves and the response of the structure to them. This affects the sensors also when they are isolated from the building foundations by means of cuts around the sensor pillars, because the ground under the pillar and the ground under the structure is the same and is continuous. The second effect lays in the fact that a foundation, typically made of reinforced concrete, acts as a layer with seismic impedance much higher than any natural soil. Seismic waves travelling upwards will be reflected downwards as they hit the foundation. On one side they shake the structure (effect 1), but on the other only a small fraction of them crosses the foundation (effect 2) and can be recorded by the instruments installed on the foundation. The same applies to the concrete pillars where seismic sensors are installed. These installations violate the basic principle of any physical measurements according to which when an interface is needed between the instrument and the object of measurement (the ground) then the interface must have an impedance as close as possible to the object of measurement, in order to minimize the perturbation of the wavefield. Clearly concrete platforms/pillars do not have this property, unless when installed on very stiff rocks. The third main effect (effect 3) concerns the back reflection of the surface waves reaching the foundation. Similarly to effect 2, when surface waves strike an extended rigid layer, such as the foundations of a building, they are mainly reflected back along the Earth's surface. This implies that, in seismic tremor recordings (or seismic events) carried out inside a structure, a fraction of surface waves will be missing.

In this work we show these effects in a number of real cases and we show the consequences that this can have in the assessment of seismic site effects, of PGA, and on the computation of attenuation laws.

How to cite: Castellaro, S., Alessandrini, G., Musinu, G., and Del Vecchio, M.: Seismic stations and soil-structure interaction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3421, https://doi.org/10.5194/egusphere-egu21-3421, 2021.

Estimation of site effects is an essential part of local seismic hazard and risk assessment, especially in densely populated urban areas. The goal of this study is to assess the site response variability in the city of Lucerne (Central Switzerland), located in a basin filled with unconsolidated deposits. Even though it is a low-to-moderate seismicity area, the long-term seismic risk cannot be neglected, in particular, because the region was struck by strong earthquakes in the past (i.e. Mw 5.9 in 1601).

To determine the spatial distribution of the soil response in the test area, we combined earthquake and ambient noise recordings using the Hybrid Standard Spectral Ratio method (SSRh) introduced by Perron et al. (2018). In the first step, we installed a temporary seismic network to record ground-motion from low-magnitude or distant earthquakes. At selected urban sites inside the sedimentary basin, the dataset was used to estimate the amplification factors with respect to a rock site using the Standard Spectral Ratio approach (SSR - Borcherdt, 1970). Then, a survey including several dozens of densely distributed single-station ambient noise measurements was performed which enabled us to estimate the basin response variability relative to the seismic stations of the temporary seismic network. Finally, we corrected the noise-based evaluation using the SSR amplification functions. To verify the useability of the presented technique in the Lucerne area, we applied the SSRh method also to the temporary stations, the resulting amplification functions largely coincide with the SSR curves. However, the daily variability of the noise wavefield due to human activities can slightly affect the results. We will also discuss the influence of the station distribution and density of the temporary network deployment.

The amplification model for the Lucerne area estimated using the SSRh method shows consistency with geological data. The results indicate that seismic waves can be amplified up to 10 times in some parts of the basin compared to the rock site. The highest amplification factors are observed for frequencies between 0.8 and 2Hz. This means a local significant increase in seismic hazard.

The presented work is a part of a detailed site response analysis study for the Lucerne area, considering 2D and 3D site effects and potential non-linear soil behaviour. This PhD project is performed in the framework of the Horizon 2020 ITN funded project URBASIS-EU, which focuses on seismic hazard and risk in urban areas.

REFERENCES

Borcherdt, R.D., 1970. Effects of local geology on ground motion near San Francisco Bay. Bull. Seismol. Soc. Am. 60, 29–61.

Perron, V., Gélis, C., Froment, B., Hollender, F., Bard, P.-Y., Cultrera, G., Cushing, E.M., 2018. Can broad-band earthquake site responses be predicted by the ambient noise spectral ratio? Insight from observations at two sedimentary basins. Geophys. J. Int. 215, 1442–1454.

