ALBERT

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 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.