Displays

Up to now, almost all of the ground motion modeling and hazard assessment for seismicity in the Netherlands focuses on horizontal motion. As a rule of thumb, the strength of vertical ground motions is taken as 2/3 of that of horizontal ground motions. In reality of course, amplifications and V/H ratios are site-dependent and thus vary regionally.  Recent studies have indeed shown that vertical ground motion is not always simply 2/3 of the horizontal motion. However, these studies are performed in areas with high magnitude (Mw>5.0) earthquakes and the question is whether vertical motion is relevant to be included in seismic hazard assessment for low magnitude earthquakes (to date, max Mw=3.6 in Groningen).

In the Netherlands, the top part of the soils is practically always unconsolidated, so the elastic waves generated by deeper (~3000m) seated earthquakes will be subject to transformation when arriving in these layers. Recordings over a range of depth levels in the Groningen borehole network show the largest amplification to occur in the upper 50 meters of the sedimentary cover. We not only observe a strong amplification from shear waves on the horizontal components, but also from longitudinal waves on the vertical component. A better understanding of vertical motion of low magnitude earthquakes aims to support the design of re-enforcement measures for buildings in areas affected by low magnitude seismicity. Furthermore, interference between the longitudinal -and shear waves might contribute to damage on structures.

This study presents observations of longitudinal wave amplification in the frequency band 1-10 Hz, corresponding to resonance periods of Dutch buildings. From 19 seismic events, with a minimum of magnitude two, we retrieved transfer functions (TFs) from the vertical component, showing a strong site response at certain locations. In addition, we calculate event V/H ratios and VH factors from the surface seismometer. These results are compared with the TFs and show a similar pattern in terms of site response. Furthermore, the sites with highest vertical amplification correspond to very low (800-900 m/s) P-wave velocities. Our study shows that vertical amplification is very site dependent. However, the question whether the vertical motion is significant enough to form a real hazard can only be answered through cooperation between seismologist and structural engineer.

How to cite: van Ginkel, J., Ruigrok, E., and Herber, R.: How significant is vertical ground motion from low magnitude earthquakes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5505, https://doi.org/10.5194/egusphere-egu2020-5505, 2020.

The identification of areas susceptible to different co-seismic instabilities is an important issue of the seismic zonation at urban scale finalized to the territory planning and its protection. Among the co-seismic permanent deformations caused by seismic shaking, the fractures, the landslides, the settlements due to liquefaction or compression/densification can be recognized.

The seismic compression or densification is a phenomenon producing permanent ground settlements in dry cohesionless soils (clean sands and sands with fine content) inducing damages to structures, infrastructures and lifelines, accordingly with well documented post-earthquake damages of past events.

 The susceptibility to this co-seismic instability in presence of dry clean sand, silty sand and sandy silty has been evaluated in the present work through the evaluation of the expected permanent ground settlements by means of non-simplified uncoupled methods computing volumetric strains from cyclic shear strains evaluated by means of site response analyses. This procedure was integrated into a parametric study of 1D seismic site response analyses varying relative density (or shear wave velocity) and thickness of compressible layers, intensity of input ground motion, depth of the seismic bedrock. The results have been then processed to define simplified charts differentiated for three different levels of input peak ground acceleration values and for the three considered lithologies (clean sands, silty sands and sandy silts).

These latter are mainly finalized to be used at urban scale, in the perspective of Seismic Microzonation (SM) studies requiring input-data commonly available in level 2 and 3 studies that have a strategic application in land use planning in the perspective of the territory protection.

A specific methodology was proposed by means of guideline based on a procedure with increasing complexity: 1) preliminary screening; 2) level 1 analyses; 3) level 3 analyses. The areas potentially susceptible to seismic compression identified in this preliminary phase are to be studied in the level 1 of SM, that identifies attention zones by checking the presence of predisposing conditions to the phenomenon. In the level 3 of SM, the susceptible zones and respect zones are identified through the estimation of the settlements by means of the charts proposed in the present work and the seismic site response analysis, respectively.

How to cite: Fabozzi, S., Porchia, A., Fierro, T., Peronace, E., Pagliaroli, A., and Moscatelli, M.: How to treat the seismic compression instability in seismic Microzonation studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5503, https://doi.org/10.5194/egusphere-egu2020-5503, 2020.

Microzonation is one of the essential tools in seismology to mitigate earthquake damage by estimating the near surface velocity structure and developing land usage plans and intelligent building design. The number of microzonation studies increased in the last few years as induced seismicity becomes more relevant, even in low risk areas. While of vital importance, especially in densely populated cities, most of the traditional techniques suffer from different short comings. The microzonation technique presented here tries to reduce the existing ambiguity of the inversion results by the combination of single-station six-component (6C) measurements, including three translational and three rotational motions, and more traditional H/V techniques. By applying this new technique to a microzonation study in Munichs (Germany) inner city using an iXblue blueSeis-3A rotational motion sensor together with a Nanometrics Trillium Compact seismometer we were able to estimate Love and Rayleigh wave dispersion curves. These curves together with H/V spectral ratios are then inverted to obtain shear wave velocity profiles of the upper 100 m. The resulting 1D velocity profiles are used to estimate the local shaking characteristics in Munich. In addition, the comparison between the estimated velocity models and the borehole-derived lithology gives a positive correlation, indicating the applicability of our method.

How to cite: Keil, S., Wassermann, J., and Igel, H.: Seismic Microzonation using 6C Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2606, https://doi.org/10.5194/egusphere-egu2020-2606, 2020.

Earthquake-induced landslides can have serious social impacts, causing many casualties and significant damage to infrastructure. They are the most destructive secondary hazards related to earthquakes. The impact of strong seismic events is not limited just to triggering of catastrophic slope failures, it also involves weakening of intact rock masses and reactivation of dormant slides. Hazard mitigation of potentially catastrophic landslides requires a thorough understanding of the mechanisms driving slope movements and seismic response.

We present an overview of the investigations on more than 25 instabilities. The results show that ambient vibration measurements allow for a rapid and objective characterization of potential slope instabilities. It is possible to distinguish unstable from stable areas, to identify slope eigen-frequencies, local amplification levels due to weak excitation, local deformation directions and properties of the internal slope structure. The ambient vibration techniques include single-station H/V ratios and polarization analyses, site-to-reference spectral ratios, array methods to identify surface-wave dispersion curves, and/or normal mode analysis using enhanced frequency domain decomposition. We analyse the seismic response of the rock slopes in different frequency bands together with its spatial and azimuthal variability, which is a fingerprint of the slope’s internal structure at different scales (tenth of meters to hundred meters). Normal mode behaviour is typically observed in structures with distinct sub-volumes, where the wave field at the resonance frequencies is oriented perpendicular to the deep persistent fractures. These structures show maximum amplification at their resonance frequency. Normal mode behaviour is also observed for rock towers, similar to what can be observed for buildings. In contrast, a highly fractured rock mass without dominant cracks is characterized by an S-wave velocity gradient with shear-wave velocity being significantly reduced close to the surface. Generally, normal modes do not develop, but surface waves propagate in such structures, which can be used for the determination of the S-wave profile. This is typical for large deep seated landslides with a layered structure. Without strong S-wave velocity contrast at depth, H/V spectral ratios show no clear peak and are not conclusive to characterize structures with highly fractured material. However, frequency-dependent ground-motion amplification from standard spectral ratios is directly related to the S-wave velocity profile and damping. Therefore, wave amplification can be a measure for the disintegration of the rock.