How to cite: Janusz, P., Perron, V., Knellwolf, C., Imperatori, W., Bonilla, L. F., and Fäh, D.: Combining recordings of earthquake ground-motion and ambient vibration analysis to estimate site response variability in the city of Lucerne, Switzerland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4380, https://doi.org/10.5194/egusphere-egu21-4380, 2021.

Local site conditions can strongly influence the level of amplification of ground-motion at the surface during an earthquake. Especially near-surface low velocity sediments overlying stiffer seismic bedrock modify earthquake ground motions in terms of amplitudes and frequency content, the so-called site response. Earthquake ground-motion site response is of great concern because it can lead to amplified surface shaking resulting in significant damage on structures despite small magnitude events. The Netherlands has tectonically related seismic activity in the southern region with magnitudes up to 5.8 measured so far. In addition, gas extraction in the Groningen field in the northern part of the Netherlands, is regularly causing shallow (3 km), low magnitude (Mw max= 3.6), induced earthquakes. The shallow geology of the Netherlands consists of a very heterogeneous soft sediment cover, which has a strong effect on seismic wave propagation and in particular on the amplitude of ground shaking.

The ambient seismic field and local earthquakes recorded over 69 borehole stations in Groningen are used to define relationships between the subsurface lithological composition, measured shear-wave velocity profiles, horizontal-to-vertical spectral ratios (HVSR) and empirical transfer functions (ETF). For the Groningen region we show that the HVSR matches the ETF well and conclude that the HVSR can be used as a first proxy for earthquake site-response. In addition, based on the ETFs we observe that most of the seismic wave amplification occurs in the top 50 m of the much thicker sediment layer. Here, a velocity contrast is present between the very soft Holocene clays and peat on top of the stiffer Pleistocene sands.

Based on the learnings from Groningen we first constructed sediment type classes for the Dutch subsurface, each class representing a level of expected amplification. Secondly, the HVSR curves are estimated for all surface seismometers in the Netherlands seismic network and a sediment class is assigned to each location. Highest HVSR peak amplitudes are measured at sites with the highest level of amplification of the sediment classification. Based on this correlation and the presence of a detailed shallow geological model at most sites in the Netherlands, a simplistic approach is presented to predict amplification at any location with sufficient lithologic information. With this approach based on the shallow sediment composition, we can obtain constraints on the seismic hazard in areas that have limited data availability but have potential risk of seismicity, for example due to geothermal energy extraction.

How to cite: van Ginkel, J., Ruigrok, E., and Herber, R.: An approach to construct a Netherlands-wide ground-motion amplification model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1657, https://doi.org/10.5194/egusphere-egu21-1657, 2021.

Measuring ground resonances is of great importance for seismic site amplification studies. The task is usually addressed with the common H/V (horizontal to vertical spectral ratio) approach, which is widely used for both microzonation studies and stratigraphic imaging. Peaks on the H/V function are used to identify ground resonance frequencies, usually assuming 1D site conditions, i.e. with plane-parallel stratigraphy. In the simple case of a horizontal soft layer overlying a bedrock, 1D resonance is linked to the local bedrock depth (as a function of the shear wave velocity of the sediment layer). Therefore, when the 1D approximation holds, spatial variations of the resonance frequency reflect changes of bedrock depth (when lateral homogeneity of the sediment cover can be assumed). However, at sites with non-plane subsurface geometries, more complex resonance patterns may develop, such as 2D resonance patterns that typically occur within sediment-filled valleys. In this case, 2D resonance involves simultaneous vibration of the whole sedimentary infill at the same frequency, which may lead to large seismic amplification. 2D ground resonances can no longer be linked to the local depth-to-bedrock directly below the measurement site, but depend on the whole valley geometry and mechanic properties. Distinguishing between the 1D and 2D nature of a site is mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment.

