Abstract
Site-specific seismic hazard studies involving detailed account of the site response require the prior estimate of the hazard at the local reference bedrock level. As the real characteristics of such local bedrock often correspond to “hard-rock” with S-wave velocity exceeding 1.5 km/s, “standard rock” PSHA estimates should be adjusted in order to replace the effects of “standard-rock” characteristics by those corresponding to the local bedrock. The current practice involves the computation of scaling factors determined on the basis of VS (S-wave velocity) and “κ0” (site specific, high-frequency attenuation parameter) values, and generally predicts larger high-frequency motion on hard rock compared to standard rock. However, it also proves to be affected by large uncertainties (Biro and Renault, Proceedings of the 15th world conference on earthquake engineering, 24–28, 2012; Al Atik et al., Bull Seism Soc Am 104(1):336–346 2014), mainly attributed to (i) the measurement of host and target parameters, and (ii) the forward and inverse conversions from the response spectrum domain to the Fourier domain to apply the VS and κ0 adjustments. Moreover, recent studies (Ktenidou and Abrahamson, Seismol Res Lett 87(6):1465–1478, 2016) question the appropriateness of current VS − κ0 scaling factors, so that the significant amplification of high frequency content for hard-rock with respect to standard-rock seems overestimated. This paper discusses the key aspects of a few, recently proposed, alternatives to the standard approach. The calibration of GMPEs directly in the Fourier domain rather than in the response spectrum domain is one possibility (Bora et al., Bull Seism Soc Am 105(4):2192–2218, 2015, Bull Earthq Eng 15(11):4531–4561, 2017). Another possibility is the derivation of GMPEs which be valid also for hard-rock conditions (e.g. Laurendeau et al., Bull Earthq Eng 16(6):2253–2284, 2018). In this latter case the host site response is first removed using theoretical site response analyses (and site velocity profile), or generalized inversions techniques. A third possibility is to use existing hard rock surface recordings to derive purely empirical scaling models from standard rock to hard rock (Ktenidou et al., PEER Report, Pacific Earthquake Engineering Research Center, Berkeley, 2016). Finally, when a sufficient amount of records are available at a given site, generic GMPEs can be scaled to the site-specific ground motion using empirical site residual (δS2Ss) (Kotha et al., Earthq Spectra 33(4):1433–1453, 2017; Ktenidou et al., Bulletin of Earthquake Engineering 16(6):2311–2336, 2018). Such alternative approaches present the advantage of a significant simplification with respect to the current practice (with thus a reduced number of uncertainty sources); their generalization calls however for high-quality recordings (including high-quality site metadata) for both host regions and target sites, especially for small to moderate magnitude events. Our answer to the question in the title is thus “No, alternative approaches exist and are promising; though, their routine implementation requires additional work regarding systematic site characterization (for the host regions) and high-quality site characterization/instrumentation (for the target site), and so do also the needed improvements of the existing HTTA procedure”.
Highlights
To account for the local site response within a seismic hazard assessment (SHA) study can be achieved following different approaches (e.g., Aristizabal, 2018)
The simplest generic methods use Ground Motion Prediction Equations (GMPEs) where site conditions are characterized only by simple site proxies such as the average shear wave velocity over the upper 30 m, VS30 (e.g. Bindi et al, 2017), soil classes based on site period (e.g., Zhao et al, 2016), or ‘‘depth to bedrock’’
Another important point that should be kept in mind is that the adjustment approach described above implicitly assumes that the ground motion predicted for the ‘‘standard-rock’’ by current GMPEs is an average one for the considered site proxy (i.e. VS30), without any specific attention to local resonance effect related to lithological or morphological effects
Summary
To account for the local site response within a seismic hazard assessment (SHA) study can be achieved following different approaches (e.g., Aristizabal, 2018). As indicated by Laurendeau et al (2018) and again emphasized later in this paper, the consequence is that the increase of the high frequency content from standard-rock to hard-rock conditions, as provided by the present adjustment practice, does not seem to be relevant anymore Another important point that should be kept in mind is that the adjustment approach described above implicitly assumes that the ground motion predicted for the ‘‘standard-rock’’ by current GMPEs is an average one for the considered site proxy (i.e. VS30), without any specific attention to local resonance effect related to lithological or morphological effects. The conclusion section will highlight the various advantages of these alternative approaches, among which their simplicity with respect to the present practice
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