ALBERT

Description: Defining the reference rock or soil condition related to ground-motion prediction is an important aspect of seismic-hazard analysis. In a previous study by the authors, a method was proposed to establish a reference rock profile for Switzerland through the comparison of empirical amplification functions with shear-wave velocity profiles at 27 selected sites of the Swiss National Seismic Network. The retrieved velocity profile served as reference for a regional ground-motion prediction equation. However, a lacking piece of information remained: the anelastic attenuation for such a reference profile. Reference attenuation is essential to correctly model and interpret amplification at high frequencies. In the present study we extended our approach to simultaneously model both the reference shear-wave velocity profile and the corresponding attenuation for Japan. We compared site-specific attenuation measurements with quarter-wavelength average velocities at 36 soil and rock sites from the Japanese KiK-net strong-motion network. The selected sites are characterized by a lack of observed resonance phenomena in order to avoid trade-off between amplification and attenuation effects. We establish a parametric model through regression analysis. The resulting model gives us the possibility to estimate anelastic attenuation of a rock site with a given velocity profile and provides the base for host-to-target adjustments of real or modeled ground motion.

Description: Large earthquakes from the intermediate-depth Vrancea seismic zone are known to produce in Bucharest ground motion characterized by predominant long periods. This phenomenon has been interpreted as the combined effect of both seismic source properties and site response of the large sedimentary basin. The thickness of the unconsolidated Quaternary deposits beneath the city is more than 200 m, the total depth of sediments is more than 1000 m. Complex basin geometry and the low seismic wave velocities of the sediments are primarily responsible for the large amplification and long duration experienced during earthquakes. For a better understanding of the geological structure under Bucharest, a number of investigations using non-invasive methods have been carried out. With the goal to analyse and extract the polarization and dispersion characteristics of the surface waves, ambient vibrations and low-magnitude earthquakes have been investigated using single station and array techniques. Love and Rayleigh dispersion curves (including higher modes), Rayleigh waves ellipticity and SH -wave fundamental frequency of resonance ( f 0 SH ) have been inverted simultaneously to estimate the shear wave velocity structure under Bucharest down to a depth of about 8 km. Information from existing borehole logs was used as prior to reduce the non-uniqueness of the inversion and to constrain the shallow part of the velocity model (〈300 m). In this study, we use data from a 35-km diameter array (the URS experiment) installed by the National Institute for Earth Physics and by the Karlsruhe Institute of Technology during 10 months in the period 2003–2004. The array consisted of 32 three-component seismological stations, deployed in the urban area of Bucharest and adjacent zones. The large size of the array and the broad-band nature of the available sensors gave us the possibility to characterize the surface wave dispersion at very low frequencies (0.05–1 Hz) using frequency–wavenumber techniques. This is essential to explore and resolve the deeper portions of the basin. The horizontal to vertical spectral ratio (H/V) curves provide important additional information about the structure and are here characterized by two major peaks. The first is attributed to the fundamental frequency of the basin, while the second can be interpreted as a mixture of the second higher mode of Rayleigh waves and other types of waves such as SH waves. This hypothesis has been verified by comparing the H/V curves with the SH -wave transfer function from the retrieved velocity structure. We could also approximate the SH transfer function with H/V ratios of earthquake recordings, providing additional verification of the robustness of the proposed velocity model. The Cretaceous bedrock depth was then inverted at each URS station from the fundamental frequency of resonance and using this model. A 3-D geophysical model for Bucharest has been constructed based on the integration of the inverted velocity profiles and the available geological information using a geographic information system.

Description: In the framework of the renewal project of the Swiss Strong Motion Network (SSMNet), a procedure for site characterization has been established. The aim of the procedure was to systematically derive realistic 1D velocity profiles at each station. It is mainly based on the analysis of surface waves, particularly from passive experiments, and includes cross checks of the derived amplification functions with those obtained through spectral modeling of recorded earthquakes. The systematic use of three component surface-wave analysis, allowing the derivation of both Rayleigh and Love dispersion curves, also contributes to the improvement of the quality of the retrieved profiles. The procedure is applied to the 30 SSMNet stations installed on various site types within the project, covering different aspects of seismic risk. The characterization of these 30 sites gives an overview of the variety of possible effects of surface geology on ground motion in the Alpine area. Such effects ranged from deamplification at hard-rock sites to amplification up to a factor of 15 in lacustrine sediments with respect to the Swiss reference rock velocity model. The derived velocity profiles are shown to reproduce observed amplification functions from empirical spectral modeling. Although many sites are found to exhibit 1D behavior, the procedure allows the detection and qualification of 2D and 3D effects. The sites are therefore classified with respect to the occurrence of 2D/3D resonance and edge-generated surface waves. In addition to the large and deeply incised alpine valleys of the Rhône, the Rhine, and the Aar, smaller structures such as local alpine valleys and alluvial fans are shown to exhibit 2D/3D behavior.