We investigated the problem in the Bolzano sedimentary basin (Northern Italy), which lies at the intersection between three valleys, using a single-station microtremor approach, the same usually applied for H/V surveys. We observed that the footprints of 1D and 2D resonances reside in different behaviors along the three components of motion. This is because, while the dynamic behavior of a 1D-site is the same along all horizontal directions, 2D resonances differ along the longitudinal and transversal directions of the resonating body, e.g. parallel and perpendicular to the valley axis. In addition, 2D resonance modes involve also a vertical component. This implies that the H/V method, by mixing the information along the three components, is not suitable to detect 2D resonances, that can be acknowledged only by looking at the individual spectral components and not at the H/V curves alone.

By analyzing several hundred single-station microtremor measurements, we identified a list of frequency and amplitude features that characterize 1D and 2D resonances on individual spectral components of motion and on H/V ratios, on a single measurement and on several measurements acquired along profiles across the investigated valleys. We identified valleys characterized by 1D-only, 1D+2D and 2D-only resonance patterns and we propose a workflow scheme to conduct experimental measurements and data analysis in order to directly assess the 1D or 2D resonance nature of a site with a single-station approach, rather than evaluating this indirectly with numerical modelling.

How to cite: Sgattoni, G. and Castellaro, S.: A workflow to discern 1D and 2D ground resonances with single-station microtremor measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3430, https://doi.org/10.5194/egusphere-egu21-3430, 2021.

Although it is nowadays desirable and even typical to characterise site conditions in detail at modern recording stations, this is not yet a general rule in Greece, due to the large number and geographical dispersion of stations. Indeed, most of them are still characterised merely through geological descriptions or proxy-based parameters, rather than through in-situ measurements. Considering: 1. the progress made in recent years with sophisticated ground motion models and the need to define region-specific rock conditions based on data, 2. the move towards large open-access strong-motion databases that require detailed site metadata, and 3. that Greek-provenance recordings represent a significant portion of European seismic data, there are many reasons to improve our understanding of site response at these stations. Moreover, it has been shown recently in several regions that even sites considered as rock can exhibit amplification and ground motion variability, which has given rise to more scientific research into the definition of reference sites. For Greece, in-situ-characterisation campaigns for the entire network would impose unattainable time/budget constraints; so, instead, we implement alternative empirical approaches using the recordings themselves, such as the horizontal-to-vertical spectral ratio technique and its variability. We present examples of 'well-behaved', typical rock sites, and others whose response diverges from what is assumed for their class.

How to cite: Ktenidou, O.-J., Gkika, F., Pikoulis, E.-V., and Evangelidis, C.: Hard as a rock? Looking for typical and atypical reference sites in the Greek network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13659, https://doi.org/10.5194/egusphere-egu21-13659, 2021.

The seismic subsoil response in terms of amplification or attenuation of the ground motion is the result of a complex combination of factors, including the vertical and horizontal subsoil heterogeneities (Fabozzi et al., 2021). In volcanic areas in particular, the vertical subsoil heterogeneities are well identified by characteristic superposition of stiffer volcanic horizons on softer levels, giving rise to stiff-soft alternating layers, also in the form of multiple Vs inversions with the depth. This condition is typical of sheet-like blankets of lava or pyroclastic deposits, extensively covering the sedimentary substratum, frequent in the peripheral areas of large basaltic stratovolcanos or in areas adjacent to large explosive acidic volcanic edifices. The aim of the present work is to study the effect of such vertical heterogeneities on the seismic site response. With this end, in correspondence of volcanic areas identified by means of a preliminary geological screening in the Italian territory, subsoil properties relevant for seismic site response analyses were extracted from the Italian database of the seismic microzonation studies (DB-SMs in DPC, 2018), which is available at www.webms.it and is developed and maintained by CNR IGAG (National Research Council of Italy, Institute of Environmental Geology and Geoengineering, www.igag.cnr. it). The collection of input data was used for an extensive one-dimensional equivalent linear numerical site response analyses, in order to evaluate the influence of stiffness inversions on ground motion at surface. In particular, different idealized subsoil 1D models of the identified geological areas were defined in terms of variation of layers thickness, shear wave velocity and nonlinear properties. The effect of the variability of these parameters on the seismic site response in terms of amplification factors (ICMS, 2008) was studied parametrically.

