Knowledge of the acceleration spectral shape is crucial to various applications in engineering seismology. Spectral amplitude decays rapidly at high frequencies. Anderson and Hough (1984) introduced the empirical factor κ to model this attenuation. This is the first time κ is studied in a vertical array consisting of more than two stations. We use 180 earthquakes recorded at a downhole array with five stations in soils and rock to investigate the effect of soil conditions on κ. Given that κ computation processes vary across literature when following the classic AndersonHough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of κ 0 range between 0.017 and 0.031 s at the surface and between 0.004 and 0.024 s at rock. This variability due to the assumptions made is larger than the error of each estimate and larger than the average difference in values between sediment and rock. For this data set, part of it can be attributed to the type of distance used. Given this variability, κ 0 values across literature may not always be comparable; this may bias the results of applications using κ 0 as an input parameter, such as ground-motion prediction equations. We suggest ways to render the process more homogeneous. We also find that κ at rock level is not well approximated by surface records from which we deconvolved the geotechnical transfer function. Finally, we compute κ on the vertical component and find a dependence of the vertical-to-horizontal κ ratio on site conditions.

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S U M M A R YAt high frequencies, the acceleration spectral amplitude decreases rapidly; this has been modelled with the spectral decay factor κ. Its site component, κ 0 , is used widely today in ground motion prediction and simulation, and numerous approaches have been proposed to compute it. In this study, we estimate κ for the EUROSEISTEST valley, a geologically complex and seismically active region with a permanent strong motion array consisting of 14 surface and 6 downhole stations. Site conditions range from soft sediments to hard rock. First, we use the classical approach to separate local and regional attenuation and measure κ 0 . Second, we take advantage of the existing knowledge of the geological profile and material properties to examine the correlation of κ 0 with different site characterization parameters. κ 0 correlates well with V s30 , as expected, indicating a strong effect from the geological structure in the upper 30 m. But it correlates equally well with the resonant frequency and depth-to-bedrock of the stations, which indicates strong effects from the entire sedimentary column, down to 400 m. Third, we use our results to improve our physical understanding of κ 0 . We propose a conceptual model of κ 0 with V s , comprising two new notions. On the one hand, and contrary to existing correlations, we observe that κ 0 stabilizes for high V s values. This may indicate the existence of regional values for hard rock κ 0 . If so, we propose that borehole measurements (almost never used up to now for κ 0 ) may be useful in determining these values. On the other hand, we find that material damping, as expressed through travel times, may not suffice to account for the total κ 0 measured at the surface. We propose that, apart from material damping, additional site attenuation may be caused by scattering from small-scale variability in the profile. If this is so, then geotechnical damping measurements may not suffice to infer the overall crustal attenuation under a site; but starting with a regional value (possibly from a borehole) and adding damping, we might define a lower bound for site-specific κ 0 . More precise estimates would necessitate seismological site instrumentation.

A ground-motion prediction equation (GMPE) specific to rock and stiffsoil sites is derived using seismic motion recorded on high V S30 sites in Japan. This GMPE applies to events with 4:5 ≤ M w ≤ 6:9 and V S30 ranging from 500 to 1500 m=s (stiff-soil to rock sites). The empirical site coefficients obtained and the comparison with the simulated site functions show that seismic motion on rock and stiff-soil sites does not depend only on V S30 , but also on the high-frequency attenuation site properties (κ 0 ). The effects of the site-specific κ 0 on site amplification are analyzed using stochastic simulations, with the need to take into account both of these parameters for rock-site adjustments. Adding the site-specific κ 0 into the GMPEs thus appears to be essential in future work. The rock-site stochastic ground-motion simulations show that the sitespecific κ 0 controls the frequency corresponding to the maximum response spectral acceleration (f amp 1). This observation is used to link the peak of the response spectral shape to κ 0 in this specific Japanese dataset and then to add the effects of high-frequency attenuation into the previous GMPE from the peak ground acceleration and up to periods of 0.2 s. The inclusion of κ 0 allows the observed bias to be corrected for the intraevent residuals and thus reduces sigma. However, this κ 0 determination is limited to a minimum number of rock-site records with M w ≥ 4:5 and to distances of less than 50 km.

