ALBERT

Description: We present a strategy for the region-specific assessment, adjustment, and weighting of ground-motion prediction models, with application to the 2015 Swiss national seismic-hazard maps. The models are provided within a logic-tree framework adopted for the probabilistic seismic-hazard analysis (PSHA). Through this framework, we consider both aleatory and epistemic uncertainties in ground-motion prediction in Switzerland, a region of low-to-moderate seismicity and consequently data poor in terms of strong-motion records. We use both empirical models developed using global strong-motion data and stochastic simulation models calibrated to local seismicity, characteristic wave-propagation effects, and site conditions. The empirical models were adjusted to account for (a) the selected hazard reference rock velocity model (using V S - 0 adjustments) and (b) the median instrumental ground-motion data at low magnitudes. The use of a carefully calibrated simulation model and V S - 0 adjusted empirical ground-motion prediction equations allowed us to precisely define the reference-rock profile, upon which subsequent analyses, such as microzonation and site-specific hazard, can be applied without uncertainty related to the reference condition. We implemented partially nonergodic aleatory uncertainties in ground-motion prediction through the single-station sigma approach. This strategy, complemented with the known reference rock condition, leads to significant reductions in the contribution of uncertainty in ground-motion characterization to PSHA in Switzerland. However, the application of the methodological framework outlined herein extends to any region, particularly those of low-to-moderate seismicity. Online Material: Tables of adjustment factors to convert selected ground-motion prediction equations (GMPEs) to the Swiss rock reference and figures showing trellis plots of all adjusted GMPEs with uncertainty estimates.

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: Ground motions of the 2011 Tohoku earthquake recorded at Onahama port (Iwaki, Fukushima prefecture) rank among the highest accelerations ever observed, with the peak amplitude of the 3-D acceleration vector approaching 2 g . The response of the site was distinctively non-linear, as indicated by the presence of horizontal acceleration spikes which have been linked to cyclic mobility during similar observations. Compared to records of weak ground motions, the response of the site during the M w 9.1 earthquake was characterized by increased amplification at frequencies above 10 Hz and in peak ground acceleration. This behaviour contrasts with the more common non-linear response encountered at non-liquefiable sites, which results in deamplification at higher frequencies. We simulate propagation of SH waves through the dense sand deposit using a non-linear finite difference code that is capable of modelling the development of excess pore water pressure. Dynamic soil parameters are calibrated using a direct search method that minimizes the difference between observed and simulated acceleration envelopes and response spectra. The finite difference simulations yield surface acceleration time-series that are consistent with the observations in shape and amplitude, pointing towards soil dilatancy as a likely explanation for the high-frequency pulses recorded at Onahama port. The simulations also suggest that the occurrence of high-frequency spikes coincided with a rapid increase in pore water pressure in the upper part of the sand deposit between 145 and 170 s. This sudden increase is possibly linked to a burst of high-frequency energy from a large slip patch below the Iwaki region.

Description: The determination of near-surface attenuation for hard rock sites is an important issue in a wide range of seismological applications, particularly seismic hazard analysis. In this article we choose six hard to very-hard rock sites ( Vs 30 1030–3000 m s –1 ) and apply a range of analysis methods to measure the observed attenuation at distance based on a simple exponential decay model with whole-path attenuation operator r . The r values are subsequently decoupled from path attenuation ( Q ) so as to obtain estimates of near-surface attenuation ( 0 ). Five methods are employed to measure r which can be split into two groups: broad-band methods and high-frequency methods. Each of the applied methods has advantages and disadvantages, which are explored and discussed through the comparison of results from common data sets. In our first step we examine the variability of the individual measured r values. Some variation between methods is expected due to simplifications of source, path, and site effects. However, we find that significant differences arise between attenuation measured on individual recordings, depending on the method employed or the modelling decisions made during a particular approach. Some of the differences can be explained through site amplification effects: although usually weak at rock sites, amplification may still lead to bias of the measured r due to the chosen fitting frequency bandwidth, which often varies between methods. At some sites the observed high-frequency spectral shape was clearly different to the typical r attenuation model, with curved or bi-linear rather than linear decay at high frequencies. In addition to amplification effects this could be related to frequency-dependent attenuation effects [e.g. Q ( f )]: since the r model is implicitly frequency independent, r will in this case be dependent on the selected analysis bandwidth. In our second step, using the whole-path r data sets from the five approaches, we investigate the robustness of the near-surface attenuation parameter 0 and the influence of constraints, such as assuming a value for the regional crustal attenuation ( Q ). We do this by using a variety of fitting methods: least squares, absolute amplitude and regressions with and without fixing Q to an a priori value. We find that the value to which we fix Q strongly influences the near-surface attenuation term 0 . Differences in Q derived from the data at the six sites under investigation could not be reconciled with the average values found previously over the wider Swiss region. This led to starkly different 0 values, depending on whether we allowed for a data-driven Q , or whether we forced Q to be consistent with existing simulation models or ground motion prediction equations valid for the wider region. Considering all the possible approaches we found that the contribution to epistemic uncertainty for 0 determination at the six hard-rock sites in Switzerland could be represented by a normal distribution with standard deviation 0  = 0.0083 ± 0.0014 s.