References

• DPC, Dipartimento della Protezione Civile, 2018. Commissione tecnica per il supporto e monitoraggio degli studi di Microzonazione Sismica (ex art.5, OPCM3907/10), (2018) WebMs; WebCLE. A cura di: Maria Sole Benigni, Fabrizio Bramerini, Gianluca Carbone, Sergio Castenetto, Gian Paolo Cavinato, Monia Coltella, Margherita Giuffrè, Massimiliano Moscatelli. In: Giuseppe Naso. Andrea Pietrosante, Francesco Stigliano.
• Fabozzi S., Catalano S., Falcone G., Naso G., Pagliaroli A., Peronace E., Porchia A., Romagnoli G., Moscatelli M. (2021) Stochastic approach to study the site response in presence of shear wave velocity inversion: application to seismic microzonation studies in Italy. Engineering Geology https://doi.org/10.1016/j.enggeo.2020.105914.
• ICMS, 2008. Indirizzi e Criteri per la Microzonazione Sismica. In: Gruppo di lavoro ICMS. Conferenza Delle Regioni E Province Autonome - Dipartimento Della Protezione Civile. https://www.centromicrozonazionesismica.it/it/download/category/7-indi rizzi-e-criteri-per-lamicrozonazione-sismica (In Italian).

How to cite: Fabozzi, S., Catalano, S., Naso, G., Pagliaroli, A., Peronace, E., Porchia, A., Romagnoli, G., and Moscatelli, M.: Effect of stiff-soft alternanting layers in volcanic areas on seismic site response, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10181, https://doi.org/10.5194/egusphere-egu21-10181, 2021.

Similar to other big cities in Central Asia (such as Tashkent, the capital of Uzbekistan, or Bishkek, the capital of Kyrgyzstan), the capital of Tajikistan, Dushanbe, is highly exposed to earthquake and associated secondary hazards due to its close vicinity to two active fault systems, the Hissar–Kokshal Fault located in the north of the city, and the Iyak–Vaksh Fault in the south. The most recent damaging earthquake near Dushanbe was located in the Tajik Depression in western Tajikistan, the Hissar Earthquake in 1989 (M = 5.5), causing small direct damage on buildings, but triggered extensive liquefaction phenomena and related landslide in loess deposits. The villages of Sharora and Okuli-Bolo were affected by mudflows destroying more than 100 houses, and 247 persons died.  

To ensure people’s safety, especially for a rapidly growing city such as Dushanbe, adequate constructions and a detailed seismic microzonation map (and related data) are the keys for sustainable urban planning. Existing estimations of seismic hazards date back to 1978; they are based on engineering geological investigations and observed macroseismic data. These were used to create the Tajik Building Code which considers seismic intensities according to the Medvedev–Sponheuer–Karnik Scale, MSK-64. However, this code does not accurately account for soil types which vary considerably in Dushanbe – not only by their nature but also due to increasing anthropogenic alteration. In this study, we performed a series of analyses on Microtremor Array Measurements, Seismic Refraction Tomography, and instrumental data recording from permanent as well as from mobile seismic stations (H/V method) in order to provide the site effect analysis for a new comprehensive microzonation of Dushanbe (and neighboring areas) accounting for the different soil types. Our results identify several critical areas where major damage is likely to occur during strong earthquakes.

How to cite: Hakimov, F., Domej, G., Ischuk, A., Reicherter, K., Cauchie, L., and Havenith, H.-B.: Site effect analysis to support the future seismic microzonation of Dushanbe, Tajikistan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16521, https://doi.org/10.5194/egusphere-egu21-16521, 2021.

