Recent Earthquake-Triggered Landslide Events in Central Asia, Evidence of Seismic Landslides in the Lesser Caucasus and the Carpathians

peer reviewedThis chapter presents an overview of earthquake-triggered landslide hazards in Central Asia, with a focus on the Tien Shan and Pamir Mountain Ranges. It essentially compiles information presented by the author in his previous publi-cations as well as other published results. This overview will show that in Central Asian mountain areas, most giant mass movements are likely to have a seismic origin—though it cannot be proved for most of them (see companion report by A. Strom). Here, we will briefly introduce those paleo-cases to establish a link with the Carpathian Mountains and the Lesser Caucasus, where old massive landslides could have a seismic origin as well and thus could provide information on ancient high-magnitude earthquakes for which no written information is available. In recent history, large earthquakes triggered only minor slope failures in the Carpathians and the Lesser Caucasus, while almost all M > 7 events that occurred last century in Central Asian mountain ranges triggered numerous landslides, including at least one two-billion m3 mass movement. However, none of these events can be compared with the 1999 Chi-Chi, the 2005 Kashmir or the 2008 Wenchuan earthquakes, which had triggered many thousands of landslides. The question is then—are such events impossible in Central Asia (maybe due to the much dryer climate than in the regions affected by the cited earthquakes) even though high-magnitude earthquakes are rela-tively common in the Tien Shan and Pamir? One possible response will be provided by a detailed outline of the largest earthquake disaster that had affected the Central Asian mountain regions in historic times: the Khait event in 1949. Some additional notes will outline the potential for future massive landslide activation during earthquakes in Armenia and Romania

Analysis of three-component ambient vibration array measurements

Both synthetic and observed ambient vibration array data are analysed using high-resolution beam-forming. In addition to a classical analysis of the vertical component, this paper presents results derived from processing horizontal components. We analyse phase velocities of fundamental and higher mode Rayleigh and Love waves, and particle motions (ellipticity) retrieved from H/V spectral ratios. A combined inversion with a genetic algorithm and a strategy for selecting possible model parameters allow us to define structural models explaining the data. The results from synthetic data for simple models with one or two layers of sediments suggest that, in most cases, the number of layers has to be reduced to a few sediment strata to find the original structure. Generally, reducing the number of soft-sediment layers in the inversion process with genetic algorithms leads to a class of models that are less smooth. They have a stronger impedance contrast between sediments and bedrock. Combining Love and Rayleigh wave dispersion curves with the ellipticity of the fundamental mode Rayleigh waves has some advantages. Scatter is reduced when compared to using structural models obtained only from Rayleigh wave phase velocity curves. By adding information from Love waves some structures can be excluded. Another possibility for constraining inversion results is to include supplementary geological or borehole information. Analysing radial components also can provide segments of Rayleigh wave dispersion curves for modes not seen on the vertical component. Finally, using ellipticity information allows us to confine the total depth of the soft sediments. For real sites, considerable variability in the measured phase velocity curves is observed. This comes from lateral changes in the structure or seismic sources within the array. Constraining the inversion by combining Love and Rayleigh wave information can help reduce such problems. Frequency bands in which the Rayleigh wave dispersion curves show considerable scatter are often better resolved by Love waves. Information from the horizontal component can be used to correctly assign the mode number to the different phase-velocity curve segments, especially when two modes seem to merge at osculation points. Such merging of modes is usually observed for Rayleigh waves and thus can be partly solved if additional information from the Love waves and the horizontal component of Rayleigh waves is considered. Whenever a site presents a velocity inversion below the top layer, Love wave data clearly helps to better constrain the solutio

Recent technological and methodological advances for the investigation of landslide dams

River-damming by landslides is a widespread phenomenon around the world. Recent advances in remote sensing technology and the rising commercial availability of their products enable the assemblage of increasingly more complete inventories and improve monitoring efforts. On the ground, multi-method dating campaigns enhance our understanding of the timelines of dam formation and failure. In comparison to single-dating methods, they reduce uncertainty by using different materials from the landslide deposit, facilitate the advantages of each method, and consider the deposit and the source area. They can pin dates on the time of lake drainage where backwater sediments are included in the dating campaign and thus inform about dam longevity. Geophysical methods provide non-invasive and rapid methods to investigate the properties and interior conditions of landslide dams. By identifying, e.g. evolving zones of weakness and saturation they can aid in the monitoring of a dam in addition to providing information on interior stratification for scientific research. To verify results from geophysical campaigns, and to add details of dam interior structures and geotechnical properties, knowledge of their sedimentology is essential. This information is gathered at sections from breached dams, other (partially) eroded landslide deposits, and through laboratory testing of sampled material. Combining the knowledge gained from all these methods with insights from blast-fill and embankment dam construction, physical and numerical modelling in multi-disciplinary research projects is the way forward in landslide dam research, assessment and monitoring. This review offers a broad, yet concise overview of the state-of-the-art in the aforementioned research fields. It completes the review of Fan et al. (2020) on the formation and impact on landslide dams

