The main concern of the occurrence of an earthquake is the ground shaking, although past events worldwide demonstrated that damage and death toll depends on both the strong ground motion and the ground effects. The variability of earthquake ground motion is caused by local geological conditions beneath a given site, due to the stratigraphic or topographic setting that can give rise to amplification and resonances. Earthquake-induced ground effects are mainly landslides, soil liquefaction, and ground subsidence. They can affect an area with damages related to the full collapse or loss in functionality of facilities, roads, pipelines, and other lifelines. The purpose of this session is to provide a forum for discussion among researchers and other professionals who study seismic amplification of the ground motion and the related hazards and to encourage multidisciplinary research in these fields.
Topics of interest include the following:
- Subsoil investigation and characterization for Seismic Microzonation mapping;
- Evaluation of seismic site response (1D-2D-3D)
- Case histories of earthquake-triggered landslides analyzed at either local or regional scale
- Slope stability analyses and runout modeling of seismically/volcanically-induced landslides;
- Studies on Soil liquefaction and earthquake-induced subsidence
A focused special issue in an EGU-journal will be edited based on the contributions of this session.
vPICO presentations: Mon, 26 Apr
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 Mw = 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 Emilia-Romagna seismic sequence in 2012 has increased the interest among Italian researchers in predicting liquefaction under seismic shaking, and in the evaluation of damage induced to structures. A number of studies were carried out during the last decade to evaluate the liquefaction susceptibility of different areas of the Italian Peninsula. Some of these studies have been focused on the territorial analysis of Naples (Evangelista & Santucci de Magistris, 2011; Silvestri & d’Onofrio, 2014), which highlighted how saturated pyroclastic soils present along the coastal areas may be interested by liquefaction phenomenon. On such a basis, the present study aims at evaluating the liquefaction susceptibility throughout the area of three municipalities (Casamicciola, Lacco Ameno and Forio) of Ischia Island in the gulf of Naples (Italy), recently hit by a Ml 4 earthquake. The coastal zones of these municipalities are characterised by the predominance of saturated pyroclastic granular deposits. The assessment was performed through a multi-level approach, i.e. by increasing level of complexity. First, the potentially liquefiable areas were delimited by combining in a Geographic Information System (GIS) data on the average seasonal depth of the water table (Piscopo et al. 2019) and on the lithological classification of the surface deposits (Seismic Microzonation, 2017). At some representative sites in these potentially liquefiable areas, simplified analyses were carried out using SPT-based semi-empirical methods (Idriss & Boulanger, 2014). The results of such analyses led to choose a specific site on which to perform non-linear ‘coupled’ dynamic analyses in time domain with the SCOSSA code (Tropeano et al. 2019). The results of the coupled analyses in terms of excess pore water pressure ratio (ru) then allowed the evaluation of the ‘Induced Damage Parameter’ (Chiaradonna et al. 2020), related to the free-field post-seismic volumetric consolidation settlement, which was classified as ‘moderate’ in this case. The procedure adopted may be a valid proposal for prompt evaluations of the liquefaction susceptibility, which allows to pass from a semi-qualitative assessment at a territorial scale to a quantitative assessment at the scale of a specific site.
Boulanger R.W.,Idriss I.M. (2014). CPT and SPT based liquefaction triggering procedures. Report No. UCD/CGM-14/01, Center for Geotechnical Modeling, University of California, Davis.
Chiaradonna A.,Lirer S.,Flora A., 2020. A liquefaction potential integral index based on pore pressure build-up. Engineering Geology, 272, 1-13.
Evangelista L.,Santucci de Magistris F. (2011). Upgrading the simplified assessment of the liquefaction susceptivity for the city of Naples, Italy. Proc of the V International Conference on Earthquake Geotechnical Engineering, Santiago, 10–13 January 2011, Paper n. 8.10.
Piscopo V.,Lotti V.,Formica F.,Lana F.,Pianese L., 2019. Groundwater flow in the Ischia volcanic island (Italy) and its implications for thermal water abstraction. Hydrogeology Journal, 28, 1-23
Silvestri F.,d’Onofrio A. (2014). Risposta sismica e stabilità dei centri abitati e infrastrutture. Relazione generale I Sessione “Analisi e gestione del rischio sismico”. Atti del XXV Convegno Nazionale AGI: La Geotecnica nella difesa del territorio e delle infrastrutture dalle calamità naturali.
