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Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.

We investigate the variation in shear-wave velocity (VS) in the shallow soil of the ARGONET vertical array in Cephalonia, Greece, utilizing an extensive 8–10-year dataset of earthquake records and applying seismic interferometry by deconvolution and Generalized Additive Models (GAMs). We identify and quantify the contributions of seasonal variation, soil anisotropy, soil nonlinearity, and long-term VS changes. Of the examined factors, nonlinearity produces the strongest VS changes in the form of reduction of up to several tens of m/s. The azimuthal and seasonal partial effects appear similar in strength. However, VS also exhibits year-to-year variation, with lower levels likely linked to the slow recovery of the soil following strong earthquakes in the broader region. When this partial effect is also considered, the temporal variation of VS is more significant than the azimuthal variation. We also observed that strong weather phenomena, such as the unusual hurricane “Ianos” that hit western Greece in 2020, are captured in our model through tensor interaction terms. Our model can identify VS drops related to nonlinear soil behavior even when masked by other effects. We demonstrate and verify this through seismic interferometry to stepwise increasing parts of earthquake recordings highlighting these within-events or coseismic VS drops.

Accelerometric data from the well-studied valley EUROSEISTEST are used to investigate ground motion uncertainty and variability. We define a simple local ground motion prediction equation (GMPE) and investigate changes in standard deviation (σ) and its components, the between-event variability (τ) and within-event variability (φ). Improving seismological metadata significantly reduces τ (30–50%), which in turn reduces the total σ. Improving site information reduces the systematic site-to-site variability, φS2S (20–30%), in turn reducing φ, and ultimately, σ. Our values of standard deviations are lower than global values from literature, and closer to path-specific than site-specific values. However, our data have insufficient azimuthal coverage for single-path analysis. Certain stations have higher ground-motion variability, possibly due to topography, basin edge or downgoing wave effects. Sensitivity checks show that 3 recordings per event is a sufficient data selection criterion, however, one of the dataset’s advantages is the large number of recordings per station (9–90) that yields good site term estimates. We examine uncertainty components binning our data with magnitude from 0.01 to 2 s; at smaller magnitudes, τ decreases and φSS increases, possibly due to κ and source-site trade-offs Finally, we investigate the alternative approach of computing φSS using existing GMPEs instead of creating an ad hoc local GMPE. This is important where data are insufficient to create one, or when site-specific PSHA is performed. We show that global GMPEs may still capture φSS, provided that: (1) the magnitude scaling errors are accommodated by the event terms; (2) there are no distance scaling errors (use of a regionally applicable model). Site terms (φS2S) computed by different global GMPEs (using different site-proxies) vary significantly, especially for hard-rock sites. This indicates that GMPEs may be poorly constrained where they are sometimes most needed, i.e., for hard rock.

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, VS30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of VS30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated VS30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.

We developed a ground-motion simulation code base on extended rupture modeling combined with the use of empirical Green’s functions (EGFs), adapted for low-to-moderate seismicity regions (with a limited set of EGFs), and extended its range of applicability to the lowest source-to-site distances. This code is based on a kinematic source description of an extended fault and is designed to allow complex fault geometries and to generate a ground motion variability in agreement with that of the recorded databases. The code is developed to work with a sparse set of EGFs. Each available EGF is therefore used in several positions on the rupture area. To be used in positions different of their original position, we applied to the EGFs some adjustments. In addition to the classical adjustments (i.e. time delay correction, geometrical spreading correction and anelastic attenuation correction), we propose here a radiation pattern adjustment. The effectiveness of it is tested in a numerical application. We showed noticeable improvements at the lowest distances, and some limitations when approaching the nodal planes of the subevents the recording of which were used as EGFs. We took advantage of the development of this code, its ability to work with a sparse set of EGFs, its ability to take into account complex fault geometries and its ability to master the general variability, to perform a ground-motion simulation scenario on the Middle Durance Fault (MDF). We perform simulations for a hard rock site (VS30 = 1800 m/s) and a sediment site (VS30 = 440 m/s) of the CEA Nuclear Research Site of Cadarache (France), and compared the computed ground motion with several ground motion prediction equations (GMPEs). The GMPEs slightly underestimate the sediment site but strongly overestimate the ground motion amplitude on the hard rock site, even when using a specific correction factor which adapts GMPEs predictions from rock site to hard rock site. This general ascertainment confirms the need to continue efforts towards the establishment of consistent GMPEs applicable to hard-rock conditions.

The present study aims dating the topmost 50 m of a 115 m-long core retrieved from a valley located in the Cadarache Area (Upper Provence, South East France). Based on burial dating by in situ produced cosmogenic nuclides (10Be, 26Al and 21Ne), and paleomagnetic analyses, a Tortonian age was determined for the sedimentary succession exhibited in the core. In addition, thin sections, grain size analyses and quartz morphoscopy were used to characterize a shallow marine environment and to correlate the sediment core to the surrounding exposed sections. When comparing the studied sedimentary record to global sea level estimates, we establish that the local Tortonian transgression was not synchronous with the global scale sea level high-stand, but happened more likely later, during a global regressive trend. The timing of the Tortonian flooding in Upper Provence was therefore most probably driven by regional tectonic forcing. Finally, the Tortonian shallow marine sediments are topped by lacustrine tight carbonates. These carbonates protect the marine sediments from the following major erosional events resulting from the Messinian sea level drop and from interglacial-glacial fluctuations that both yielded to the present-day valley topography.