In this work we analyzed the three-dimensional seismic site response of the Central Archaeological Area of Rome, which includes the Palatine Hill, Roman Forum, and Coliseum area. The study area is characterized by complex site conditions (stratigraphy, dynamic properties, surficial and buried morphology, etc). Detailed three-dimensional large-scale model was built in order to evaluate site response using dynamic numerical modelling approach. The explicit finite‐difference code FLAC3D (ITASCA Consulting group Inc., 2017) was used for numerical simulations.

The area of Rome is affected by earthquakes from different seismogenic districts: (1) the central Apennine mountain chain, located about 90–130km east of Rome (M = 6.7–7.0); (2) the Colli Albani volcanic area located 20km to the south of the city (M=5.5); and (3) the Rome area itself characterized by rare, shallow, low-magnitude events (M < 5). Both artificial and natural accelerograms were then simulated to be compatible with the reference spectra associated to the three earthquake scenarios.

This study highlights the role of local geological and geotechnical conditions producing amplification of seismic ground motion. The analyses show maximum amplification factors, defined in terms of Housner Intensity ratio for three periods range (0.1-0.5; 0.5-1.0 and 1.0-2.0), as high as 2.2–2.4 over the period range of 0.1–1.0 s. Such values can be significantly relevant for the monumental and archaeological heritage of this area, as many are highly vulnerable due to their great age. Physical phenomena controlling site response are discussed on the basis of buried and surficial morphology and lithostratigraphic conditions.  Finally, microzonation maps are produced in order to ascertain the seismic hazard of the examined area and, consequently, to assess possible parameters for seismic retrofitting of the monuments.

How to cite: Razzano, R., Moscatelli, M., Pagliaroli, A., Mancini, M., Stigliano, F., and Lanzo, G.: Three-dimensional numerical modelling of site effects in the Palatine Hill, Roman Forum, and Coliseum archaeological area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9647, https://doi.org/10.5194/egusphere-egu21-9647, 2021.

Earthquakes are – amongst many others – one type of triggering factors for mass movements in mountainous regions such as landslides, deep-seated gravitational slides (DSGSD), rockfalls, mudflows, etc. Hence, the emerging hazard would require an area-wide assessment of seismogenic impact to better apprehend the interplay of different triggering factors contributing to mass movement activity. However, seismicity itself is difficult to assess for several reasons. On the one hand, there are various parameters describing ground motion, and not all of them are suitable for area-wide assessments due to their availability or complexity. On the other hand, phenomena such as attenuation and topographic amplification must be taken into account, especially when the region of interest is an orogen.

Considering the criteria mentioned above and aiming for a mapping approach ascribing one value of seismogenic impact to one geographic location, we developed a strategy based on two empirical laws approximating Arias Intensity: the first law estimates Arias Intensities for a particular location as a function of earthquake magnitudes and focal depths; the second law corrects these estimated Arias Intensities in relation to the height differences to the nearest channel beds. Finally, we sum all corrected Arias Intensities resulting from different earthquakes in one particular location. Values obtained in this last step do not represent a physical entity; nevertheless, they allow for quantitative assessment of seismic exposure with respect to a given earthquake dataset covering a specific time frame, also allowing for color coding and comparative mapping approaches in GIS for other factors triggering mass movements.

In our case study, we assess the seismic exposure of a set of several hundreds of landslides, DSGSD, and rockfalls located in a rectangular area in the Italian Central Alps. In a first step, the area was discretized using a quadratic grid with increments of 1 km in order to assign points of evaluation to the previously mapped polygons representing landslides, DSGSD, and rockfalls. Additionally, to each polygon, a centroid point was attributed to avoiding the loss of polygons smaller than 1x1 km. In a second step, we computed the seismic exposure in each point resulting from two earthquake datasets covering the Alps, including a 500 km wide buffer zone: instrumental earthquake data of the ISC Bulletin covering a period from 1900 to 2019; macro-seismic earthquake data of the SHARE European Earthquake Catalog covering a period from 1000 to 2006.