Tropeano G.,Chiaradonna A.,d’Onofrio A.,Silvestri F. (2019). A numerical model for non-linear coupled analysis of the seismic response of liquefiable soils. Computers and Geotechnics, 105(2019):211–227, doi.org/10.1016/j.compgeo.2018.09.008
How to cite: Gargiulo, F., Sorvillo, G., d'Onofrio, A., and Silvestri, F.: Multi-level approach analysis of liquefaction susceptibility: an application to three municipalities of Ischia Island, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3184, https://doi.org/10.5194/egusphere-egu21-3184, 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.
Most of the towns, villages and infrastructures settled in Central Italy are placed nearby active faults and, consequently, the ground motion evaluation and the ground failures characterization under near-fault earthquakes are noteworthy issues to be investigated. The Madonna delle Fornaci - MDF – site, close to Pizzoli village (L’Aquila in Central Italy), has been selected as an emblematic site for assessing the effects induced by near-fault earthquakes, because it is located very close to the Pizzoli-Barete active Fault accountable for the February 2, 1703 Mw 6.67 earthquake. After this historical earthquake, remarkable surface manifestations, attributed to soil liquefaction and coseismic ground sinkholes, were observed at the MDF site, occurred in the Holocene alluvial deposit of the Aterno River, as witnessed by several written sources (among which Uria De Llanos, 1703). As concerns the geological setting, the MDF site is placed in the Plio-Quaternary NW-SE elongated L’Aquila intramontane basins which is bounded by a framework of active NW-SE trending and SW-dipping extensional faults which includes also the above mentioned Pizzoli-Barete active Fault. A comprehensive geophysical, geological, and geotechnical campaign has been carried out at the MDF site with the goal to obtain the seismic site characterization and the shallow and deep subsoil model preparatory to the quantitative estimation of the near-fault ground motion and the evaluation of the soil liquefaction potential induced by the 1703 seismic event.
The field survey consisted of three shallow continuous core drilling 15-20 m-deep boreholes; in one of the them, a down hole test and SPT measurements were conducted every 1 m depth; an open tube piezometer at the 11-12 m depth was installed in one of the boreholes; a couple of undisturbed samples were sampled for geotechnical laboratory tests; a MASW, Seismic refraction and ERT investigations were performed along two perpendicular 70-m long alignments; several single station microtremor measurements performed also in the neighbouring area. These data permitted preliminary to elaborate a quite confident 1-2D litho- and seismo-stratigraphic model for the MDF test site.
The MDF site is characterized by mainly calcareous grain-supported Holocene alluvial deposit: sandy gravel and gravelly sand with a silty component, sometimes predominant, in the matrix with water table level about 8-12 m b.g.l. Moreover, the following horizons are noteworthy to mention: an orange sand level at 11-12 m b.g.l. which could be considered preliminary as a liquefaction-prone level and an organic reddish-brown silty clay at 14-15 m b.g.l., which could be used for C14 dating.
Further, a 200 m-deep continuous core drilling borehole, executed nearby the MDF site by ISPRA for the mapping of the Italian geological sheet 348 “Antrodoco”, was also taken into consideration to obtain the complete 1D subsoil model for the near-fault ground motion amplification modelling.
The near-fault ground motion evaluation of the MDF site, considered as paradigmatic of the Central Italy seismicity, will go on through the geotechnical characterization of the alluvial deposits, the shear wave velocity versus depth profile and the seismic input evaluation to use for the numerical modelling.
How to cite: Tallini, M., Monaco, P., Spadi, M., Chiaradonna, A., and Papasodaro, F.: Seismic characterization of Pizzoli (Central Italy) to estimate site effects induced by near-fault earthquakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3169, https://doi.org/10.5194/egusphere-egu21-3169, 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.
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.
Assessing seismic site effects is essential in earthquake hazard studies. Local seismic amplification is strongly related to the site stratigraphy and topography, the dynamic properties of the subsoil deposits, and the earthquake features. The evaluation of these factors is mandatory to achieve a consistent model of the seismic hazard at small scale. Here we discuss the case of Castelnuovo village (L’Aquila, central Italy). Located on a small ridge, approximately 60 m higher than the valley floor, the village was heavily struck by April 6, 2009, Mw 6.3 L’Aquila earthquake, with catastrophic collapse of several buildings. Previous studies ascribed the observed damage to the presence of shallow caves beneath the buildings or to the topographic amplification.
