SUMMARY
The difficulty and the high cost to assess the subsurface properties led to the development of several geophysical techniques. Generally, the focus of a site study is the reconstruction of ...the S-wave velocity profile down to few tens to hundreds of metres (e.g. 30–300 m), but not the investigation of deeper structures, such as the transition to the crystalline basement. However, such deeper structures are of interest when seismic hazard products have to relate to a reference rock-velocity profile, for example in regional seismic hazard assessment and microzonation studies. To estimate the S-wave velocity profiles down to several kilometres, we study the potential of Rayleigh and Love waves at low (down to 0.1 Hz) and high (up to 20 Hz) frequencies using two seismic arrays of increasing size. The small array, with a maximum inter-station distance of 900 m and a recording time of 3 hr, was aimed at constraining the shallow subsurface down to about 350–400 m, while the big one, with a maximum inter-station distance of more than 29 km and 23 hr of recording had the goal to constrain the deeper structure. The arrays were deployed in northern Switzerland (east of the village of Herdern) within the Swiss Molasse basin, a sedimentary basin north of the Alps stretching from Lake Constance to Lake Geneva; its thickness increases from 800 to 900 m in the northeast to more than 5 km in the southwest. The seismic data recorded by the two arrays were analysed using the techniques developed for the analysis of small-aperture arrays. The results were inverted for the S-wave velocity profile in two steps: first, the Rayleigh and Love wave phase dispersion curves were inverted together. Secondly, the previous dispersion curves were jointly inverted with the measured Rayleigh wave ellipticity angle. The resulting S-wave velocity profiles are similar and show agreement with the available geological and geophysical data, confirming the potential of surface waves to investigate deep structures. Moreover, our analysis proves the feasibility of site characterization techniques to large arrays and the possibility to estimate the P- and S-wave velocity profiles down to 5 km, deeper than the contrast between Molasse basin and crystalline rock at around 2.1 km.
Summary
Physical properties of near-surface soil and rock layers play a fundamental role in the seismic site effects analysis, being an essential element of seismic hazard assessment. Site-specific ...mechanical properties (i.e. shear- and compressional-wave velocities and mass density) can be inferred from surface wave dispersion and horizontal-to-vertical or ellipticity data by non-linear inversion techniques. Nevertheless, results typically exhibit significant inherent non-uniqueness as different models may fit the data equally well. Standard optimization inversion techniques minimize data misfit, resulting in a single representative model, rejecting other models providing similar misfit values. An alternative inversion technique can be formulated in the Bayesian framework, where the posterior probability density on the model space is inferred. This paper introduces an inversion approach of surface wave dispersion and ellipticity data based on a novel multizonal transdimensional Bayesian formulation. In particular, we parametrize 1-D layered velocity models by the varying number of Voronoi nuclei, allowing us to treat the number of layers as an unknown parameter of the inverse problem. The chosen parametrization leads to the transdimensional formulation of the model space, sampled by a reversible jump Markov chain Monte Carlo algorithm to provide an ensemble of random samples following the posterior probability density of model parameters. The used type of the sampling algorithm controls a model complexity (i.e. the number of layers) self-adaptively based on the measured data's information content. The method novelty lies in the parsimonious selection of sampling models and in the multizonal formulation of prior assumptions on model parameters, the latter allows including additional site-specific constraints in the inversion. These assumptions may be based on, e.g. stratigraphic logs, standard penetration tests, known water table, and bedrock depth. The multizonal formulation fully preserves the validity of the transdimensional one, as demonstrated analytically. The resultant ensemble of model samples is a discrete approximation of the posterior probability density function of model parameters and associated properties (e.g. VS30, quarter-wavelength average velocity profile and theoretical SH-wave amplification function). Although the ultimate result is the posterior probability density function, some representative models are selected according to data fit and maximum of the posterior probability density function. We first validate our inversion approach based on synthetic tests and then apply it to field data acquired from the active seismic survey and single-station measurements of ambient vibrations at the SENGL seismic station site in central Switzerland.