The study serves as a preliminary test for assessing wider areas across the Alps, which either geologically or geographically belong together. We illustrate our mapping approach in a series of maps discussing the effects of the number of earthquakes, magnitudes, distances, topography, and time frame.

How to cite: Domej, G., Frattini, P., Valbuzzi, E., and Crosta, G. B.: Quantifying the seismogenic impact on mass movements in the Alps in terms of Arias Intensity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1511, https://doi.org/10.5194/egusphere-egu21-1511, 2021.

The morphological evolution of landslide slopes is generally controlled by the combination of weathering, tectonics, gravity and river erosion. Among them, seismic shaking plays a fundamental role in landslide activity and mobility in high seismicity regions. It can result in important modifications of landslide geometry and consequently, of its response to external loadings. In particular, morphological changes in landslide slope can imply changes in the interactions between seismic waves and landslide mass, which could theoretically modify the hazard related to the earthquake-induced effects. This study aims at pointing out the effects of slope morpho-evolution on the long-term modification of earthquake-induced landslide dynamics, which is here quantified in terms of expected seismically induced displacements, considering unaltered seismic hazard conditions. The Albuñuelas landslide was selected, located in Andalusia (South Spain) which is one of the most seismic regions of Spain. This landslide is a large roto-translational process whose last earthquake-induced reactivation occurred during the 1884 Andalusia Earthquake (M_w 6.5), causing relevant damages to the Albuñuelas village. Data available from field surveys and geophysical investigations, allowed to derive the current engineering-geological model of the landslide slope. According to the available geological and geomorphological data, the slope shape was back-deformed to reproduce the landslide geomorphological evolution sequence over time, until its first-time failure. The reconstructed sequence is consistent with a geomorphological evolution mainly driven by the combination of earthquake-induced re-activations and low rates of deformation caused by the intense incision of the Albuñuelas River, responsible for the valley deepening. 2D-dynamic stress-strain numerical simulations were performed on several stages of such sequence applying 17 equivalent signals derived following the LEMA_DES (Levelled-Energy Mutifrequential Analysis for Deriving Equivalent Signals) approach with an Arias Intensity of 0.1 m/s, according to the Andalusia regional seismic hazard. The outputs were expressed in terms of seismically induced displacements vs. characteristic periods diagrams, in order to highlight the role of signal frequency content as well as the effect of the landslide 2D-geometry (T_l) and thickness (T_s) on the resulting displacements. Since the morpho-evolution resulted in a progressive increasing of the landslide mass length and its dislodgment into several blocks since the first-time failure, the landslide mobility was analysed over time at each single-block scale. The comparison revealed a not neglectable modification of the Albuñuelas landslide susceptibility to the local seismic hazard over time, highlighting the necessity to understand the mechanisms driving the natural system evolution to provide more reliable earthquake-induced hazard scenarios.

How to cite: Mita, M., Bourdeau, C., Delgado, J., Lenti, L., and Martino, S.: Influence of morpho-evolution on the earthquake-induced mobility of the Albuñuelas landslide (Granada, Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12886, https://doi.org/10.5194/egusphere-egu21-12886, 2021.

Seismic stability evaluation plays a crucial role in landslide disaster risk reduction. Related modeling also has to consider the potential influences of the rainfall on the hillslopes. This study aims at understanding the relative influence of the seismic loading and extreme cumulative rainfall on a massive active landslide in the seismically active Vrancea-Buzau region of the Romanian Carpathians (45° 30' 23" N, 26° 25' 05" E). This region has been subjected to more than 700 earthquakes (M>4) events with the highest magnitude of 7.2 (M_w) during the year 1960-2019. Rainfall data of the year 2000-2019 revealed the occurrence of relatively intense rainfall events, especially during the last ten years. The landslide has an aerial dimension of ~9.1 million m². It hosts the small village of Varlaam at the toe along the Bisca River. The slope (with an average gradient of 15-20°) is covered by shrubs and scattered trees near its borders and is relatively barren in the central part. Shales with some intercalated sandstone layers belonging to the Miocene thrust belt constitute the rocks of the slope.   