In this work, an updated and detailed subsoil model for Castelnuovo site has been provided, based updated geological surveys, such as borehole logs and geophysical data consisting in microtremor measurements and down-hole.
These measurements identified resonant frequencies occurring in the range of 0.7-3.0 Hz. These frequency peaks are related to the presence of a velocity contrast at depth between the San Nicandro silts and the Madonna della Neve breccias, as indicated by the performed deep boreholes. Thanks to analytical, numerical, and geostatistical techniques, we identified the main impedance contrast at approximately 210 m depth from the top of the hill, much deeper than previous studies. These new findings allowed to create an accurate and consistent subsoil model summarized by two geological cross-sections of the Castelnuovo ridge, showing that the seismic site effects at the Castelnuovo village are mainly related to stratigraphic amplification.
How to cite: Spadi, M., Tallini, M., Albano, M., Cosentino, D., Nocentini, M., and Saroli, M.: Local Seismic site effects estimated by detailed seismic surveys: the case of Castelnuovo village (L’Aquila Basin, central Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6078, https://doi.org/10.5194/egusphere-egu21-6078, 2021.
A reliable prediction of the response of a soil column subjected to earthquake excitation is a basic although challenging achievement in geotechnical earthquake engineering problems.
A critical step is the analysis type selection. Nowadays, the equivalent linear approach is extremely widespread, mainly for its low computational demand and for its suitability to simulate soil behaviour up to the medium-strain level. However, this approach approximates the hysteresis loop exhibited by soils during a load cycle through an average shear modulus and damping ratio. Consequently, the nonlinear approach would more adequately describe the real soil behaviour, but it requires a large amount of data to be correctly calibrated that is not always available.
To address the differences by using these approaches, a comparison between the surficial acceleration response spectra of a single-layered 20 m-thick soil column of Messina Gravels (gravel and sand with occasionally silty levels) underlain by a rigid bedrock, subjected to strong-motion recordings, is presented.
The software STRATA was used for the equivalent linear analyses. The nonlinear site response analyses were performed with the OpenSees framework, while the bounding surface-based SANISAND constitutive model was selected to reproduce the soil nonlinear behaviour.
A single column in 3D space with periodic boundaries to simulate 1D conditions was considered. The input excitation was applied at the base nodes of the column and the parameters to be assigned to the model were obtained from Gorini (2019).
For both analysis methods, linear elastic analyses were performed by applying a 0.3g sine sweep with frequencies up to 30 Hz. The obtained results were interpreted in terms of acceleration transfer function and a satisfactory congruence was achieved.
The non-linear behaviour of the soil was triggered by applying three accelerograms from strong-motion events (Kobe, Kocaeli and Chi-Chi), downloaded from the PEER database. As results, for periods higher than 1.5 s neglectable amplification effects are observed, so a good accordance is highlighted between equivalent linear and nonlinear analyses. For the period range 0.3-1.5 s, amplification occurs but it is still correctly caught by both the approaches. Strong differences are, instead, observed in the lower periods range, up to 0.3 s, where the equivalent linear approach returns essentially similar spectral accelerations as those of the input motions, while nonlinear analysis highlights amplification and eventually deamplification effects.
In conclusion, it appears that the soil non-linearity should be carefully evaluated for high-seismicity areas because the equivalent linear method tends to underestimate the response, assuming a stiffer behaviour. This was clear for a single-layered soil column and it becomes certainly more complex for stratified soil deposits. To this end, the non-linear approach appears more appropriate to avoid underpredictions of the input motion to be applied for design purposes, but a high effort should be made to properly characterize the soil for the calibration of the selected nonlinear model as well.
How to cite: Fierro, T., Castiglia, M., and Santucci de Magistris, F.: Bounding surface models in site response analysis: comparison with the equivalent linear approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4928, https://doi.org/10.5194/egusphere-egu21-4928, 2021.
Assessment of seismic risk at a local scale is fundamental to the adoption of efficient risk mitigation strategies for urban areas with spatially distributed building portfolios and infrastructure systems. An important component of such a study is to estimate the spatial distribution of the expected seismic ground motion induced by site response. The current work presents a detailed seismic site response study at urban scale, performed in the context of developing an earthquake risk model for the Swiss canton of Basel-Stadt. Different studies undertaken over last two decades in the area concluded that unconsolidated sediments were responsible for inducing resonances and significant amplification of seismic waves over a range of frequencies pertinent to engineering interest. Therefore, we make a step forward in this study by attempting to develop a three-dimensional (3D) integrated geological-seismological model, which will explicitly account for the complex geological conditions at the surface and at depth. Thanks to the past projects, there is an abundance of geological, geophysical and seismological data for Basel. Earthquake recordings are available from an operating network of more than 20 permanent stations as well as from several former and six current temporary stations. Ambient noise measurements are available from several hundred single stations and more than 25 passive seismic arrays. In addition, a number of active seismic measurements and borehole logs are also available. An updated 3D model of subsurface geological structure of the area has been provided by the team of Applied and Environmental Geology (AUG) of University of Basel.