A key component in seismic hazard assessment is the estimation of ground motion for hard rock sites, either for applications to installations built on this site category, or as an input motion for site response computation. Empirical ground motion prediction equations (GMPEs) are the traditional basis for estimating ground motion while V S30 is the basis to account for site conditions. As current GMPEs are poorly constrained for V S30 larger than 1000 m/s, the presently used approach for estimating hazard on hard rock sites consists of ''host-to-target'' adjustment techniques based on V S30 and j 0 values. The present study investigates alternative methods on the basis of a KiK-net dataset corresponding to stiff and rocky sites with 500 \ V S30 \ 1350 m/s. The existence of sensor pairs (one at the surface and one in depth) and the availability of P-and S-wave velocity profiles allow deriving two ''virtual'' datasets associated to outcropping hard rock sites with V S in the range [1000, 3000] m/s with two independent corrections: 1/down-hole recordings modified from withinThe following softwares are employed in this study: 1) the one written by J.-C. Gariel and P.-Y.Bard of the 1D reflectivity approach (Kennett 1974); 2) the site_amp v.5.6 program package provided by Dave Boore (U.S. Geological Survey); and the pikwin software developed by Perron et al. (2017). DOI 10.1007/s10518-017-0142-6 motion to outcropping motion with a depth correction factor, 2/surface recordings deconvolved from their specific site response derived through 1D simulation. GMPEs with simple functional forms are then developed, including a V S30 site term. They lead to consistent and robust hard-rock motion estimates, which prove to be significantly lower than host-to-target adjustment predictions. The difference can reach a factor up to 3-4 beyond 5 Hz for very hard-rock, but decreases for decreasing frequency until vanishing below 2 Hz. Electronic supplementary materialBull Earthquake Eng

Site effects for hard-rock sites are typically computed using analytical models for the effect of κ 0 , the high-frequency attenuation parameter. New datasets that are richer in hard-rock recordings allow us to evaluate the scaling for hard-rock sites (e.g., V S30 > 1500 m=s). The high-frequency response spectra residuals are weakly correlated with κ 0 , in contrast to the strong scaling with κ 0 in the analytical models. This may be due to site-specific shallow resonance patterns masking part of the effect of attenuation due to damping. An empirical model is developed for the combined V S30 and κ 0 scaling for hard-rock sites relative to a reference site condition of 760 m=s (i.e., correction factors that should be used for going from soft rock to hard rock, taking into account the net effect of V S and κ 0 ). This empirical model shows high-frequency amplification that is more similar to the analytical prediction corresponding to a hard-rock κ 0 of 0.020 s rather than the typical value of 0.006 s, which is commonly used for hard-rock sites in the centraleastern United States. Compared to the current analytical approach, this leads to a reduction of high-frequency (> 20 Hz) scaling of about a factor of 2.

We investigate a seismic crisis that occurred in the western Gulf of Corinth (Greece) between December 2020 and February 2021. This area is the main focus of the Corinth Rift Laboratory (CRL) network, and has been closely monitored with local seismological and geodetic networks for 20 yr. The 2020–2021 seismic crisis evolved in three stages: It started with an Mw 4.6 event near the northern shore of the Gulf, opposite of Aigion, then migrated eastward toward Trizonia Island after an Mw 5.0 event, and eventually culminated with an Mw 5.3 event, ∼3  km northeast of the Psathopyrgos fault. Aftershocks gradually migrated westward, triggering another cluster near the junction with the Rion–Patras fault. Moment tensor inversion revealed mainly normal faulting; however, some strike-slip mechanisms also exist, composing a complex tectonic regime in this region dominated by east–west normal faults. We employ seismic and geodetic observations to constrain the geometry and kinematics of the structures that hosted the major events. We discuss possible triggering mechanisms of the second and third stages of the sequence, including fluids migration and aseismic creep, and propose potential implications of the Mw 5.3 mainshock for the seismic hazard of the region.

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