Repeated measurements on slopes allow for the detection of possible changes in their properties. Semi-permanent installations on instabilities of interest allow for a continuous assessment of the dynamic response in order to understand variations due to weather conditions and potential long-term changes. This includes the measurement of site-amplification during earthquakes derived from empirical spectral modelling. When measuring in the same season and weather condition, the seismic response of rock instabilities in general remains unchanged over years, as long a no external trigger affects the instability, including a strong earthquake, partial failure of the slope or permafrost degradation.

How to cite: Fäh, D., Häusler, M., Glueer, F., Burjanek, J., and Kleinbrod, U.: Characterization of unstable rock slopes using ambient vibration analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9997, https://doi.org/10.5194/egusphere-egu2020-9997, 2020.

The majority of numerical landslide models are designed in 2D. In particular, models based on finite difference methods (FDM) are time-consuming and – as a result – in most cases also cost-intensive. 3D models, therefore, increase the processing time significantly. Another contributing factor to long processing times in the context of modeling of seismically-induced displacements is the fact that mesh grid increments must be small due to the necessity of correct wave propagation through the material. The larger the frequency range of the applied seismic signal should be, the smaller has to be the mesh grid increment. 3D models are, however, considered as more realistic.

In this work, we present a comprehensive study on numerical 2D and 3D models of the Diezma Landslide, Southern Spain. The Landslide is represented in its shape as it appeared at the time of the main rupture on 18^th of March in four model layouts: (1) a simplified model in 3D that outlines the landslide body with planar triangular tiles, (2) a longitudinal cross section through this simplified 3D model representing the simplified 2D model, (3) a smooth model in 3D that envelops the landslide body according to the main topographic features, and (4) a longitudinal cross section through this smooth 3D model representing the smooth 2D model.

On both the simplified and the smooth 2D models, a series of 11 seismic scenarios was applied as SV-waves assuming a source sufficiently far for vertical incidence at the model bottoms in order to produce horizontal shear inside the landslide body with respect to the underlying bedrock. All 11 signals are characterized by different frequency contents, Arias Intensities from 0.1 to 1 m/s, moment magnitudes from 5.0 to 7.0 and peak ground accelerations from 0.8 to 1.2 m/s², and therefore correspond to scenarios that represent the local seismicity in Southern Spain.
Because of time-related limitations, only four of these signals were respectively applied to the simplified and smooth 3D model. Newmark-Displacements were calculated using all 11 signals with the classic Newmark-Method that approximates the landslide body in 2D by a rigid block on an inclined plane, and with Newmark’s Empirical Law as spatial information covering the landslide area across the slope in regular intervals.

We present a systematic comparison of all models and obtained displacements, showing that the Newmark-Methods deliver very similar results to the maximum displacements obtained by FDM. Moreover, we discuss on a particular example that – although seeming more accurate in the layout – smooth models lead not necessarily to realistic results.

How to cite: Domej, G. and Bourdeau, C.: Numerical modeling of seismically-induced slope displacements: a comparison between 2D and 3D finite difference models and Newmark-Displacements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4186, https://doi.org/10.5194/egusphere-egu2020-4186, 2020.

                Zagros Mountains form a seismically active fold and thrust belt in western Iran. In addition to the high levels of seismicity, slope failures are common throughout the region, where historical records of very large landslides (> 30 km³) are documented. On the November 12^th 2017, the largest earthquake (Mw 7.3) ever recorded in the Zagros occurred near the town of Sarpol-Zahab (NW Zagros/Iraq border). Following the earthquake, only one large co-seismic rockslide and some small rockfalls were documented near the epicenter. This rather small landslide activity for such a large earthquake raises the question of both the observation completeness and the controlling factors of the landslide triggering in this arid mountainous environment.

            We conducted an original inventory mapping of the landslides induced by this event along 200 km of the Iran-Iraq border. The landslides were detected by different methods: the scars of rapid co-seismic landslides were mapped using a comparison of pre- and post-seismic Planetlab images (3 m resolution), whereas slow-moving landslides (cm/yr-m/yr) were detected by deriving time-series of ground deformation from radar and optical satellite images. Interferometric measurements were constructed for 3 ascending and descending Sentinel-1 SAR tracks, over a time period of 15 months (spanning 6 months before and 9 months after the main shock), allowing the detection and monitoring of very-slow-moving landslides (cm/yr), while slow-moving landslides of higher velocities (m/yr) were detected from correlation of pre and post-earthquake optical satellite images (Planet and SPOT67 imagery; 3 m and 1.5 m resolution, respectively), orthorectified over precise DEMs.

            We detected 8 giant rotational rockslides (3.10⁶ to 3.10⁷ m²) and 360 small rockfalls (2.10² to 2.10⁴ m²) in our study area. The small slope-failures were concentrated in the steepest areas around the epicenter (within a radius of 45 km) while the giant ones were situated in far fields (150 km far from the epicenter). Geomorphological analysis of the giant landslides revealed the reactivation of huge masses with several hundreds meters scarps at their top and runout distance of several hundreds meters, advancing over a river at their toe. The geodetical analysis of these giant landslides, show their co-seismic acceleration by few cm.  Furthermore, the analysis of the displacement time-series of these giant rockslides shows that four of them are destabilized over the longer term. This observation raises question both of the risk posed by these rockslides and the controlling factors of their initiation. A geological and seismological analysis suggests that the triggering of these giant rockslides can be controlled by the geological structure (stratigraphy and folding) and the resulting topography, as well as by the fault mechanism of major earthquakes. Finally, the landslide reactivation mechanism during the Sarpol-Zahab earthquake is discussed.

How to cite: Cheaib, A., Lacroix, P., Zerathe, S., Jongmans, D., Ajorlou, N., Doin, M.-P., Hollingsworth, J., Rauscher, G., and Abdallah, C.: Reactivation of giant paleo-rockslides during the Sarpol-Zahab Mw7.3 earthquake, Iran, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10503, https://doi.org/10.5194/egusphere-egu2020-10503, 2020.

The 2016 M_w 7.8 Kaikoura Earthquake in Canterbury, New Zealand produced one of the most complex fault ruptures observed in the historical period and produced strong ground shaking. As a consequence, over twenty-nine thousand landslides were triggered over a total area of about 10,000 km² with the majority concentrated in a smaller area of about 3,600 km² (Massey et al 2018). In addition, hillslopes in the affected area were severely damaged by tension cracking and dilation. Large volumes of landslide debris generated during the earthquake remain stored in the landscape and the potential for rainfall to trigger landslides on the failed and partially failed hillslopes is anticipated to be elevated for the foreseeable future. Despite this little is known about the increase in landslide hazard and the timeframe over which this hazard will be elevated.

