Geotechnical characterization and stability analysis of subaqueous slopes in Lake Lucerne (Switzerland).

Tsunamis occur not only in marine settings but also in lacustrine environments. Most of the lacustrine tsunamis are caused by seismically- or aseismically-triggered mass movements. Therefore, an assessment of the stability of subaqueous slopes is crucial for tsunami hazard assessment in a lake. We selected Lake Lucerne (Switzerland) as a natural laboratory to perform an in-depth geotechnical characterization of its subaqueous slopes. This lake experienced documented tsunamis in 1601 and 1687. Some of its slopes still bear sediment volumes with a potential for tsunamigenic failure. To identify such slopes, we interpreted available reflection seismic data and analyzed the bathymetric map. Then, we performed 152 dynamic Cone Penetration Tests with pore pressure measurement (CPTu) and retrieved 49 sediment cores at different locations in the lake. These data were used to characterize the failure-prone sediments and to evaluate the present-day static stability of subaqueous slopes. Obtained results allowed the definition of three classes of slopes in terms of static stability: unstable slopes, stable slopes close to the unstable state, and stable areas. Non-deltaic slopes with thicker unconsolidated fine-grained sediment drape and moderate-to-high slope gradients (> 5-10°) have the lowest Factor of Safety. In agreement with previous studies, the failure plane for the non-deltaic slopes is embedded within the fine-grained glaciolacustrine sediments. Deltaic slopes with prevailing coarse-grained sediments mostly appear statically stable. Finally, we generalized the measured undrained shear strength profiles into the depth-dependent power-law models. These models define the of Lake Lucerne's sediments and can be applied to other lakes with similar sedimentation history. Supplementary Information The online version contains supplementary material available at 10.1007/s11069-022-05310-1

Seismic Characterization of Swiss Strong-Motion Borehole-Station Sites by Inversion of Full Microtremor Horizontal-to-Vertical Spectral Ratios [H/V(z,f)]

The assessment of the local site amplification during an earthquake requires, among other input information, a reliable estimate of the shear-wave velocity profile, including the con-tact with engineering and seismic bedrocks. We determine the shear-wave velocity (VS) profiles at two Swiss strong-motion borehole-station sites at Visp (Valais) and Buochs (Nidwalden) by inversion of microtremor horizontal-to-vertical spectral ratio [H/V(z,f)] curves measured at the surface and at different depths. These borehole stations were built to monitor not only the seismic activity in Switzerland and the surrounding areas but also the nonlinear site response, especially liquefaction processes during strong local and regional earthquakes. The boreholes are equipped with accelerometers at various depths, with the deepest borehole located at 102 m below the surface. In the first part, we review the forward modeling algorithm of the full-microtremor H/V(z,f), with a focus on the com-putational cost and accuracy. In the second part, we perform a temporal analysis of the H/V(z,f) curves obtained from the accelerometers. The results show seasonal variabilities in H/V between summer and winter. The third part presents the inversions of the H/V curves for a single day in summer and winter at both sites. From the full H/V(z,f) inversion, we obtain shear-wave velocities in the upper 30 m (VS30) of 216 and 209 m/s at Visp in winter and summer, respectively. At Buochs, the corresponding VS30 are 269 and 345 m/s. The depths of the seismic bedrock are at 219 and 210 m at Visp, and at Buochs they are at 293 and 213 m. The estimated velocity profiles compare well with independent estimates from array measurements of ambient seismic vibrations, gravimetry, and geological log-ging information. Finally, we use the obtained seismic velocity profile information to model the theoretical 1D shear wave transfer function. The latter result compares well with amplification function results obtained using earthquake recordings.ISSN:0037-1106ISSN:1943-357

Project LAKE TSUNAMIS: ocean-bottom seismometer operations in Lake Lucerne (Switzerland) in 2018-2023

The project "Lake Tsunamis: Causes, Controls, and Hazard (Characterization of subaqueous unstable slopes with geophysical and geotechnical measurements.)" was initiated to understand their governing mechanisms of genesis, propagation, frequency, and the related hazard within an interdisciplinary context. Swiss lakes served as a laboratory for this holistic approach. The project was divided into five work packages (WP), where WP1 comprised a large number of geophysical (using amongst other techniques ocean bottom seismometers (OBS)) and geotechnical measurements to characterize the structure and stability of potentially unstable subaqueous lake slopes. To evaluate the potential and applicability of ambient vibration techniques in a shallow-water offshore environment, multiple single-station and array OBS measurements were performed on subaqueous slopes in Lake Lucerne. Eight DEPAS Lobster type broadband OBS from the German Instrument Pool for Amphibian Seismology (DEPAS) and one Nammu type OBS from ETH Zürich were successfully deployed and recovered at more than 170 distinct locations in 2018-2020. In 2020-2023 the single Nammu OBS was deployed several times for supplemental measurements. Surveys with an airgun of 1-inch³ volume were used on top of the deployment locations to determine the misorientation of the horizontal components. In addition, multibeam bathymetric surveys were performed to locate the positions of the OBS on the lake floor with high accuracy. A workflow for passive seismic investigations with OBS in such shallow-water settings was developed. The seismic response and its variability at the measured sites in terms of amplification functions during earthquakes and resonance frequencies was determined. Shear-wave velocity profiles at different morphological types of slopes down to a depth of 100-150 m below the lake floor were resolved and interpreted. Combining geophysical and geotechnical measurements and interpretation, static and dynamic slope-stability analyses were performed. Thresholds for the subaqueous slope-failure triggering in terms of earthquake magnitude and epicentral distance, macroseismic intensity, and ground-motion intensity measures were derived using earthquake ground-motion modelling