We use dispersion characteristics of surface waves from ambient vibration array data for imaging subsurface shear wave velocity (Vs) profiles. We apply a novel approach based on a Multizonal Transdimensional Inversion (MTI), formulated in the Bayesian probabilistic framework, in order to retrieve 1D Vs profiles from ambient vibration arrays. A joint inversion of multimodal Rayleigh and Love wave dispersion curves along with Rayleigh wave ellipticity curve is performed. This is a major improvement as such joint inversions were performed only for few sites in this area. The key advantages of MTI are that the model complexity in terms of number of layers and distribution of associated parameters are determined self-adaptively from the measured data, and model uncertainties can be assessed quantitatively. Additional constraints on the depths of intermediate layers are drawn from the 3D geological model and boreholes for the multizonal inversion. Moreover, the solution of the transdimensional Bayesian inversion enables reconstruction of the posterior probability density function of prior model parameters and their properties from the ensemble of inverted models. Hence, the model uncertainty can be duly propagated from dispersion curves to Vs profiles. The initial results seem very promising in resolving the interfaces corresponding to major velocity contrasts, especially in the complex sedimentary structure of the Rhine Graben formation. The ongoing analysis will also better identify composition, geometry, thickness and topography of the surficial unconsolidated sediments as well as the underlying more consolidated layers, which will form the basis for future numerical simulations of earthquake ground motion.
How to cite: Imtiaz, A., Panzera, F., Hallo, M., Dresmann, H., Steiner, B., and Fäh, D.: Developing an integrated 3D geological-seismological model at urban scale in Basel, Switzerland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12674, https://doi.org/10.5194/egusphere-egu21-12674, 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.
Estimation of site effects over large areas is a key-issue for land management and emergency system planning in a risk mitigation perspective. In general, site-conditions are retrieved from available global datasets and the ground-shaking estimation is based on ground motion prediction equations.
An advanced procedure to estimate site effects over large areas is here proposed with reference to the Italian territory. Site-condition were defined for homogenous morpho-geological areas in accordance to the borehole logs and the geophysical data archived in the Italian database for seismic microzonation (https://www.webms.it/). Ground motion modifications were determined by means of about 30 milion of one-dimensional numerical simulations of local seismic site response. Correlations between amplification factors (i.e. the ratio between free-field and outcrop response spectra), AF, and site-condition (i.e. harmonic mean of the shear wave velocity in the upper 30 m of the deposit, VS30) were determined for each morpho-geological homogeneous area depending on the reference seismic intensity (i.e. referred to the outcropping stiff rock characterised by VS30 ≥ 800 m/s). The AF-VS30 correlations were proved to satisfactory forecast the site effects when compared with the results of site specific estimation of local seismic site response.
How to cite: Falcone, G., Acunzo, G., Mendicelli, A., Mori, F., Naso, G., Peronace, E., Porchia, A., Romagnoli, G., Tarquini, E., and Moscatelli, M.: Evaluation of stratigraphic effects on seismic site response over large areas: the case study of Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2014, https://doi.org/10.5194/egusphere-egu21-2014, 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.