In 2011, an amplification map achieved by macroseismic information was developed for Switzerland using the collection of macroseismic intensity observations of past earthquakes. For each village, a
...ΔIm
was first derived, which reflects the difference between observed and expected macroseismic intensities from a region-specific intensity prediction equation. The
ΔIm
values are then grouped into geological/tectonic classes, which are then presented in the macroseismic amplification map. Both, the intensity prediction equation and the macroseismic amplification map are referenced to the same reference soil condition which so far was only roughly estimated. This reference soil condition is assessed in this contribution using geophysical and seismological data collected by the Swiss Seismological Service. Geophysical data consist of shear-wave velocity profiles measured at the seismic stations and earthquake recordings, used to retrieve empirical amplification functions at the sensor locations. Amplification functions are referenced to a generic rock profile (Swiss reference rock condition) that is well defined, and it is used for the national seismic hazard maps. Macroseismic amplification factors
Af
, derived from empirical amplification functions, are assigned to each seismic station using ground motion to intensity conversions. We then assess the factors
dΔf
defined as the difference between
Af
and
ΔIm
. The factor
dΔf
accounts for the difference between the reference soil condition for the intensity prediction equation and the Swiss reference rock. We finally analysed relationships between
Af
and proxies for shear-wave velocity profiles in terms of average shear-wave velocity over defined depth ranges, such as
V
S
,30
, providing an estimate of the reference shear velocity for the intensity prediction equation and macroseismic amplification map. This study allows linking macroseismic intensity observations with experimental geophysical data, highlighting a good correspondence within the uncertainty range of macroseismic observations. However, statistical significance tests point out that the seismic stations are not evenly distributed among the various geological–tectonic classes of the macroseismic amplification map and its revision could be planned merging classes with similar behaviour or by defining a new classification scheme.
Abstract
Strong ground-motion databases used to develop ground-motion prediction equations (GMPEs) and calibrate stochastic simulation models generally include relatively few recordings on what can ...be considered as engineering rock or hard rock. Ground-motion predictions for such sites are therefore susceptible to uncertainty and bias, which can then propagate into site-specific hazard and risk estimates. In order to explore this issue we present a study investigating the prediction of ground motion at rock sites in Japan, where a wide range of recording-site types (from soil to very hard rock) are available for analysis. We employ two approaches: empirical GMPEs and stochastic simulations. The study is undertaken in the context of the PEGASOS Refinement Project (PRP), a Senior Seismic Hazard Analysis Committee (SSHAC) Level 4 probabilistic seismic hazard analysis of Swiss nuclear power plants, commissioned by swissnuclear and running from 2008 to 2013. In order to reduce the impact of site-to-site variability and expand the available data set for rock and hard-rock sites we adjusted Japanese ground-motion data (recorded at sites with 110 m s−1 < Vs30 < 2100 m s−1) to a common hard-rock reference. This was done through deconvolution of: (i) empirically derived amplification functions and (ii) the theoretical 1-D SH amplification between the bedrock and surface. Initial comparison of a Japanese GMPE's predictions with data recorded at rock and hard-rock sites showed systematic overestimation of ground motion. A further investigation of five global GMPEs’ prediction residuals as a function of quarter-wavelength velocity showed that they all presented systematic misfit trends, leading to overestimation of median ground motions at rock and hard-rock sites in Japan. In an alternative approach, a stochastic simulation method was tested, allowing the direct incorporation of site-specific Fourier amplification information in forward simulations. We use an adjusted version of the model developed for Switzerland during the PRP. The median simulation prediction at true rock and hard-rock sites (Vs30 > 800 m s−1) was found to be comparable (within expected levels of epistemic uncertainty) to predictions using an empirical GMPE, with reduced residual misfit. As expected, due to including site-specific information in the simulations, the reduction in misfit could be isolated to a reduction in the site-related within-event uncertainty. The results of this study support the use of finite or pseudo-finite fault stochastic simulation methods in estimating strong ground motions in regions of weak and moderate seismicity, such as central and northern Europe. Furthermore, it indicates that weak-motion data has the potential to allow estimation of between- and within-site variability in ground motion, which is a critical issue in site-specific seismic hazard analysis, particularly for safety critical structures.