Investigating the subsurface in a shallow water environment using array and single-station ambient vibration techniques

Single-station and array ambient vibration techniques are widely used in onshore environments, in particular to retrieve the subsurface structure and shear-wave velocity profiles. We apply these techniques offshore in Lake Lucerne (Switzerland) using single-station and array Ocean Bottom Seismometer (OBS) data. This lake has experienced tsunamigenic subaquatic slope failures in the past and still has sediment-charged slopes that might fail in the presence of a seismic or aseismic trigger. The application of traditional onshore methods offshore brings additional challenges related to the processing of recorded data. To overcome these challenges, we perform multibeam bathymetry surveys to precisely locate the OBS on the lake floor and airgun shootings to determine the orientation of the horizontal components of the seismometer and to correct the time drift of the recorder. Then we obtain surface-wave phase velocity dispersion curves of Scholte and Love waves, and Scholte wave ellipticity curves at six subaquatic slopes. After the estimation of the dispersion curves, we deal with their modal identification using mode attribution analysis. The shear-wave velocity and thickness of the sedimentary layers at the investigated slopes are inferred using a transdimensional Bayesian inversion algorithm. The resolved velocity profiles show very low shear-wave velocities in shallow lake sediments and allow us to improve the understanding of the local stratigraphy. This research contributes to the assessment of stability and tsunamigenic potential of subaquatic slopes in Lake Lucerne.ISSN:0956-540XISSN:1365-246

On the seismic response and earthquake-triggered failures of subaqueous slopes in Swiss lakes

Seismically triggered subaqueous mass movements in lakes may generate tsunamis that can cause significant damage on the shore. In this study, we assess the seismic response and stability of subaqueous slopes in Swiss lakes based on recorded seismological data, historical and geological information and geotechnical surveys. We performed seismic investigations at multiple locations in Lake Lucerne using Ocean Bottom Seismometers (OBS). For these locations, we derived ground-motion amplification functions from local and regional earthquakes and horizontal-to-vertical spectral ratios (H/V) from the earthquake and ambient vibration recordings. The results show (1) very high amplification levels, often exceeding values of 50–100 in the frequency range between 1 and 10 Hz, (2) the fundamental frequency of resonance in the range of 0.5–3.5 Hz and (3) laterally variable site response even for closely located stations. We sought also the signatures of non-linear site response in the H/V curves or ground-motion amplification functions but found only weak indicative effects and no clear evidence. This is most likely due to the low levels of ground motion recorded during the OBS campaigns. We conducted back analyses of historical earthquakes in Switzerland with available documental and/or geological evidence of induced (tsunamigenic) subaqueous slope failures in Swiss lakes. The data set of historical events was complemented with a selection of instrumentally recorded earthquakes in Switzerland. For the analyses, we selected multiple sites in Swiss lakes which failed in the past or are prone to failure in the future. We modelled the ground motion at these locations assuming Swiss standard reference rock conditions (vs30 = 1105 m s−1). The modelled ground motion intensity measures (IM) included peak ground acceleration (PGA), peak ground velocity (PGV) and pseudospectral acceleration (PSA) at 0.3, 1 and 2 s. We estimated the minimum ground motion and macroseismic intensity at reference rock conditions required to trigger the failures of subaqueous slopes. In addition, we defined a threshold for the seismic triggering of such failures in terms of moment magnitude (Mw) and epicentral distance (Re) as: Mw = 2.891 + 1.904 log10(Re + 5.166) for Re ≥ 3.7 km. Our results are consistent with previous studies based on worldwide observations. Furthermore, we related the modelled ground motions to the Swiss seismic hazard products and estimated the return period of critical ground shaking responsible for triggering subaqueous slope failures (with potential for tsunami generation) to be in the range of 36–224 yr. Finally, based on previously collected geotechnical data (in situ Cone Penetration Testing and laboratory sediment analysis), we determined the most likely values of the seismic coefficient k to be used with the ground motion IMs modelled at reference rock conditions in infinite slope stability analyses to estimate the factor of safety (FS). For PGA, we found a k = 1; for PGV, k = 2; for PSA0.3s, k = 0.6; for PSA1s, k = 2 and for PSA2s, k = 5.5. These estimates are conservative and affected by the trade-off between the thickness of unconsolidated sediments and the slope angle. Thus, we recommend applying them to slopes with a low-to-moderate gradient (<15◦) and sediment thickness of more than 2 m.ISSN:0956-540XISSN:1365-246