The availability of a unique database, where all data of the seismic microzonation studies carried out in about 1900 municipalities of Italy (https://www.webms.it/) are achieved with a standardized format, allowed statistical elaborations in terms of subsoil parameters. In particular, we analysed borehole logs and geophysical data in order to characterize them with the shear wave velocity (Vs) vertical profile, and the code of standardized engineering geological units, according to the Italian Guidelines for Seismic Microzonation (Seismic Microzonation Working Group, 2015; 2018). The Vs parameter, extracted from about 3700 geophysical surveys, was correlated to the engineering geological units from the borehole logs, with 1meter step. The correlation was performed for about 1700 available Down-Hole (DH) surveys and for about 2000 Multichannel Analyses of Surface Waves (MASW). For these latter, we selected only MASW surveys located near boreholes, no more than 100 m away. The statistical analysis on the distribution and dispersion of Vs parameter allowed to calculate the Vs values related to the mode, mean, median, standard deviation, first quartile, third quartile, minimum and maximum, and the trend with depth of Vs for each engineering geological unit. Validation with external datasets (e.g. Italian Vs30 map, Mori et al., 2020) demonstrates that the characterization of engineering geological units in term of Vs, based on velocity profiles extracted by the Italian seismic microzonation dataset, allow to reliably characterize the engineering geological model, where no geophysical data are available. Statistics of subsoil parameters will represent a fundamental tool for computing local seismic ground motion parameters (e.g. PGA, HSM) in the areas not covered by seismic microzonation studies.
- Mori, F., Mendicelli, A., Moscatelli, M., Romagnoli, 796 G., Peronace, E., Naso, G., 2020. A new Vs30 map for Italy based on the seismic microzonation dataset. Engineering Geology 275, 105745. https://doi.org/10.1016/j.enggeo.2020.105745.
- Seismic Microzonation Working Group, 2015. Guidelines for Seismic Microzonation http://www.protezionecivile.gov.it/httpdocs/cms/attach_extra/GuidelinesForSeismicMicrozonation.pdf
- Seismic Microzonation Working Group, 2018. Standard di rappresentazione e archiviazione informatica Versione 4.1. http://www.protezionecivile.gov.it/attivita-rischi/rischio-sismico/attivita/commissione-supporto-monitoraggio-studi-microzonazione/standard-rappresentazione-archiviazione-informatica
How to cite: Romagnoli, G., Carbone, G., Catalano, S., Cesarano, M., Fabozzi, S., Falcone, G., Moscatelli, M., Naso, G., Peronace, E., Porchia, A., Tarquini, E., and Albarello, D.: Characterization of the engineering geological units with the shear wave velocity parameter: statistical analysis of data from the Italian seismic microzonation studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10341, https://doi.org/10.5194/egusphere-egu21-10341, 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.
- 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.
The Mw 6.9 earthquake that took place offshore between the Greek island of Samos and Turkey’s İzmir province on 30 October 2020 came hardly as a surprise. Due to the extensional tectonic regime of the Aegean and high deformation rates, earthquakes of similar size frequently occur in the Aegean Sea on fault segments close to the shores of Turkey, affecting the settlements on mainland Turkey and on the Greek Islands. Samos-Sigacik earthquake had a normal faulting mechanism. It was recorded by the strong motion networks in Turkey and Greece. Although expected, the earthquake was an outstanding event in the sense of highly localized, significant levels of building damage as a result of amplified ground motion levels. This presentation is an overview of strong ground motion characteristics of this important event both regionally and locally. Mainshock records suggest that local site effects, enhanced by basin effects could be responsible for structural damage in central Izmir, the third largest city of Turkey located at 60-70 km epicentral distance. We installed a seven-station network in Bayraklı and Karşıyaka districts of İzmir within three days of the mainshock in search of site and basin effects. Through analysis of recorded aftershocks we explore the amplification characeristics of soils in the two aforementioned districts and try to understand the role basin effects might have played in the resulting ground motion levels and consequently damage.
How to cite: Çakti, E., Sesetyan, K., Hancilar, U., Caglar, M., Dar, E., Suleyman, H., Malcioglu, F. S., and Tetik, T.: 30 Ooctober 2020 Samos-Sigacik Earthquake: On Strong Ground Motion and Local Site Amplification, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16250, https://doi.org/10.5194/egusphere-egu21-16250, 2021.
Estimation of site effects over large areas is a key-issue in a seismic risk mitigation perspective.
We prove here that the IGAG20 approach (Falcone et al., 2021), developed for the estimation of the stratigraphic Amplification Factors (AF) at a national scale for Italy, can be used in international context, as it is based on AF-Vs30 laws developed according to 40 geo-morphological clusters available globally after Iwahashi et al. (2018) and Vs30 proxy laws after Mori et al. (2020).
The availability of AF maps is fundamental for the improvement of the estimates of surface shaking for the "shakemaps" produced after the seismic events, and for the consequent improvement of the preliminary estimates of coseismic effects (i.e. landslides and liquefaction) and damage of residential buildings.