Moment magnitudes (MW) are computed for small and moderate earthquakes using a spectral fitting method. 40 of the resulting values are compared with those from broadband moment tensor solutions and ...found to match with negligible offset and scatter for available MW values of between 2.8 and 5.0. Using the presented method, MW are computed for 679 earthquakes in Switzerland with a minimum ML= 1.3. A combined bootstrap and orthogonal L1 minimization is then used to produce a scaling relation between ML and MW. The scaling relation has a polynomial form and is shown to reduce the dependence of the predicted MW residual on magnitude relative to an existing linear scaling relation. The computation of MW using the presented spectral technique is fully automated at the Swiss Seismological Service, providing real-time solutions within 10 minutes of an event through a web-based XML database. The scaling between ML and MW is explored using synthetic data computed with a stochastic simulation method. It is shown that the scaling relation can be explained by the interaction of attenuation, the stress-drop and the Wood–Anderson filter. For instance, it is shown that the stress-drop controls the saturation of the ML scale, with low-stress drops (e.g. 0.1–1.0 MPa) leading to saturation at magnitudes as low as ML= 4.
The geometry of three-dimensional subsurface structures plays an important role in determining local seismic site effects as in the case of alpine valleys. Detailed knowledge of these structures is ...fundamental in seismic hazard and risk studies. In this study we investigate an area in the upper Rhone valley around Visp, in the southwestern part of Switzerland. A large dataset of geological and geophysical data, consisting of borehole logs, microtremor horizontal to vertical spectral ratios and shear-wave velocity measurements, was compiled to build a detailed 3D model of the subsurface. By combining fundamental frequency information from noise recordings and shear-wave velocity profiles, three main geophysical discontinuities were identified and their physical properties constrained through a stepwise process. First, the bedrock depth was estimated; in a second step a generic velocity model was defined and finally, combining all the available geological and geophysical information, we developed a 3D geophysical model. The model was compared with a local 3D geological model and a model derived from gravimetric data. The study area is a complex alpine valley where 2D/3D wave propagation phenomena occur. In such case a purely 1D response assumption is considered to be invalid. In order to test the 3D model, we modelled different ambient-vibration wave fields and compared observed and synthetic H/V spectral ratios. We slightly modified our 3D geophysical model in some areas based on this comparison. Finally, a good match between simulated and empirical spectral ratios corroborated the model. The results suggest that the use of ambient vibration techniques are a powerful and cost-effective tools to reconstruct three-dimensional models of the subsurface. Finally, we used the 3D model to predict amplification of earthquake ground motion in the basin. Again, the match between observed and modelled amplification at the locations of the seismic stations is good. This allows us to map amplification inside the study area.
•A detailed 3D model of the subsurface in Visp (Switzerland) is build using geophysical and geological data.•Three main geophysical discontinuities were identified and their physical properties constrained through a stepwise process.•Ambient-vibrations wave field is modelled and comparison between observed and synthetic H/V spectral ratios is made.•The 3D model is used to predict amplification of earthquake ground motion in the basin.
SUMMARY
Modelling the attenuation of shear wave energy is an important component of seismic hazard analysis. Previous studies have shown how attenuation, particularly in the uppermost layers of the ...crust, is regionally dependent. The impact of this is that the decay of energy radiating from an earthquake will vary from place to place. To quantify the regional attenuation in Switzerland we model the Fourier spectral amplitude of small‐to‐moderate earthquakes, recorded on the local seismic networks. High‐frequency decay is parametrized by Q and κ, while apparent geometrical spreading models account for the frequency‐independent decay of energy. We analyse ground motion encompassing the significant duration of shaking to provide models that are useful for the purpose of seismic hazard analysis. Two methods are used to estimate the whole path attenuation parameter, t*: first, a simultaneous fit of the source model and attenuation effects across the entire spectral bandwidth for earthquakes with M > 2; and secondly, a linear fit of an attenuation model to the high‐frequency part of the spectrum for earthquakes with M > 3.5. The t* parameter is found to vary with hypocentral distance consistent with a weakly attenuating crust and strongly attenuating uppermost layer. 1‐D tomographic inversions indicate a profile of increasing Q with depth down to the Moho. Frequency‐independent decay is parametrized using a three‐part model which allows for the inclusion of Moho reflection phases in the spectrum in the range of 20–140 km in the Swiss Foreland and from 70 to 140 km in the Swiss Alps.