We used airborne LiDAR captured immediately after the earthquake (November 2016), and at six consecutive dates between November 2017 and April 2019  to develop high resolution surface change models to construct an inventory of rainfall-induced landslides and reactivated landslides following the earthquake. The results were compared with landslide inventories for a series of significant storm events between 1880 and 2019 which were compiled from various sources, including mapping from available aerial photography and satellite imagery collected between 1961 and 2019.

Analysis of the landslide inventories indicates that rainfall triggering thresholds for landslides on these highly cracked and dilated slopes is lower than before the earthquake which has resulted in a significant increase in landslide frequency for a given rainfall amount through the initiation of new landslides on weakened slopes, reactivation of existing landslides and reworking of landslide debris stored on the landscape. Most of the landslides triggered by rainfall following the earthquake were highly mobile debris flows that were strongly coupled to the channel network. Preliminary results suggest that the highest rates of post-earthquake landslide initiation (for both new and reactivated landslides) occurred in the first major storm event following the earthquake and the rate has reduced with time since the earthquake. Maximum landslide size (area) also decreased with time following the earthquake. Quantification of rates of post-EQ rainfall-induced landsliding using LiDAR differencing and aerial photo interpretation will further our understanding of post-earthquake landscape recovery.

How to cite: Rosser, B., Jones, K., Massey, C., Ashraf, S., Strawbridge, G., and Morris, S.: Quantifying the effects of the M7.8 November 14, 2016 earthquake on rainfall-induced landslide triggering and reactivation, Kaikoura, New Zealand , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12409, https://doi.org/10.5194/egusphere-egu2020-12409, 2020.

The characterization of seismic site response represents one of the most important issues of seismic hazard assessment and risk mitigation planning. Characterizing the site conditions involves the measurement of several soil properties such as the shear-wave velocity (Vs), density and damping properties as a function of depth. Therefore, most of the site-effect studies in earthquake ground motions are based on the properties of the upper 30 meters and the anti-seismic building codes propose in most cases a simplified analysis based on shear wave velocity of the shallow subsoil. From a seismological perspective, the upper 30 meters would almost never represent more than 1% of the distance from the source. This should be taken into account especially for large and deep alluvial basins, representing the most inhabited geological environment of the world, where could be difficult to estimate the thickness and the velocity profile of the soft sediment overlying the rigid seismic bedrock.

The common approach adopted to characterize greater depths is then an extrapolation of shear wave velocity in depth, considering a selected linear or non-linear velocity gradients till the depth of the considered seismic bedrock (usually set to Vs ≥ 800 m/s). These gradients are generally derived from geological information or from literature, but how much the gradients choice affects the final site response analyses is often a neglected aspect.

In this work we try to investigate the generic case of deep alluvial basins. We consider the shallow subsoil as characterized by several in-situ tests in northern Italy. We extrapolate the deeper soil structure considering different literature velocity gradients obtained for deep basins in different geological contests: tectonic basins (Lower Rhine Basin and Po Plain) and Alpine basins (Grenoble and Lucerna Basins). We perform one-dimensional analysis of shear waves with the Linear Equivalent Method. The study demonstrates how relevant can be the role of velocity gradient choice for the ground response scenario. Starting from the same shallower Vs structures, the computed seismic motion at surface can present variation in the order of 50% varying the velocity gradients in depth. The results are of relevant interest for the analysis of seismic hazard in the deep alluvial basins environments, which host the main urban areas around the world.

How to cite: Cascone, V. and Boaga, J.: The influence of velocity gradients choice in deep alluvial basin seismic site response, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5123, https://doi.org/10.5194/egusphere-egu2020-5123, 2020.

When an earthquake occurs, the propagation of the seismic waves is conditioned by local conditions, e.g., depth to seismic bedrock and impedance ratio between soft soil and seismic bedrock. Bearing in mind that the maximum depth of site prospections generally does not extend up to seismic bedrock depth, a parametric study was carried out with reference to ideal case studies in order to investigate the effect on local seismic amplification of the depth to bedrock.

The results are presented in terms of charts of amplification factors (i.e., ratio of integral quantities referred to free-field and reference response spectra) and minimum depth to investigate vs building type. These charts will allow defining the thickness of the cover deposit that should be characterised in terms of geophysical and geotechnical parameters in order to perform seismic site response analysis according to a precautionary approach, in areas where depth to seismic bedrock is higher than conventional maximum depth of site surveys.

How to cite: falcone, G., naso, G., fabozzi, S., mori, F., moscatelli, M., peronace, E., and romagnoli, G.: Evaluation of the effect of depth to bedrock on seismic amplification phenomena, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5848, https://doi.org/10.5194/egusphere-egu2020-5848, 2020.

In this work, a review of the seismic microzonation for the city of Campobasso, conducted after the 2002 Molise earthquake (Italy), is made. Fourteen sites to perform 1D analysis were selected. The stratigraphy and the physical and mechanical properties of the soils were available from both direct tests and literature survey. Down-hole profiles were accessible for the area and additional MASW tests were conducted in 2016.

A seismic hazard analysis was performed by accounting for the characteristics of the active faults located in a range of about 50 km, the disaggregation of PGA with a probability of exceedance of 10% in 50 years and the Gutenberg-Richter recurrence law for a return period of 475 years based on the Parametric Catalogue of Italian Earthquakes. The magnitude and distance ranges that are most probable to contribute to the seismic hazard of the municipality are 5.5-7.5 and 0-50 km, respectively. These ranges were used for the selection of a set of design earthquake motions to be representative of the seismicity of the site, which, consequently, matches the requirements of the Italian code in terms of target spectrum. Eight earthquake motions were selected from the ESM and PEER databases; the target spectrum refers to a Safe Life Limit State (SLV) with return period of 475 years, topographic category T1 and soil type A. The compatibility is verified by fitting the mean spectrum obtained from the accelerograms within a tolerance of 10 % in the lower bound and 30% in the upper bound for a specific range of periods of the design spectrum. The software InSpector was used to check the match.

1D local seismic response analyses were performed in the frequency domain by using the software STRATA.

There was a good agreement between the shear wave velocity profiles obtained from down-hole and MASW tests, except for few cases in which problems during the test execution or high environmental noise could have affected the down-hole results by providing meaningless profiles. Even though the shear wave velocity profiles have a good agreement, the transfer functions computed with both profiles show different resonance frequencies as expected. From the 1D seismic response analyses, the importance of the superficial layers in the amplification of the earthquake motion was highlighted, thus showing a substantial difference in the acceleration profile at the surface and a few meters below the top ground. The spectra at the surface were compared with the relative target spectra for the same site class of the considered soil deposit and the accelerations were found to be higher than those provided by the code spectra for the small periods range and the design spectra become instead much conservative for periods higher than 0.4 s. The latter two considerations underline the importance of conducting site response analyses in engineering applications to optimize the design seismic forces on the structures.