The IGAG20 approach was implemented for evaluating the shaking maps for the recent Mw=6.4 Croatian seismic event, with a focus on the three most affected localities: Petrinjia, Sisak, and Glina. From the OpenQuake engine, Silva et al. (2014), a stochastic scenario analysis was performed and PGV and PGA shaking maps amplified with AF maps were produced. With the PGV map, landslide and liquefaction probability maps are produced respectively with the Nowicki et al. (2018) and Zhu et al. (2017) models. With the PGA map, a preliminary residential buildings damage estimation is produced and compared with the EMS98 damage distribution available from the grading maps produced by COPERNICUS (https://emergency.copernicus.eu/mapping/list-of-components/EMSR491 ). Finally, all the shaking maps are compared with USGS products (https://earthquake.usgs.gov/earthquakes/eventpage/us6000d3zh/executive).
Falcone, G., Mendicelli, A., Moscatelli, M., Romagnoli, G., Peronace, E., Naso, G., Acunzo G., Porchia, A., Tarquini, E., 2021. Seismic amplification maps of Italy based on site-specific microzonation dataset and one-dimensional numerical approach Eng. Geol. - Under review
Iwahashi, J., Kamiya, I., Matsuoka, M., Yamazaki, D., 2018. Global terrain classification using 280 m DEMs: segmentation, clustering, and reclassification. Prog. Earth Planet. Sci. https://doi.org/10.1186/s40645-017-0157-2
Mori, F., Mendicelli, A., Moscatelli, M., Romagnoli, G., Peronace, E., Naso, G., 2020. A new Vs30 map for Italy based on the seismic microzonation dataset. Eng. Geol. https://doi.org/10.1016/j.enggeo.2020.105745
Nowicki Jessee, M.A., Hamburger, M.W., Allstadt, K., Wald, D.J., Robeson, S.M., Tanyas, H., Hearne, M., Thompson, E.M., 2018. A Global Empirical Model for Near-Real-Time Assessment of Seismically Induced Landslides. J. Geophys. Res. Earth Surf. https://doi.org/10.1029/2017JF004494
Silva, V., Crowley, H., Pagani, M., Monelli, D., Pinho, R., 2014. Development of the OpenQuake engine, the Global Earthquake Model’s open-source software for seismic risk assessment. Nat. Hazards. https://doi.org/10.1007/s11069-013-0618-x
Zhu, J., Baise, L.G., Thompson, E.M., 2017. An updated geospatial liquefaction model for global application. Bull. Seismol. Soc. Am. https://doi.org/10.1785/0120160198
How to cite: Mori, F., Mendicelli, A., Falcone, G., Peronace, E., Moscatelli, M., and Naso, G.: Contribution of a new seismic amplification factor map approach for shakemaps improvement: the Croatia Mw=6.4 earthquake scenario. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2905, https://doi.org/10.5194/egusphere-egu21-2905, 2021.
The proposed research aims at the investigation of large mass movements on volcanic islands, like the San Andres landslide on El Hierro island (Canary Islands, Spain). These coastal and submarine landslides are extremely large (with run-out exceeding tens of km) and voluminous (up to hundreds of km3). They represent therefore a major geological hazard with direct consequences for the population of the islands. Volcanic activity and large earthquakes, as well as factors unrelated to the growth of the island like heavy precipitations and sea level change must be considered among the important triggering factors. Recent studies also evidenced that these large instabilities and failure mechanisms are linked to the geomechanical characteristics of the volcanic rocks, especially the formation of low strength and high deformability rocks.
San Andres landslide, formed between 176 and 545 ka, has been interpreted as the result of an aborted giant collapse and it represents one of the rarest sites where it is possible to investigate the landslide mass and fault planes of a volcanic collapse structure onshore. While several studies have been performed for the surface characterization, there is still a lack of knowledge about the subsurface properties of the San Andres landslide. For this purpose we conducted a seismological survey on El Hierro island in October 2020 aimed at the characterization of the internal properties of the terrestrial part of the landslide through seismological measurements.
Three temporary seismic arrays and two seismic profiles were deployed in order to retrieve the elastic properties of the subsurface through the analysis of seismic ambient noise. We applied the f-k analysis and cross-correlation techniques to measure the dispersion of the surface waves, the features of which were successively inverted to retrieve 1D shear-wave velocity profiles. Furthermore we analysed 3D signals to investigate the site resonance frequencies and thus identified impedance contrasts at depth. We therefore determined the degree of (de)-consolidation of the sliding mass itself estimated and and then compared it to the surrounding rocks of the volcanic island. During the aforementioned campaign, we also performed UAV flights to establish a 3D model of the investigated site. These recent investigations contributed to the construction of a 3D-geomodel by including existing geological information.
