Coupling Between Stratigraphic and Topographic Effects on Slopes (original) (raw)
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Advances and problems in understanding the seismic response of potentially unstable slopes
Engineering Geology, 2011
The influence of site effects on landslide triggering during earthquakes has been inferred in several studies, but its evaluation is made difficult by the complexity of factors controlling the dynamic response of potentially unstable slopes and also by the lack of local ground motion instrumental observations. This work explores this problem and reports new findings based on an ongoing long term accelerometric monitoring conducted on a landslide-prone test area in the Apennine Mountains, Italy, where the presence of site effects enhancing seismic susceptibility of local slopes has been invoked on the basis of historic accounts of landsliding triggered at large epicentral distance. The recordings relative to low-to-moderate magnitude earthquakes showed significant amplifications affecting hillslope portions covered by thick (N 5 m) colluvia and pronounced amplification maxima oriented along the local maximum slope direction on a recent deep-seated landslide. While the amplifications seem most likely linked to high impedance contrast between surface materials and underlying substratum, the causes of directivity are less clear. The case of the monitored test site together with evidence of site response directivity identified on other hillslopes, suggest that the directivity phenomena can result from a combination of topographic, lithological and structural factors that act together to re-distribute shaking energy, focusing it on site-specific directions. Thus, it is difficult to single out the critical factors controlling such phenomena and no general criterion for the identification of sites affected by directivity is proposed here. Nevertheless, the presence and orientation of site response directivity can be revealed through reconnaissance techniques by using recordings of seismic weak motion and/or ambient microtremors, and calculating azimuthal variation of shaking energy and horizontal-to-vertical ground motion spectral ratios. A comparison with the recordings obtained during the recent M W = 6.3 earthquake that hit the Abruzzo region in April 2009 demonstrated that analysis relying on data from low energy events can furnish valid indications for slope behaviour also under stronger shaking, provided the data are well differentiated in terms of distance, azimuth and source characteristics. Furthermore, the comparative analysis of the Abruzzo earthquake recordings at a landslide and reference sites showed that directivity properties of strong shaking had been correctly anticipated using earlier weak motion observations. However, under the complex slope conditions the identification of resonance frequencies from horizontal-to-vertical spectral ratios estimated from weak-motion accelerometric recordings does not seem reliable, and better results have been obtained by velocimetric microtremor recordings.
Geophysical Journal International, 2014
The recent growth of seismic monitoring networks allows for systematic studies of local seismic effects at sites with pronounced topography. We applied a terrain classification method to identify such sites within Swiss and Japanese networks and compiled a data set of highquality earthquake recordings. As a number of recent studies have found local effects to be directional at sites with strong topographic features, polarization analysis of particle motion was performed and azimuthally dependent resonant frequencies were estimated. The same procedure was also applied for available ambient vibration recordings. Moreover, average residuals with respect to ground motion prediction models for a reference bedrock were calculated to estimate the average amplification or deamplification for each station. On one hand, observed amplifications are found to be tightly linked with ground motion directionality as estimated by polarization analysis for both earthquake and ambient vibration recordings. On the other hand, we found no clear relation between local topographic features and observed amplification, so the local subsurface properties (i.e. shear wave velocity structure) seem to play the key role and not the geometry itself.
On the role of topographic amplification in seismic slope instabilities
Journal of Rock Mechanics and Geotechnical Engineering, 2015
Surface wave generation due to body wave propagation near ground surface has been discussed in the literature. This phenomenon, typically occurring in topographic changing areas, along with its interaction with body waves (SV), decreases precision of formulas for evaluation of slope displacement. This significant fact caused the researchers not only to investigate the combined surface and SV waves motion pattern, but also to consider its effect on structures built on the slopes. In order to reveal the phenomenon, several finite element numerical studies have been performed by ABAQUS programme. Besides, two physical model slopes simulating the landslide occurrence have been constructed and tested by shaking table device. The results of induced and calculated accelerations obtained by two approaches have been compared and Rayleigh wave generation has been proved. Furthermore, the slope displacements have been calculated by various empirical methods and the results were compared with numerical ones. The results proved that in order to increase the precision of empirical formulas for displacement prediction, surface wave effect should be taken into account. Finally, a concept of "effective depth of surficial amplification" is introduced and its effect on dynamic slope stability is analysed.
Journal of Seismology, 2003
Increased structural damage caused byearthquakes on hilltops and along ridgeshas often been related to amplification ofground motion due to the presence oftopography. However, comparison betweenobservations and numerical modeling hasshown that amplification is only partlydependent on the prominent surfacemorphology. Strong effects are also inducedby soft layers, such as weathered rockmaterial or colluvium, covering thetopographies.Numerous seismically triggered landslidesare reported to occur in the same materialsthat are likely to amplify ground motions.Therefore, it can be suspected that groundmotion dynamics significantly contribute tothe observed slopes failures. Thispotential relationship is the subject ofthe present case study, the Ananevorockslide in the northeastern Tien Shanmountains. The survey included geophysicalprospecting, earthquake recordings andstructural analyses of the rock fabric. Onthe basis of the field data, observedamplification effects could be related tothe local geological conditions andparticularly to the surface morphology andto the presence of low-velocity layers –deeply weathered rocks – on the top of thebedrock. Surface layer- andtopography-dependent amplification has alsobeen studied numerically by 2D and 3Dfinite element modeling of ground motiondynamics. The present paper focuses onadditional effects that may be induced bythe presence of a fault zone and of thelandslide scarp. Further, observed andcomputed ground motion dynamics areconnected with slope failuresusceptibility: 2D numerical simulationsreveal that strain localization is closelyrelated to wave amplification in surficiallow-velocity layers, particularly belowconvex surface morphologies.