How to cite: Fierro, T., Castiglia, M., and Santucci de Magistris, F.: Observations on 1D local seismic response analyses: a case study in the Molise region, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10474, https://doi.org/10.5194/egusphere-egu2020-10474, 2020.

Microtremor measurements represent a useful tool to study the seismic amplification in urban areas. One of the methods that permits characterizing seismic properties of soils is the H/V spectral ratio. This technique is especially useful when dealing with shallow low velocity layers, allowing an effective determination of its velocity and thickness. The H/V technique is very convenient to realize microzonation surveys because of its simplicity and low cost. However, it is recommended to combine it with other geophysical methods and geological information to better constrain the resulting models. In recent years the use of ambient noise cross-correlation has been widely used to retrieve Rayleigh wave dispersion curves between pairs of stations. These curves carry an important information about the subsoil velocity structure and have been already exploited for seismic microzonation purposes.

The aforementioned methods, H/V spectra and Rayleigh wave dispersion curves, in principle allow obtaining 1D body wave and density profiles. However, one of the most important problems when inverting H/V and dispersion curves, is the poor constraint on density and P wave velocities. This difficulty can be partially solved by imposing some constraints over the inverse problem (e.g. fixing the Vp/Vs ratio) or by devising inverse methods allowing the different parameters to be determined in different steps.

We propose a novel approach which consists of a joint inversion of H/V spectra and Rayleigh wave dispersion curves, realized simultaneously for all the elements of the mini-array. This allows increasing the ratio between the number of available data and the number of parameters to invert, improving the stability of the inverse problem and reducing the uncertainties on the estimated parameters. For the evaluation of the retrieved model, we used the trans-dimensional Monte Carlo exploration which has shown to be very efficient in evaluating the quality of the resulting model, through an intensive exploration of the “a posteriori” probability density function over the model parameter space.

We show the improvement in the obtained results on synthetic tests as well as on actual data. In particular we apply this method, named method MARISMA (Mini ARrays for seISmic MicrozonAtion) on a dataset recorded in the town of San Cristóbal de La Laguna (Tenerife, Canary Islands, Spain) during the summer of 2019.

How to cite: D'Auria, L., Alfaro Rodríguez, M., Bermejo López, D., Crowther, J., Kennett, L., Cabrera-Pérez, I., Martínez van Dorth, D., and Soubestre, J.: Robust determination of S-wave velocity profiles by using mini-arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4629, https://doi.org/10.5194/egusphere-egu2020-4629, 2020.

After the M = 7.0 Haiti earthquake in 2010, many teams completed seismic risk studies in Port-au-Prince to better understand why this not extraordinarily strong event had induced one of the most severe earthquake disasters in history (at least in the Western World). Most highlighted the low construction quality as the main cause for the disaster, but some also pointed to possible soil and topographic amplification effects, especially in the lower and central parts of Port-au-Prince (e.g., close to the harbor). Therefore, we completed a detailed site effect study for Gros-Morne hill located in the district of Pétion-Ville, southeast of Port-au-Prince by using near surface geophysical methods. The horizontal to vertical spectral ratio technique was applied to ambient vibrations and earthquake data, and multichannel analysis of surface waves and P-wave refraction tomography calculation were applied to seismic data. Standard spectral ratios were computed for the S-wave windows of the earthquake data recorded by a small temporary seismic network. Electrical resistivity tomography profiles were also performed in order to image the structure of the subsurface and detect the presence of water.

Different site effect components are represented for the entire survey area; we present maps of shear wave velocity variations, of changing fundamental resonance frequencies, and of related estimates of soft soil/rock thickness, of peak spectral amplitudes and of ambient ground motion polarization. Results have also been compiled within a 3D surface-subsurface model of the hill that helps visualize the geological characteristics of the area, which are relevant for site effect analyses. From the 3D geomodel we extracted one 2D geological section along the short-axis of the hill, crossing it near the location of the Hotel Montana on top of the hill, which had been destroyed during the earthquake and has now been rebuilt. This cross-section was used for dynamic numerical modelling of seismic ground motion and for related site amplification calculation. The numerical results are compared with the site amplification characteristics that had been estimated from the ambient vibration measurements and the earthquake recordings. Related results only partly confirm the strong seismic amplification effects highlighted by previous papers for this hill site, which had been explained by the effects of the local topographic and soil characteristics.

How to cite: Havenith, H.-B. and Ulysse, S.: Site effects on Gros Morne Hill, Port-au-Prince, Haïti, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2098, https://doi.org/10.5194/egusphere-egu2020-2098, 2020.

_{(ilaria.primofiore@gmail.com; jbaron@inogs.it; glaurenzano@inogs.it; pklin@inogs.it; cristina.muraro@isprambiente.it; g.vessia@unich.it)}

The 2016 Italian seismic sequence showed, once again, the relevant role of the differentiated seismic effects at short distance in varied geological environments. In the case study of Arquata del Tronto hamlet, several response analyses have been performed in order to reproduce the ground response through 2D finite element numerical codes (Primofiore, 2019; Pagliaroli et al., 2019). According to the Italian Guidelines for Seismic microzonation ICMS (2010), in the case of hills, the topographic effects of seismic amplification must be studied by numerical methods. In those cases, when the relieves are made up of soil deposits, 2D numerical analyses are used, indeed. Instead, when rocky hills are considered, the amplification effects due to the topography are considered by means of 1D simplified analyses or at most, 2D ground response analyses. The recent damages of old settlements located on the top of rocky hills, such as Arquata del Tronto hill, put in evidence the relevant role of three-dimensional movements of asymmetrical isolated rocky reliefs in generating heavy disruptions during the seismic shaking. In addition, on surface there are commonly fracturing layers of rocks, which played an important role in amplifying seismic waves according to their thicknesses. 3D numerical analyses at Arquata del Tronto hill have been carried out through the spectral element method implemented in SPECFEM3D code. Results suggested that an accurate simulation of the topographic effects of isolated asymmetrical rocky hills can be appreciated only through 3D numerical analyses, because they capture the out-of-plane bending moment (torsional effect) that asymmetry induces. The results showed that seismic behaviour of articulated morphology of the isolated relieves cannot be simulated by means of 2D seismic response analyses.

_References

_{Pagliaroli, A., Pergalani, F., Ciancimino, A., et al. (2019). Site response analyses for complex geological and morphological conditions: relevant case-histories from 3rd level seismic microzonation in Central Italy.}

_{Bulletin of Earthquake Engineering, 1-37.}

_{Primofiore, I. (2019). Studio della risposta sismica in località Arquata del Tronto mediante modellazioni numeriche 3D. Master Degree thesis (in Italian), University “G. d’ Annunzio” of Chieti-Pescara.}

_{Working group M. S. (2010). ICMS - Indirizzi e Criteri per la Microzonazione Sismica. In Conferenza delle Regioni e delle Provincie autonome. Dipartimento della protezione civile, Roma (Vol. 3).}

How to cite: Primofiore, I., Baron, J. M.-P., Laurenzano, G., Klin, P., Muraro, C., and Vessia, G.: Evaluation of the seismic response at the Arquata Del Tronto hamlet through 3D numerical analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22163, https://doi.org/10.5194/egusphere-egu2020-22163, 2020.