In regions with moderate seismicity and large intervals between strong earthquakes, paleoseismological archives that exceed the historical and instrumental timescale are needed to establish reliable ...estimates of earthquake recurrence for long return periods. In several regions, lake sediments have shown to be suitable for paleoseismological studies by causally linking characteristic sedimentological features to historic earthquakes. Studies on single lakes, however, do neither allow determining the paleoepicentre nor the paleomagnitude for the potential paleoearthquakes. Here we compile, using shaking-induced mass movements and micro deformations (summarized as Sedimentary Event Deposits SEDs), the sedimentary paleoseismic record of 11 lakes from Switzerland over the last 10,000 years. The large dating uncertainty attributed to such deposits (up to 250 years) does not allow us to conclusively test for one large earthquake hypothesis when comparing the different lake records and therefore represents one of the major limitations of this approach. Instead, using a new approach of exploring the normalized frequency of occurrence averaged over a larger area, the compiled dataset reveals striking periods of enhanced occurrence of SEDs in the studied lakes during several phases of the past 10,000 years, centered at 9700, 6500 and during the last 4000 cal yr BP. Moreover, we use a calibrated intensity attenuation relation in order to model scenarios of possible epicentral areas and ranges of magnitudes of paleoearthquakes. We differentiate two cases: (i) a ‘single-earthquake’ scenario if SEDs occur simultaneously in various studied lakes, or (ii) a ‘multi-earthquake’ scenario if SEDs in the studied lakes cluster within a time interval. The modelled scenarios allow us to propose maximally possible magnitudes of large paleoearthquakes, constituting an important input for seismic hazard assessment in the Swiss Alps.
•Catalogue of event deposits from the sediment record of 11 lakes over 10,000 years.•Coeval deposits in multi-lake settings as proxy for seismic activity close to lakes.•The earthquake-related deposit frequency curve shows peaks at 9700 and 4000 to 0 BP•Using seismic attenuation relation we reconstruct scenarios of paleoearthquakes.
Rock slope failures can lead to huge human and economic loss depending on their size and exact location. Reasonable hazard mitigation requires thorough understanding of the underlying slope driving ...mechanisms and its rock mass properties. Measurements of seismic ambient vibrations could improve the characterization and detection of rock instabilities since there is a link between seismic response and internal structure of the unstable rock mass. An unstable slope near the village Gondo has been investigated. The unstable part shows strongly amplified ground motion with respect to the stable part of the rock slope. The amplification values reach maximum factors of 70. The seismic response on the instable part is highly directional and polarized. Re-measurements have been taken 1 year later showing exactly the same results as the original measurements. Neither the amplified frequencies nor the amplification values have changed. Therefore, ambient vibration measurements are repeatable and stay the same, if the rock mass has not undergone any significant change in structure or volume, respectively. Additionally, four new points have been measured during the re-measuring campaign in order to better map the border of the instability.
Graphical abstract
.
The ambient vibration analysis is a non-invasive and low-cost technique used in site characterization studies to reconstruct the subsurface velocity structure. Depending on the goal of the research, ...the investigated depth ranges from tens to hundreds of meters. In this work, we aimed at investigating the deeper contrasts within the crust and in particular down to the sedimentary-rock basement transition located at thousands of meters of depth. To achieve this goal, three seismic arrays with minimum and maximum interstation distances of 7.9 m and 26.8 km were deployed around the village of Schafisheim. Schafisheim is located in the Swiss Molasse Basin, a sedimentary basin stretching from Lake Constance to Lake Geneva with a thickness ranging from 800 to 900 m in the north to 5 km in the south. To compute the multimodal dispersion curves for Rayleigh and Love waves and the Rayleigh wave ellipticity angles, the data were processed using two single-station and three array processing techniques. A preliminary analysis of the inversion results pointed out a good agreement with the fundamental modes of Rayleigh and Love waves used in the inversion and a quite strong disagreement with the higher modes. The impossibility to explain at the same time most of the dispersion curves was interpreted as the co-existence, within the investigated area, of portions of the subsurface with different geophysical properties. The hypothesis was confirmed by the Horizontal-to-Vertical spectral analysis (H/V) which indicated the presence of two distinguished areas. The observation allowed a new interpretation and the identification of the Rayleigh and Love wave fundamental modes and of the S-wave velocity profiles to be reconstructed for each investigated zone. It results in two S-wave velocity profiles with similar velocities down to 15 km deferring only in their shallow portions due to the occurrence of a low velocity zone at a depth of 50–150 m at the centre of the investigated area.
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