S-wave velocity measurements applied to the seismic microzonation of Basel, Upper Rhine Graben

An extensive S-wave velocity survey had been carried out in the frame of a recent seismic microzonation study of Basel and the border areas between Switzerland, France and Germany. The aim was to better constrain the seismic amplification potential of the surface layers. The survey included single station (H/V spectral ratios) and ambient vibration array measurements carried out by the Swiss team, as well as active S-wave velocity measurements performed by the German and French partners. This paper is focused on the application of the array technique, which consists in recording ambient vibrations with a number of seismological stations. Several practical aspects related to the field measurements are outlined. The signal processing aims to determine the dispersion curves of surface waves contained in the ambient vibrations. The inversion of the dispersion curve provides a 1-D S-wave velocity model for the investigated site down to a depth related to the size of the array. Since the size of arrays is theoretically not limited, arrays are known to be well adapted for investigations in deep sediment basins, such as the Upper Rhine Graben including the area of the city of Basel. In this region, 27 array measurements with varying station configurations have been carried out to determine the S-wave velocity properties of the geological layers down to a depth of 100-250 m. For eight sites, the outputs of the array measurements have been compared with the results of the other investigations using active sources, the spectral analysis of surface waves (SASW) and S-wave reflection seismics. Borehole information available for a few sites could be used to calibrate the geophysical measurements. By this comparison, the advantages and disadvantages of the array method and the other techniques are outlined with regard to the effectiveness of the methods and the required investigation depth. The dispersion curves measured with the arrays and the SASW technique were also combined and inverted simultaneously to use the advantages of both methods. Finally, the paper outlines and discusses the contribution of the S-wave velocity survey to the new seismic microzonation of the Basel region. In this regard one major outcome of the survey is the quantification of vertical and lateral changes of the S-wave velocity, due to changing lithology or changing compaction and degree of weathering of the layer

Earthquake-induced landslide hazard assessment in the Vrancea Seismic Region (Eastern Carpathians, Romania): Constraints and perspectives

peer reviewedIn seismically-active regions, earthquake-induced landslides (EqIL) are likely to enhance slope denudation and sediment delivery both over short and longer terms, which might strongly condition landscape evolution in general. In mountain regions marked by a medium to high seismicity, co-seismic slope failures typically present a relatively low frequency but also high magnitude pattern which should be addressed accordingly within landslide hazard assessment, considering the already high frequency of precipitation-triggered landslide events. The Vrancea Seismic Region located in the curvature sector of the Eastern Carpathians (Romania) is the most active intermediate-depth seismic zones (focal depth > 70 km) in Europe. It represents the main seismic energy source throughout Romania with significant transboundary effects recorded as far as Ukraine and Bulgaria. During the last 300 years, the region featured 13 earthquakes with magnitudes (Mw) above 7, out of which seven events had Mw above 7.5 and three between 7.7 and 7.9. Apart from the direct damages, the Vrancea earthquakes are also responsible for causing numerous other geohazards, such as ground fracturing, groundwater level disturbances and deep-seated landslide occurrences (e.g. rock slumps, rock-block slides, rock falls, rock avalanches). The previous large earthquake-induced deep-seated landslides of the Vrancea region were found to affect the entire slope profile. They often formed landslide dams which strongly influenced the river morphology, posing a serious threat to human life and human facilities of the downstream rural communities through the imminent lake outburst floods. Despite the large potential of this research issue, the correlation between the region's seismotectonic context and landslide geomorphic predisposing factors has not been extensively documented and fully understood yet. Presently, the available geohazard inventories provide limited historical information to quantify the triggering role of seismic activity for observed slope failures across the Vrancea region. However, it is acknowledged that the morphology and geology of numerous large, deep-seated and dormant landslides of this region, which may be reactivated in future, with head scarps near mountain tops and located close to faults, in anti-dip slope conditions show significant similarities to the large mass movements with a proven seismic origin (such as in the Tien Shan, Pamir, Longmenshan, etc.). Thus, the relationship between landslide occurrences and the joint action of triggers and preparing factors (seismotectonic or climatic or both) needs to be investigated in more detail and further considered in the regional multi-hazard risk assessments. The purpose of this paper is to outline the current knowledge level of the landslide-earthquake relationship by accounting for the possible effects of the previous major earthquakes in the Vrancea region. The key findings contribute to the gain of the baseline knowledge for an improved assessment framework of multi-hazard (earthquake-landslide) risks, as required by the Sendai Framework for Disaster Risk Reduction (SFDRR)