Although rotational seismology has progressed in recent decades, the links between rotational ground motion and site soil conditions are poorly documented. New experiments were performed on Kefalonia Island (Greece) following two large earthquakes ( M W  = 6.0, M W  = 5.9) in early 2014 on two well-characterized sites (soft soil, V S30  ~ 250 m/s; rock, V S30  ~ 830 m/s, V S30 being harmonic average shear-wave velocity between 0 and 30 m depth). These earthquakes led to large six-component (three translations and three rotations) datasets of hundreds of well-recorded events. The relationship between peak translational acceleration versus peak rotational velocity is found sensitive to the site conditions mainly for the rotation around the vertical axis (torsion; dominated by Love waves): the stiffer the soil, the lower the torsion, for a given level of translational acceleration. For rotation around the horizontal axes (rocking; dominated by Rayleigh waves), this acceleration/rotation relationship exhibits much weaker differences between soft and rock sites. Using only the rotation sensor, an estimate of the Love-to-Rayleigh energy ratios could be carried out and provided the same results as previous studies that have analyzed the Love- and Rayleigh-wave energy proportions using data from translational arrays deployed at the same two sites. The coupling of translational and rotational measurements appears to be useful, not only for direct applications of engineering seismology, but also to investigate the composition of the wavefield, while avoiding deployment of dense arrays. The availability of new, low-noise rotation sensors that are easy to deploy in the field is of great interest and should extend the use of rotation sensors and expand their possible applications. Graphical abstract .

Modern probabilistic seismic hazard assessment (PSHA) focuses on the separation and different treatment of epistemic and aleatory uncertainties. Recent site-specific PSHA studies have pointed out that, if the site response and its epistemic uncertainties can be appropriately considered by adjustments to median estimates from ground motion prediction equations (GMPEs), the aleatory variability (sigma) of the GMPEs can be replaced by the single-station sigma thus partially relaxing the ergodic assumption employed in the PSHA. The site-specific partially nonergodic approach, correctly applied, provides a more accurate representation of the seismic hazard at a specific site and a more rigorous treatment of uncertainties. This paper presents the strategy followed to apply this relatively recent approach to a critical infrastructure in Southern France located on hard-rock site conditions (Vs30 ≈ 2000 m/s). The target site conditions are defined in terms of shear-wave velocity (Vs) profiles and high-frequency attenuation (κ 0 ) based on the results of site investigations and on the exploitation of earthquake records at seismic stations in the target site area. The host-to-target Vs-κ 0 adjustment of median estimates for the selected GMPEs is performed by using the inverse random vibration theory approach (Al Atik et al. in Bull Seismol Soc Am 104:336–346, 2014 ) considering epistemic uncertainties in target Vs profile and κ 0 . The single-station sigma model is developed based on Rodriguez-Marek et al. (Bull Seismol Soc Am 104:1601–1619, 2013 ) due to the lack of local data. The results of the site-specific partially nonergodic PSHA are discussed by means of a sensitivity analysis and are compared to the results from standard ergodic PSHA. We found that, for the considered site, the site-specific approach provides a substantial reduction (up to 50%) of the uniform hazard spectra at 10,000-year return period compared to the ergodic approach.

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”.

Alluvial valleys generate strong effects on earthquake ground motion (EGM). These effects are rarely accounted for even in site-specific studies because of (a) the cost of the required geophysical surveys to constrain the site model, (b) lack of data for empirical prediction, and (c) poor knowledge of the key controlling parameters. We performed 3D, 2D and 1D simulations for six typical sedimentary valleys of various width and depth, and for a variety of modifications of these 6 “nominal models” to investigate sensitivity of EGM characteristics to impedance contrast, attenuation, velocity gradient and geometry. We calculated amplification factors, and 2D/1D and 3D/2D aggravation factors for 10 EGM characteristics, using a representative set of recorded accelerograms to account for input motion variability. The largest values of the amplification and aggravation factors are found for the Arias intensity and cumulative absolute velocity, the lowest for the root-mean-square acceleration. The aggravation factors are largest for the vertical component. For each model, at least one EGM characteristic exhibits a significant 2D/1D aggravation factor, while all EGM characteristics exhibit significant 2D/1D aggravation factor on the vertical component. For all investigated sites, there is always an area in the valley for which 1D estimates are not sufficient. 2D estimates are insufficient at several sites. The key structural parameters are the shape ratio and overall geometry of the sediment-bedrock interface, impedance contrast at the sediment-bedrock interface, and attenuation in sediments. The amplification factors may largely exceed the values that are usually considered in GMPEs between soft soils and rock sites.