Description: We present a stochastic ground-motion model for Switzerland. A model for the far-field Fourier amplitude spectra of earthquakes is first proposed, constrained by ground motions observed from small local and regional earthquakes in addition to macroseismic intensities observed from large damaging events in Switzerland. We then use this model in a stochastic simulation technique to generate predictions of peak ground acceleration and velocity and 5%-damped response pseudo-spectral acceleration. We facilitate the prediction of ground motions from finite-fault sources through the use of the R EFF distance metric and show that magnitude and distance scaling-behavior of events up to M w  7.5 is broadly consistent with Next Generation Attenuation and Japanese models. Ground-motion prediction uncertainty is described in terms of between- and within-event uncertainties through residual analysis of response spectra. Finally, single-station sigma is derived, accounting for the prediction uncertainty at a given site without the ergodic assumption. Consistency of the model is emphasized through its compatibility with other seismic hazard products: the model is referenced to a generic rock profile that was developed by utilizing velocity profiles of the sites of seismic stations and is calibrated at higher magnitudes to the Swiss regional macroseismic intensity prediction model. Furthermore, the model is based on moment magnitudes from the recently developed Earthquake Catalogue of Switzerland 2009 (ECOS09). The well-defined reference for the model means that it can easily be adapted for a site-specific application, for instance, through the use of proxies such as quarter-wavelength velocity and travel-time average shear-wave velocity in the upper 30 m ( V S 30 ).

Description: High-frequency acceleration pulses recorded during recent damaging earthquakes show that the evolution of pore water pressure in liquefiable soils may have a significant effect on earthquake ground motions. Such observations suggest that advanced constitutive soil models capable of treating the phase transformation behavior of liquefiable soils should be used for reliable predictions of earthquake site response. Advanced constitutive models require knowledge of the dilatancy parameters that describe the potential of soils to generate excess pore water pressure. We demonstrate that these dilatancy parameters can be determined directly from field observations by inverting strong motions recorded on vertical arrays (i.e., installation of surface and borehole accelerometers). We analyze the records of the 1987 M  6.6 Superstition Hills earthquake, the 1993 M  7.8 Kushiro-Oki, Japan, earthquake, and the 2011 M  9.0 Tohoku, Japan, earthquake to quantify the dilatancy parameters at the Wildlife liquefaction array (WLA), at Kushiro port (KP), and the KiK-net site FKSH14, respectively. Synthetic acceleration time series obtained from the minimum misfit models are describing the time and frequency evolution of the observations more precisely than previously published models. Dilatancy parameters obtained for WLA and KP suggest that soils at these sites were more resistant to liquefaction than predicted from field and laboratory tests. We also infer a high liquefaction resistance (CRR 7.5 =0.5) for the site FKSH14, which exhibited dilation pulses of up to during the Tohoku earthquake. These findings indicate that even soils with a strong liquefaction resistance may exhibit cyclic mobility effects during strong and prolonged ground motions.

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: 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 high-quality 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.

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 analyse the raw, unfiltered acceleration time-series strong-motion data used for the European Strong Motion Database. After selecting high-quality recordings, suitable for Fourier analysis, we estimate crustal and site attenuation properties, Q 0 and 0 , respectively, using two methods: a broad-band spectral modelling approach and a high-frequency linear fit. We find 0 varies strongly, from negligible to 0  = 0.09 s, with an average of 0  = 0.032 s or 0  = 0.033 s depending on the method employed. This is consistent with the wide variety of recording-site conditions from hard-rock to very-soft soil. Using the attenuation model, we then proceed to determine site-class amplification, seismic moments and 2 stress parameters for several events with M w values between 5 and 7.6. Site amplification is shown to vary strongly within a single site-class, although average amplification is consistent with resonance expected at soft-soil sites and theoretical crustal amplification at hard-rock sites. We show that seismic moments determined from Fourier spectra are consistent with database M w values from moment tensor analysis, and that the resulting stress parameters are independent of magnitude or depth. Finally, we show that using the results of our analyses, along with the R eff distance metric to account for the geometry of the finite fault, we can predict pseudospectral acceleration (peak ground acceleration to 10 s) of the Izmit 1999 ( M w  7.5–7.6) event using a point-source stochastic simulation.