Investigation of deep geohazard sites with seismic and ambient noise methods, combined with 3D Geomodelling

peer reviewedDuring the last years our group has studied a series of massive slope failures as well as deep geo-sites for engineering projects (dam and possible tunnel construction sites) located in various geohazard contexts. Therefore, we tested various combinations of active seismic and ambient noise methods and representation of related outputs in complex 3D Geomodels. Investigated sites include massive, likely seismically landslides in the Romanian Carpathian Mountains, slopes near the Rogun Dam construction site in Tajikistan, a very large active landslide in the Swiss Alps, an incipient volcanic flank collapse on El Hierro Canary Island and a site selected for the possible installation of an Einstein Telescope in the BE-NL-DE border region

Belgium

The stability of rock slopes is often guided significantly by the structural geology of the rocks composing the slope. In this work, we analyse the influence of structural characteristics, and of their seismic response, on large and deep-seated rock slope failure development. The study is focused on the Tamins and Fernpass rockslides in the European Alps and on the Balta and Eagle’s Lake rockslides in the southeastern Carpathians. These case studies are compared with catastrophic rock slope failures with ascertained or very likely seismic origin in the Tien Shan Mountains. The main goal is to identify indicators for seismically induced rock slope failures based on the source zone rock structures and failure scar geometry. We present examples of failures in anti-dip slopes and along-strike rock structures that were potentially (or partially) caused by seismic triggering, and we also consider a series of mixed structural types, which are more difficult to interpret conclusively. This morpho-structural study is supported by 2D and 3D distinct element numerical models of the Balta site (with reconstructed initial mountain morphology) showing that seismic shaking typically induces deeper-seated deformation in initially ‘stable’ rock slopes. In addition, for failures partially triggered by seismic shaking, these studies can help identify the contribution of the seismic factor to slope instability. The identification of the partial seismic origin on the basis of the dynamic response of rock structures can be particularly interesting for case histories in less seismically active mountain regions (in comparison with the Andes, Tien Shan, Pamirs), such as in the European Alps and the Carpathian Mountains. 3D models were also run to simulate the full rock avalanche process, including the formation of a dam on the valley floor

2D and 3D dynamic numerical modelling of seismically induced rock slope failure

peer reviewedThe stability of rock slopes is often guided by the structural geology of the rocks composing the slope. Especially, discontinuities can significantly influence slope stability according to their orientation with respect to the one of general slope. Here, we will focus on the triggering of giant rockslides. The final goal is to identify failure characteristics allowing us to distinguish seismic trigger modes from climatic ones, notably on the basis of the source zone rock structures. This study is supported by dynamic numerical modelling. More specifically, we will present results based on a parametric numerical study, using distinct element codes designed for 2D and 3D dynamic analysis. This study was applied to the Balta rockslide in the SE Carpathian Mountains (Romania) that has been extensively studied by Mreyen et al. (2019) during the last years

Landslide Susceptibility Mapping in the Vrancea-Buzau Seismic Region, Southeast Romania

peer reviewedThis study presents the results of a landslide susceptibility analysis applied to the Vrancea- Buzau seismogenic region in the Carpathian Mountains, Romania. The target area is affected by a large diversity of landslide processes. Slopes are made-up of various types of rocks, climatic conditions can be classified as wet, and the area is a seismically active one. All this contributes to the observed high landslide hazard. The paper analyses the spatial component of the landslide hazard affecting the target area, the regional landslide susceptibility. First, an existing landslide inventory was completed to cover a wider area for the landslide susceptibility analysis. Second, two types of methods are applied, a purely statistical technique, based on correlations between landslide occurrence and local conditions, as well as the simplified spatial process-based Newmark Displacement analysis. Landslide susceptibility maps have been produced by applying both methods, the second one also allowing us to simulate different scenarios, based on various soil saturation rates and seismic inputs. Furthermore, landslide susceptibility was computed both for the landslide source and runout zones—the first providing information about areas where landslides are preferentially triggered and the second indicating where landslides preferentially move along the slope and accumulate. The analysis showed that any of the different methods applied produces reliable maps of landslide susceptibility. However, uncertainties were also outlined as validation is insufficient, especially in the northern area, where only a few landslides could be mapped due to the intense vegetation cover