Description: A predictive equation to obtain the vertical-to-horizontal ratio (V/H) of ground motion for rock sites has been established in a previous article. The method was based on the comparison between V/H of Fourier and response spectra of earthquakes with the quarter-wavelength average velocity at discrete frequencies. We extend this approach to account for resonance phenomena at soft-sediment sites. In order to do so, a new parameter is defined and included in the comparison with the V/H spectra. The new parameter is directly derived from the quarter-wavelength velocity and represents the frequency-dependent seismic impedance contrast at the site. We show that extending the correlation in this three-dimensional space is beneficial to reconstruct V/H of the 5%-damped response spectra at soft-sediment sites ( V S 30 〈800 m/s) for which a shear-wave velocity profile is available. In this study we analyze 220 sites of the Japanese KiK-net strong-motion network. These sites were selected from the entire network through comparison of the fundamental frequencies estimated from the recordings and by indirect modeling methods. From the analysis, two types of predictive equations are then established, the first based on frequency-dependent and the second on frequency-independent correlations. These can subsequently be used to reconstruct the V/H spectrum at any site with a known shear-wave velocity profile. For both equations, uncertainties of the V/H models are provided, and a sensitivity study to magnitude–distance dependence is presented. Finally, we show an example of the application of the model at four selected soft-sediment sites of the Swiss Seismic Network.

Description: We investigated the sequence of 2-D resonance modes of the sediment fill of Rhône Valley, Southern Swiss Alps, a strongly overdeepened, glacially carved basin with a sediment fill reaching a thickness of up to 900 m. From synchronous array recordings of ambient vibrations at six locations between Martigny and Sion we were able to identify several resonance modes, in particular, previously unmeasured higher modes. Data processing was performed with frequency domain decomposition of the cross-spectral density matrices of the recordings and with time-frequency dependent polarization analysis. 2-D finite element modal analysis was performed to support the interpretation of processing results and to investigate mode shapes at depth. In addition, several models of realistic bedrock geometries and velocity structures could be used to qualitatively assess the sensitivity of mode shape and particle motion dip angle to subsurface properties. The variability of modal characteristics due to subsurface properties makes an interpretation of the modes purely from surface observations challenging. We conclude that while a wealth of information on subsurface structure is contained in the modal characteristics, a careful strategy for their interpretation is needed to retrieve this information.

Description: 〈span〉〈div〉Abstract〈/div〉We explore the role of scenario‐dependent site amplification on local magnitude (ML) and possible bias it may introduce. ML is strongly influenced by local site response, which is conditioned by unique local geological factors. To isolate the effect of the near‐surface amplification on ML, relative differences between station‐specific ML at the surface and borehole (ΔML,STN) are studied for 34 sites from the KiK‐net network, Japan. We find strong moment magnitude (M) dependent scenario‐specific ΔML,STN trends over the range 3.0〈M〈6.5. To model these trends, we employ the stochastic method, initially using empirical surface‐to‐borehole (S/B) Fourier spectral ratios for the site term. Simulated data, ΔML,STN(M), based on the available site‐response information are shown to closely match the empirical ΔML,STN trends. Subsequently, the site term is replaced with (a) linear 1D shear‐wave (horizontal) transfer function (1D‐SHTF) amplification, (b) horizontal‐to‐vertical ratios, and (c) quarter wavelength amplification to calculate ΔML,STN(M) in the absence of S/B. We find that ΔML,STN(M) trends are best estimated with S/B as the site term, but in many cases using a linear 1D‐SHTF model is adequate. Furthermore, we discuss how this phenomenon may be related to the observed inequality between M and ML at low magnitudes and how ΔML,STN(M) may be used in the future to compute unbiased ML with greater confidence.〈/span〉