The majority of casualties associated with historical eruptions on Tenerife (Canary Islands) were linked to the seismicity preceding and accompanying the eruptive activity. Therefore, the volcano-tectonic seismicity constitutes a relevant hazard. Moreover, the tectonics of the archipelago and paleoseismological evidences in the southern part of the island, suggest the possibility of destructive earthquakes on the island and its surroundings.

The complex geology of the island also affects seismic wave propagation and can lead to local seismic amplification phenomena. Actually, a recent moderate earthquake (Ml=4.4) located east of the island, has been recorded by a dense broadband network: Red Sísmica Canaria (C7) operated by Instituto Volcanológico de Canarias (INVOLCAN) showing relevant local seismic amplification effects at different sites. For this reason, in the spring of 2019, INVOLCAN started a research program, named TFsismozon, aimed at characterizing the local seismic response of the urban areas of Tenerife with the aim of mitigating the seismic risk of the island.

The first site selected for this purpose was the town of San Cristóbal de La Laguna, declared World Heritage Site by UNESCO 1999 and partially built over lacustrine sediments, which can be responsible for seismic wave amplification. For this purpose, during the summer of 2019, INVOLCAN realized a dense seismic survey of the town, performing seismic noise measurements on 453 sites located in the downtown and its surroundings, for a total surface of about 11 km². The measurements were realized by deploying mini-arrays, composed of 3-4 elements, for a duration of 2-3 hours. These measurements were realized with the goals  of obtaining H/V ratios and also to get the surface waves dispersion curves through the cross-correlation of the seismic noise. The amplification frequencies are obtained through the H/V ratio, while the joint inversion of both H/V and dispersion curve data allows for obtaining Vs profiles for each point.

This survey therefore represents the first extensive mapping of seismic amplification effects in the Canary Islands. It also allows for improving the geological models of the town, in particular providing a high-resolution map of the lacustrine deposits on which part of the town lies. The preliminary results of the survey evidenced a clear relation between the sediment thickness and the frequency of the dominant peaks in H/V ratio. Moreover, the preliminary data analysis, on the basis of the H/V ratios, showed that the south-eastern area of the survey may be similar to the lacustrine basin, although previous geological maps indicated the presence of basalts.

How to cite: Cervigón Tomico, G., Fernández del Campo, D. P., Fernández Agudo, E., García Salamanca, A. F., Tisdall, R., Cabrera-Pérez, I., Martínez van Dorth, D., Soubestre, J., Bidaurrazaga Aguirre, G., Ortega Ramos, V., D'Auria, L., and Pérez, N. M.: High resolution seismic microzonation of San Cristóbal de La Laguna (Tenerife, Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6835, https://doi.org/10.5194/egusphere-egu2020-6835, 2020.

We analyzed the seismic site response of the Amatrice village, that experienced extensive and very high level of damage after the 24th of August 2016 earthquake, further aggravated by the following shocks of October 2016. In particular, site response was investigated by simulating seismic wave propagation through an advanced 3D subsurface model of the site. Availability of experimental site transfer functions allowed validating simulation results and evaluating advantages and drawbacks of this approach.

The 3D subsoil model was developed based on the available dataset of borehole stratigraphic logs, shear wave velocity profiles obtained from Down-Hole tests and 2D ARRAY measurements as well physical and mechanical properties measured by means of laboratory tests (EmerTer Project, 2018; CNR IGAG Report, 2018).

The model was forced by 3-component (3C) input constituted of acceleration time histories that were selected from the European Strong-Motion database (www.esm.mi.ingv.it ; Luzi et al., 2016) by considering a return period = 975y.

The explicit finite-difference code FLAC3D (ITASCA Consulting group Inc., 2017) was used for numerical simulations; this code operates in the time domain, incorporates a compliant base, free-field lateral boundaries and uses a fully nonlinear approach to model the dynamic soil properties. The identification of the seismic bedrock depth was carried out by an iterative procedure that minimizes the difference between recorded motions after deconvolution at depth. A hysteretic-damping model and Rayleigh damping formulation were used to account for viscous damping in dynamic condition. Rule by Kuhlemeyer and Lysmer (1973) was adopted for element size definition to achieve a satisfactory level of accuracy up to 10 Hz. The finite difference mesh consists of about 1.1 million tetrahedral-shaped elements.

Three control points in correspondence with three temporary seismic stations, i.e., MZ12, MZ28 and MZ31, were considered in order to compare the simulated 3D transfer functions with the experimental ones. In particular, MZ12 was located in the historical center of Amatrice village, MZ28 in the southeastern part of the village, while MZ31 in the western sector. Available Standard Spectral Ratios (Borcherdt, 1970, Milana et al., 2019) were used to determine the experimental frequencies and amplifications. The results showed that the average amplification is about 4 for MZ12 in the frequency range 5-7Hz and about 2 for MZ28 station at 3Hz, while amplification function is essentially flat at MZ31. In the historical part of the village, only Horizontal-to-Vertical Spectral Ratio (Nakamura, 1989) measurements were available. Reasonable agreements were found in the considered frequency range 1-10Hz.

This approach, which simulated the 3C ground motion field, demonstrated to be useful to evaluate the most important 3D model features relevant for site amplification.

The present work was performed in the frame of the SISMI Project, funded by Regione Lazio and devoted to developing new technologies for improving the security and the reconstruction process of the historical centers in central Italy. All the activities were carried out under in the framework of the “DTC Lazio” (https://dtclazio.it/).

How to cite: Razzano, R., Gaudiosi, I., Moscatelli, M., Luigi, C., Lanzo, G., Martini, G., and Hailemikael, S.: Modelling the three-dimensional site response in the village of Amatrice, Central Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22483, https://doi.org/10.5194/egusphere-egu2020-22483, 2020.

In 2005, the capital of Myanmar was moved to the newly designed city of Nay Pyi Taw, some 300 km north of Yangon. Both Yangon as well as the capital Nay Pyi Taw are situated along the 1200 km long north-south trending Sagaing Fault, an active strike-slip fault which showed large and disastrous earthquakes in the past. Almost nothing is known about details of the Sagaing Fault in the area of Nay Pyi Taw, neither the precise location of different branches of the Sagaing Fault, nor the precise location of recent seismic events along different branches of the fault, nor the distribution and depth of the sedimentary layers in and around Nay Pyi Taw.