A first survey involving the multi-station array and related Horizontal-to-Vertical noise Spectral Ratio (HVSR) measurements was completed in summer 2019. The findings of the HVSR were processed using the inversion process to infer the shear wave velocity distribution with depth and to detect the sliding surface of the landslide. These velocities were further used to estimate the geotechnical properties of the subsurface using the empirical equations. The HVSR based depth profiles and the Unmanned Air Vehicle based topographic information were used to take four 2D slope sections. These sections were considered for 2D discrete element modeling based stability evaluation under static and dynamic condition along with sensitivity analysis. Static simulation was used to determine the Factor of Safety (FS) using the shear strength reduction approach. Ricker wavelet was used as input seismic load in the dynamic simulation. Potential run-out and flow characteristics of the slope material were explored using the Voellmy rheology based RAMMS software. The relationship between rainfall, surface runoff, and soil moisture was also explored to understand the hydrogeological influence on slope stability.

Though the slope reveals meta-stability (1.0<FS<2.0) condition under static loading, displacement in the soil reaches up to 1.5 m that further increases to 2.8 m under dynamic loading. According to the topographic characteristics of the slope and to the presence of landslide material or intact bedrock near the surface, acceleration along the slope reaches a Peak Ground Acceleration in the range of 0.6 to 1.3g. Eight extreme rainfall events (>50mm/24 hours) during the year 2000-2019 are noted to temporally coincide with enhanced surface runoff and increased soil moisture in the region. Debris flow runout modeling indicated that the slope material may attain a maximum flow height and flow velocity of 13±0.8 m and 5±0.5 m/sec, respectively, along the river channel.

Keywords: Landslide; Earthquake; Slope stability; Runout; SE Carpathian

How to cite: Kumar, V., Cauchie, L., Mreyen, A.-S., Cerfontaine, P., Micu, M., and Havenith, H.-B.: Inferring hillslope response under seismic loading and rainfall: A case study from the SE Carpathian, Romania, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3276, https://doi.org/10.5194/egusphere-egu21-3276, 2021.

The seismic sequence that struck Central Italy in 2016 was characterized by three main shocks respectively occurred on August 24^th Mw 6.0; October 26^th Mw5.9 and October 30^th Mw 6.5. The seismic sequence caused several ground effects over a large area of ​​the central Apennine mountain range, mainly affecting transportation routes.

In the aftermath of the sequence, several research groups mapped around 820 landslides involving road cuts in rock and fill slopes over an area of about 2000 km² (GEER,ISPRA, C.E.R.I. by Roma La Sapienza). These data are summarized in the CEDIT catalog by Martino et al., (2017), where almost 150, 250 and 420 instability phenomena were respectively triggered by each mainshock. Further updates were carried out by the Authors in the framework of the Reluis projects of the Department of Civil Protection. In particular, other 550 phenomena were mapped by interpretation of aero photos provided by google-earth. For some of the largest ones, field surveys were carried out for mechanical, structural, and geometrical characterization.

The dataset distribution was analyzed with geological, geomorphological, and seismic parameters, such as lithology, fault distance, landslide run-out, estimates of mobilized volumes, distance from the epicenter, PGA, and damages.

The triggered events are mainly characterized by Category I of Keefer (1984) classification, namely rockfalls and rockslides. The maximum triggering distance resulted as high as 50 km far from the epicenter. The most affected areas are characterized by ridge crests or flanks of valleys in carbonate rocks.

This study permitted to highlight the most relevant parameters for the assessment of earthquake-triggered susceptibility for the study area and identify some meaningful and critical case studies for the future development of the research.

How to cite: Forte, G., De Falco, M., Iannicelli, F., and Santo, A.: Analysis of a database of landslides triggered by the 2016 Central Italy seismic sequence , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1597, https://doi.org/10.5194/egusphere-egu21-1597, 2021.