How sensitive the effects of lateral heterogeneity on seismic ground motion are
This paper discusses site effects due to complex local geology in terms of seismic building codes. Our discussion is based on Euroseistest. The reasons for this choice is the very detailed information available on the subsoil structure at this shallow, alluvial valley, and the existence of a large database with earthquake records of small and moderate intensity, which makes Euroseistest one of the best documented cases of site effects studies. We take as our starting point previous studies of site effects at this valley ) that showed the large impact of the 2D valley structure on its site response. In this study, we pursue those ideas but extend the objectives along two lines. First, we quantify the additional amplification introduced by the complex geology relative to the predictions of a 1D model in terms of an "aggravation factor". Second, we check the sensitivity of the 2D/1D aggravation factor on the basis of a thorough parametric analyses on the most important parameters that condition the seismic response of 2D soil structures that is dynamic properties (shear wave velocity and attenuation distribution with depth) and shape of geometric characteristics of the basin.
Directivity of slope dynamic response to seismic shaking
Geophysical Research Letters, 2007
1] A four year-long accelerometric monitoring of a landslide-prone slope area in Italy provided evidence of amplifications with systematic directional differences in shaking energy by a factor of 2 -3, apparently correlated with the directions of local topographic features. The examination of regional-scale data revealed similar site response directivity phenomena also on some steep rock slopes not affected by amplification. The spectral properties of the site responses were investigated using HVSR analysis and a new approach based on the multiplication of spectra from several events. The results suggest that such phenomena are caused by a directional redistribution of spectral energy controlled by a combination of interrelated topographic and geologic factors acting in similar directions. Citation: Del Gaudio, V., and J. Wasowski , Directivity of slope dynamic response to seismic shaking,
Two-dimensional assessment of topographical site effects on earthquake ground response
4th International FLAC Symposium on Numerical Modeling in Geomechanics, Paper: 04-08
Amplification factors resulting from earthquakes were assessed for different sites of two real cases: the earthquakes in Colombia (1999) and El Salvador (2001). Major damage was reported concentrated towards the top of slopes and ridges. In El Salvador, a big landslide was triggered during the earthquake in a combined failure mechanism. The geological materials in both cases are medium to highly plastic, and consist of pyroclasts, epyclasts, paleosoils, and residual soils of volcanic origin. From both areas example sites were numerically modeled with the real acceleration records (frequency-independent) and with artificial sinusoidal functions with different frequencies. Results show higher amplification patterns along the top of the hills with an irregular amplification/deamplification trend within narrow ranges. Along the slopes, the amplification generally decreases towards the base with an about exponential trend, and in depth decrease linearly. The results of the landslide section were similar to the failed slope.
Effects of Vertical Heterogeneity of Soil Sediments on Seismic Soil Response
Site effects are resulted from the non-linear filtering mechanisms within the soil sedimentary layers overlying bedrock. In contemporary design codes, site effects are taken into account by introducing different site factors for different site classes. The prescribed site classification systems are based on averaging shear wave velocity in the soil sediments. However, significant amplification of the seismic displacement demand may be developed from mechanisms which can result in resonance behaviour. In such situations, soil amplification cannot be determined accurately by considering the average shear wave velocity of the sediments alone. The effects of vertical heterogeneity in the soil sediments have not been explicitly parameterised in the conventional code provisions. This paper presents results from parametric studies showing the influence of vertical heterogeneity in the soil sediments on the soil amplification behaviour. A methodology for modelling soil heterogeneity is desc...
Experimental Investigation of the Topographic Modification of Earthquake Ground Motion
2017
Experimental Investigation of the Topographic Modification of Earthquake Ground Motion Jacob Dafni Chair of the Supervisory Committee: Chair Joseph Wartman Civil and Environmental Engineering Topographic modification of earthquake ground motion can significantly impact ground motion amplitude and frequency content. While previous studies have investigated topographic modification of ground motion, also called “topographic effect”, there are discrepancies between the results of field and numerical investigations. A new experimental approach involving physical modeling in a geotechnical centrifuge was used to study topographic effects. The centrifuge captures the complexity of a physical process and shares many of the advantages of a numerical model (e.g., material properties, instrumentation location, and ground motions can be controlled). Experimental results show that topographic amplification can exceed amplification due to subsurface geology (i.e., “site amplification”). The resu...