Since 2014, 4 shallow earthquakes with magnitudes larger than ML=4 are reported near Nay Pyi Taw. Some were clearly felt in the capital. The different location solutions reported by local and international agencies indicate a location accuracy not better accurate than 5 km. We derived re-locations and moment tensor analyses as well as meaningful model uncertainties for these events. The results show that the Sagaing Fault near Nay Pyi Taw may follow different active branches. While geological mapping indicates an active branch west of Nay Pyi Taw, the event locations and source mechanisms of the recent seismic activity indicate an active branch under and east of Nay Pyi Taw. Here, a geological mapping is complicated as sediments of unknown thickness cover the basement. Therefore, a microzonation study has been started with the aim to determine the fundamental resonant frequencies of the sedimentary layers, their spatial variability, and the amplification factors. First results of this ongoing project with more than 50 noise recordings in and around Nay Pyi Taw indicate amplification of ground motion with a factor up to 10 in distinct frequency ranges from 0.3 – 10 Hz.

How to cite: Milkereit, C., Tun, P. P., Than, O., Oo, K., Naing, K. Z., Aung, M. M., Htwe, Y. M. M., and Heimann, S.: Source mechanism of recent seismic events and microzonation studies along Sagaing Fault near Nay Pyi Taw, capital of Myanmar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2856, https://doi.org/10.5194/egusphere-egu2020-2856, 2020.

Soil conditions affect ground motion amplification. Thus, seismic site classification is a critical issue to predict ground motion parameters in the context of both probabilistic seismic hazard analysis and real-time generation of shaking maps. Especially on large areas, simplified procedures for estimating the seismic soil amplification can be advantageous. In order to account for these local effects, some proxies which account for the soil behaviour can be identified; e.g., the average shear-wave velocity of the upper 30 m (VS,30), or the equivalent shear-wave velocity from the depth of the seismic bedrock (VS,eq). 
In this study, two maps of seismic shallow soil classification for Italy according to Eurocode 8 (EC08) and the new Italian Building Code (ItBC2018) are presented. The methodology from which the maps are derived is described in Forte et al. (2019) and accounts for two sources of information: site-specific measurements and large-scale geological maps. The soil maps are obtained via a four-step procedure: 
(1) a database of about four-thousand shear-waves velocity (Vs) measurements coming from in-hole tests, surface geophysical tests and microtremors is built, covering (unevenly) the whole national territory; 
(2) twenty geo-lithological complexes are identified from the available geological maps; 
(3) the investigations are grouped as a function of the geo-lithological complex and the distribution of measured VS,30, VS,eq are derived;
(4) medians and standard deviations of such distributions are assumed to be representative of the corresponding complexes that are consequently associated to soil classes. 
The EC08 soil class map and the available database of Vs measurements were compared with the seismic soil map provided by the USGS based on a topographic slope-proxy (Allen and Wald, 2007). The latter is obtained by the correlation between topographic slope and VS,30, assuming morphometrical characteristics of the terrain as representative of the lithology. The slope-based method appears less reliable than the proposed approach, because its predictions resulted in a slight but systematic overestimation of the measured soil classes. Therefore, the proposed map can be more suitable for large-scale seismic risk studies, despite it is not a substitute of seismic microzonation and local site response analyses.
To make the results of the study available, a stand-alone software “SSC-Italy” has been developed and is freely available at http://wpage. unina.it/iuniervo/SSC-Italy.zip. 

How to cite: Forte, G., Chioccarelli, E., De Falco, M., Cito, P., Santo, A., and Iervolino, I.: Seismic soil class map for Italy according to European and Italian codes map , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18087, https://doi.org/10.5194/egusphere-egu2020-18087, 2020.

The topography effect has been thriving investigated based on numerical modeling. It impacts the seismic ground shaking, usually amplifying the amplitude of shaking at top hills or ridges and de-amplifying at valleys. However, the correlation between the earthquake-induced landslide and the topographic amplification is relatively unexplored. To investigate the amplification of seismic response on the surface topography and the role in the Chi-Chi earthquake-induced landslide in the JiuJiu peaks area, we perform a 3D ground motion simulation in the JiuJiu peaks area of Taiwan based on the spectral element method. The Lidar-derived 20m resolution Digital Elevation Model (DEM) data was applied to build a mesh model with realistic terrain relief. To this end, in a steep topography area like the JiuJiu peaks, the designed thin buffer layers are applied to dampen the mesh distortion. The three doubling mesh layers near the surface accommodate a more excellent mesh model. Our results show the higher amplification of PGA on the tops and ridges of JiuJiu peaks than surrounding mountains, while the de-amplification mostly occurs near the valley and hillside. The relief topography could have a ±50% variation in PGA amplification for compression wave, and have much more variety in PGA amplification for shear wave, which could be in the range between -50% and +100%. We also demonstrate that the high percentages of the landslide distribution right after the large earthquake are located in the topographic amplified zone. The source frequency content interacts with the topographic feature, in general, small-scale topography amplifies the higher-frequency seismic waves. It is worthy of further investigating the interaction between the realistic topography and the velocity structure on how to impact the seismic response in the different frequency bands. We suggest that the topographic seismic amplification should be taking into account in seismic hazard assessment and landslide evaluation.

How to cite: Chen, C.-T., Lee, S.-J., and Chan, Y.-C.: Surface topography effects on seismic ground motion and correlation with earthquake-induced landslide: An example of the JiuJiu peaks in 1999 Chi-Chi Taiwan earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6510, https://doi.org/10.5194/egusphere-egu2020-6510, 2020.

In many seismically active areas of the world, earthquake‐induced landslides commonly account for a significant portion of the total impact of earthquakes. When landslides occur offshore, in coastal areas, they can generate proximal tsunami waves that reach the coastlines in only a few minutes, and can be very dangerous.

The triggering power of earthquakes on landslides is often estimated on seismic wave amplitude (Peak ground acceleration, Arias intensity …), which is usually computed simply from the magnitude and distance of the earthquake using ground motion prediction equations (GMPEs). In this study we show that the local amplification due to site effect can be very strong offshore, and then should not be neglected.

In order to test and quantify the potential amplification of seismic waves offshore, we installed a broadband seismometer (PRIMA station) near the transition between the continental shelf and the upper continental slope, at a water depth of 18 m, offshore Nice city airport (southeastern France).  Situated at the mouth of the Var River, this zone is unstable and prone to landslides. A catastrophic landslide and tsunami already occurred in 1979, causing 10 casualties and large damages.

We analyze the recordings of earthquakes and seismic noise at the PRIMA station by comparing them to nearby inland stations. We find that the seismic waves are strongly amplified at PRIMA at some specific frequencies (with an amplification factor greater than 10 at 0.9 Hz). Using geological and geophysical data, we show that the main amplification frequency peak (at 0.9Hz) is due to the velocity contrast between the Pliocene sedimentary layer and fine-grained sediments dated from the Holocene. This offshore site effect could have a crucial impact on the triggering of a submarine landslide by an earthquake in this region. 

It is therefore crucial to detect and quantify the seismic amplification effects caused by superficial offshore sediment, in order to take them into account in predictive model.