In prospect, the estimation of the landslide geometry will contribute to the evaluation of the flank stability as well as to the assessment of the risks associated to any possible reactivation.
How to cite: Cauchie, L., Cerfontaine, P., Mreyen, A.-S., and Havenith, H.-B.: Imaging of an incipient volcanic flank collapse by passive seismic methods: El Hierro, Canary Islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7951, https://doi.org/10.5194/egusphere-egu21-7951, 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.
To better comprehend mechanisms at the origin of natural slope failures, a vast number of potential slope weakening and failure triggering factors ought to be considered. Especially for rather ancient slope failures, such factors can be difficult to identify and strongly depend on the regional to local climatic as well as seismo-tectonic context.
An example of such ancient failure of unknown origin is the Balta rockslide that is located in the seismic region of Vrancea-Buzau, Romanian Carpathians. Even though more superficial landslides are found abundantly in the studied valley, the Balta failure stands out in terms of magnitude and observed geomorphological markers (profound detachment scarp, debris mass accumulation). During the last years, the Balta rockslide has been intensively studied with geophysical measurements (seismic and electrical methods) in order to characterise the landslide and in-situ rock material as well as the extensive dimension of the failure (with an estimated volume of 28.5-33.5 million m³).
In this work, we show the results of a numerical back-analysis of the Balta rockslide based on its reconstructed slope topography, implemented with 3D geomodelling, and on prior established geophysical and geomorphological studies; the reconstruction was furthermore conditioned by the morphology of neighbouring slopes in order to better constrain related uncertainties. The structural aspect of the anti-dip bedding of the sandstone dominated flysch slope was remodelled with 40° dipping discontinuities, while 55° dipping crossing discontinuities represent the main joint family observed in the field. The back-analysis was performed with the 3D distinct element code 3DEC (version 5.2, developed by Itasca) and aims at both, understanding static factors affecting slope stability, as well as the behaviour of the pre-failure slope if subjected to dynamic loading by using a synthetic Ricker multiplier as well as real earthquake acceleration data. The actual slope shape in its post-failure state could be approximated after 120 seconds of ground acceleration and is highlighted by lateral spreading of debris mass as well as towards the valley; the latter supposedly caused a temporary landslide dam formation, and possibly accounts for the river diversion observed in the field. This numerical approach furthermore allows us to outline the main controlling factors during seismic slope excitation that are predominated by topographic and structural site effects.
How to cite: Mreyen, A.-S., Cauchie, L., Micu, M., and Havenith, H.-B.: 3D distinct element back-analysis (static and dynamic) of a reconstructed rock slope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9559, https://doi.org/10.5194/egusphere-egu21-9559, 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 (Mw 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 (Tl) and thickness (Ts) 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 (Mw) 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 24th Mw 6.0; October 26th Mw5.9 and October 30th 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.
Shake maps, produced a few hours after a seismic event, represent the key input for the rapid assessment of earthquake triggered landslides scenario maps in near real time.
The IGAG20 approach (Falcone et al., 2021) improves the prediction of these by contemplating the site effects that are calculated as a function of the Vs30 (Mori, 2020) and the intensity of the shaking.
The method originally calculates the amplification factor for some intensity measures at the surface level for the national hazard, in Italy.
Here, we present applications of the method, in terms of scenarios, for a few main shocks of past seismic events in Italy: Friuli 1976, Umbria-Marche 1997 and L’Aquila 2009. We used the OpenQuake engine (Silva et al., 2014), to produce PGV and PGA stochastic maps including amplification factors. The PGV map helped calculating landslide probability maps within the Nowicki et al. (2018) model, while the PGA map was a key input for landslide rockfall maps obtained within the STONE model (Antonini et al., 2002, Guzzetti et al., 2002; Alvioli 2020).
Results of both models were compared with available landslide records for the corresponding earthquake events, either in the form of points or polygons (Govi 1977; Guzzetti et al 2009).
How to cite: Mendicelli, A., Mori, F., Falcone, G., Peronace, E., Moscatelli, M., Giuseppe, N., and Alvioli, M.: New simplified models for earthquake-triggered landslides in large area: application to Italian case studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15552, https://doi.org/10.5194/egusphere-egu21-15552, 2021.
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