How to cite: Courboulex, F., Mercerat, E. D., Larroque, C., Migeon, S., Deschamps, A., Hello, Y., Baques, M., Rivet, D., and Ambrois, D.: Offshore Landslides could be favored by Seismic Amplification due to Site Effects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19565, https://doi.org/10.5194/egusphere-egu2020-19565, 2020.

Three landslide inventories were prepared for the area affected by the 7.8 Mw Nepal earthquake (April 25, 2015). The first inventory contains 21,151 earthquake-induced landslides (EQL), directly associated to the 7.8Mw earthquake, mapped by using Google Earth’s pre and post-earthquake images, helicopter footage and Google Crisis data. Landslides were classified as debris flows, shallow translational landslides and rotational landslides. This last class included a relatively small number of events.  The second inventory includes only pre-event shallow landslides (PESL) to evidence those landslides which were already active before the 2015 earthquake. This inventory includes more than 2,500 landslides. The third inventory includes almost 20,000 large landslides (LL), consisting mostly of rock avalanches, slumps, rockslides, and deep-seated gravitational slope deformations (DSGSD). The spatial distribution of the three inventories was analysed with respect to land surface parameters. The EQL inventory shows in general a different spatial distribution with respect to the other two inventories. This is probably related to the seismic triggering and to the characteristics of the geographic area. A joint analysis of the LL and the EQL inventories shows that only a few earthquake-induced landslides (about 15 %) are directly associated to reactivation of LL.

A Principal Component Analysis (PCA) and a Discriminant Analysis were performed to analyse the controlling parameters on EQL and PESL. The analyses were based on: 1) land surface parameters, 2) hydrological parameters, 3) seismic parameters, 4) lithological parameters, 5) land cover, and 6) meteorological parameters. The statistical analyses show that the most critical variables for landslide triggering during an earthquake are associated to the land surface parameters, in association with the cosesimic displacement and the PGA,  that show an effect on the landslide size and density respectively. PESL seem to be mainly controlled by land surface parameters, with some of them (e.g. elevation) showing a slightly inverse relationship with landslide density. Agricultural land use, slope gradient and rainfall (reference period 1980-2000) show a high correlation with the PESL landslide triggering in absence of earthquakes.

How to cite: Valagussa, A., Frattini, P., Valbuzzi, E., Billinge-Jones, M., and Crosta, G. B.: 2015 Nepal Earthquake: A Comparison between Landslide Inventories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8293, https://doi.org/10.5194/egusphere-egu2020-8293, 2020.

Following the 7.8 Mw earthquake that struck Nepal on April 25th, 2015, a high-resolution earthquake-induced landslide inventory was prepared. 21,151 landslides have been mapped using Google Earth’s pre- and post-earthquake images, helicopter footage and Google Crisis data. For a representative subset of landslides (~7%), the main scar area was manually distinguished from the landslide transport and deposition areas. Starting from this subset of scar areas, six different relationships between scar area and total landslide area were attained for six different intervals of the landslide aspect ratio (AR, i.e. ratio between landslide length and width) which is used as a proxy of landslide mobility. These relationships were used to estimate the scar area for the entire dataset. For landslides with AR lower than 3 (i.e. low-mobility landslides) the total volume was calculated with the equations proposed by Larsen et al. (2010) by using the total landslide area values. For landslides with an AR larger than 3 (i.e. high-mobility landslides) the volume was computed by applying the equation by Larsen et al. (2010) to landslide scar area only, and considering a constant thickness for the runout area (1m based on field activities). By comparing the landslide denudation and mass wasting to uplift and subsidence measured by InSAR (ALOS-2 satellite data) following the Nepal earthquake, the net volume change in the earthquake-affected area was calculated.

How to cite: Frattini, P., Valagussa, A., Valbuzzi, E., and Crosta, G. B.: 2015 Nepal Earthquake: A Mass Wasting Balance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19419, https://doi.org/10.5194/egusphere-egu2020-19419, 2020.

     The Jinsha river fault zone in the eastern margin of the Tibetan Plateau is an old suture structure after the shutting of the proto-Tethys and a large scale ultra-lithosphere fault zone consisted of  5 to 6 fault branches with a width of 50km, have a long  geological evolution history. Since late Quatery, this fault zone is mainly dominated by dextral strike slip with partial thrusting component, absorbing  partial energy of the extrusion movement of  Tibetan Plateau. Along the fault zone, lower terraces of Jinsha river at Muronglou, Buzhong, Langzhong, Guxue, etc. were displaced, indicating the fault zone is active in late Quaternary, with an average rate of 3.5～4.3mm/ /yr. horizontally and 0.9-1.1mm/yr. vertically respectively in Holocene. Influenced by the intense fault activity of Jinsha river fault zone, this region is characterized by fractured rocks, strongly weathered surfaces.

      The Jinsha river, the upstream of the Yangtze river, parallel to Jinshajiang fault zone, flows from north to south, forming deep river valley and huge terrain elevation difference. Numerous huge landslides have developed along the river, for example, there are 23 giant avalanches in the 38 km long reach from Narong to Rongxue, with general volumes of 10~70 million m³ and even up to several hundreds million m³. Moreover, the landslides produce many loose clastic fragments which detonate many debris flows and river blocking. The latest disaster event is the Baige barrier lake in 2018 caused by landslide, with a water storage capacity of 524 million m³, causing tens of billions of yuan of economic losses. These landslides are distributed along the fault and its two sides, suggesting that these huge avalanches are closely related to the intense activity of the fault zone and special topography.

Keywords: Huge landslide, Jinsha River, Jinsha River Fault Zone, late Quatery activity

How to cite: Chang, Z., Chang, H., Mao, Z., and Guo, R.: Huge Landslides along the Jinsha River in Southeastern Tibetan Plateau and Their Association with the Recent Activity of Jinsha River Fault Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2815, https://doi.org/10.5194/egusphere-egu2020-2815, 2020.

Various mechanisms are proposed to explain landslide recovery time in the time following major earthquakes. However, research on prescribing possible recovery times following an earthquake is still relatively new. This paper provides an insight into factors governing landslide recovery time, which could be considered as a step forward in predictive modeling for landslide recovery time. To accomplish this, we examined 11 earthquake-affected areas based on the characteristics of both landslide events and landslide sites associated with diverse morphologic and climatic conditions. Our analyses indicate that the dominant characteristics of post-seismic landslide mechanisms determine the recovery time. The characteristics can be identified based on: (i) the fraction of area affected by landslides (%), (ii) mean relief and its standard deviation (m), (iii) average daily accumulated precipitation (mm) and (iv) rainfall seasonality index. If there are not enough co-seismic landslide deposits or not enough relief to trigger large deposits on hillslopes, then the recovery processes are mostly controlled by new landslides caused by a strength reduction of hillslope materials. In most of the cases, this brings a relatively quick recovery process in which the majority of post-seismic landslides may happen within a year or even in a month if sufficient intense rainfalls occur soon after the earthquake. If the predisposing factors create large co-seismic landslide deposits, then remobilization of material takes the role of the dominant mechanism and recovery may take years. Overall, our analyses show that the recovery takes relatively longer if a large amount of co-seismic landslide material is deposited within a high-relief mountainous environment where precipitation rate is low and not persistent.

How to cite: Tanyas, H., Kirschbaum, D., Lombardo, L., and Gorum, T.: A closer look at factors governing landslide recovery time in post-seismic periods , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8640, https://doi.org/10.5194/egusphere-egu2020-8640, 2020.

In tectonically active mountain belts, landslides contribute significantly to erosion. Statistical analysis of regional inventories of earthquake-triggered-landslides after large earthquakes (Mw>5.5) reveal a complex interaction between seismic shaking, landslide material, and rainfall. However, the contributions of each component have never been quantified due to a lack of in-situ data for active landslides. We exploited a 3-year geodetic and seismic dataset for a slow-moving landslide in Peru affected by local earthquakes and seasonal rainfalls. Here we show that in combination, they cause greater landslide motion than either force alone. We also show the rigidity of the landslide’s bulk clearly decreasing during Ml≥5 earthquakes. The recovery is affected by rainfall and small earthquakes (Ml<3.6), which prevent the soil from healing, highlighting the importance of the timing between forcings. These new quantitative insights into the mechanics of landslides open new perspectives for the study of the mass balance of earthquakes.

How to cite: Bontemps, N., Larose, E., Lacroix, P., Jara, J., and Taipe, E.: Rain and small earthquakes maintain a slow-moving landslide in a persistent critical state, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4464, https://doi.org/10.5194/egusphere-egu2020-4464, 2020.

Semi-empirical procedures for evaluating liquefaction potential (e.g. Seed & Idriss, 1971) require the estimation of cyclic resistance ratio (CRR) and cyclic shear stress ratio (CSR). The first can be obtained using empirical relationships based on in situ tests (e.g. CPT, SPT), the latter can be expressed as function of the maximum horizontal acceleration at ground surface (a_max), total and effective vertical stresses at the depth of interest (σ_v0, σ’_v0) and a magnitude-dependent stress reduction coefficient (r_d) that accounts for the deformability of the soil column (Idriss & Boulanger, 2004). All these methods were developed referring to a moment magnitude (M_w) equal to 7.5 and therefore require a magnitude scale factor (MSF) to make them suitable for different magnitude values. Usually, MSF and r_d are computed with reference to the mean or modal value of M_w taken from a disaggregation analysis, while a_max is obtained from a seismic hazard curve, including the contribution of various combinations of magnitudes and distances (Kramer & Mayfield, 2005). Thus, there might be inconsistency between the magnitude values used to evaluate either MSF or r_d and a_max. To overcome this problem, Idriss (1985) suggests to directly introduce the MSF in the probabilistic hazard analysis of the seismic acceleration. In this contribution, an alternative method is proposed, by properly modifying the acceleration seismic hazard curve conventionally adopted by the code of practice on the basis the disaggregation analysis, so that i) the contribution of the different magnitudes and the associated MSF and r_d-values are considered, ii) the computational effort is reduced since a CSR-hazard curve is straightforward obtained. This alternative method is used to carry out a simplified liquefaction assessment of a sand deposit located in the municipality of Casamicciola Terme (Naples, Italy), where the results of SPT tests are available from recent seismic microzonation studies. The CSR computed using the proposed procedure is lower than that obtained adopting the classical method suggested by Idriss & Boulanger (2004). This can be explained considering that the suggested method takes into account all the magnitudes that contribute to the definition of the seismic hazard, instead of considering the mean or modal value of the disaggregation analysis. Such an accurate prediction of the seismic demand may represent a basis for more reliable seismic microzonation maps for liquefaction and for a less conservative design of liquefaction risk mitigation measures.

References

Idriss, I.M. (1985). Evaluation of seismic risk in engineering practice, Proc. 11th Int. Conf. on Soil Mech. and Found. Engrg, 1, 255-320.

Idriss, I.M., Boulanger, R. W. (2004). Semi-Empirical Procedures for Evaluating Liquefaction Potential During Earthquakes, Proceedings of the 11th ICSDEE & 3rd ICEGE, (Doolin et al. Eds.), Berkeley, CA, USA, 1, 32-56.

Kramer, S.L., Mayfield, R.T. (2005) Performance-based Liquefaction Hazard Evaluation, Proceedings of the Geo-Frontiers Congress, January 24-26, Austin, Texas, USA.

Seed H.B., Idriss M. (1971). Simplified procedure for evaluating soil liquefaction potential, J. Soil Mech. Found. Div., 97, 1249-1273.

How to cite: Alleanza, G. A., de Silva, F., d'Onofrio, A., Gargiulo, F., and Silvestri, F.: Liquefaction assessment based on CSR-hazard curve through empirical procedure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21167, https://doi.org/10.5194/egusphere-egu2020-21167, 2020.

Numerical simulations of granular flows have been widely developed and used during the last two decades. Depending on the situation and scale of the simulations, different methods are used, each having specific pros and cons. Among them, three main methods can be distinguished such as; discrete, continuous or depth-averaged approach. At the laboratory scale, discrete approach consists of representing all the grains and contacts. When the amount of grains are important enough to consider the granular medium as an effective fluid, Navier-Stokes simulations can be performed using an appropriate rheology for the fluid, like the -rheology. However, when simulations are performed on geophysical scales none of these two methods can be used because of the enormous computation time required to solve them. To cope up with this issue, the depth-averaged approachs wherein the normal velocities are neglected, considerably reduce the computation time.

Even though all these models have been widely used, it is not clear exactly what information can be extracted about the forces exerted to the ground. These forces represent a new way of visualising a geophysical granular flow. Indeed, very recently, the recorded seismic signals from geophysical granular flows were used to interpret these forces. As a result, seismic data can be used to extract information on the flow dynamics which was missing due to the difficulties of direct observation (ashes, dust, etc…). Being able to compute and interpret the forces generated by a granular flow on
the ground represents a new way for calibrations of numerical methods and is a key point in analysing seismic data generated by granular flows and subsequently in understanding the landslide dynamics at the geophysical scale.

After a quick presentation of the numerical differences between the three models, we present comparisons between discrete, continuous [1] and depth-averaged [2] models. Besides, we put forward this study on the values taken by the forces generated on the ground during the evolution of granular dam breaks. Although, these three methods give relatively the same final deposits, in good agreement with the experiments, we observe they lead to very different dynamics in terms of flow acceleration, forces and histories.

1. http:basilisk.fr.

2. A. Mangeney et al., JGR 112 F02017 (2007)

How to cite: Martin, H., Viroulet, S., Peruzzetto, M., and Mangeney, A.: Simulations of the Basal Forces Generated by Dam Breaks: Comparison Between Continuous and Discrete Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22493, https://doi.org/10.5194/egusphere-egu2020-22493, 2020.