A study on the variability of Kappa (κ) in a borehole: Implications of the computation process

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 Anderson–Hough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of κ0 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, κ0 values across literature may not always be comparable; this may bias the results of applications using κ0 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

Rock and stiff-Soil site amplification: Dependency on VS30 and Kappa (κ0)

A ground‐motion prediction equation (GMPE) specific to rock and stiff‐soil sites is derived using seismic motion recorded on high VS30 sites in Japan. This GMPE applies to events with 4.5≤Mw≤6.9 and VS30 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 VS30, but also on the high‐frequency attenuation site properties (κ0). The effects of the site‐specific κ0 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 κ0 into the GMPEs thus appears to be essential in future work. The rock‐site stochastic ground‐motion simulations show that the site‐specific κ0 controls the frequency corresponding to the maximum response spectral acceleration (famp1). This observation is used to link the peak of the response spectral shape to κ0 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 κ0 allows the observed bias to be corrected for the intraevent residuals and thus reduces sigma. However, this κ0 determination is limited to a minimum number of rock‐site records with Mw≥4.5 and to distances of less than 50 km

Reference stations for Christchurch

During the Canterbury earthquake sequence, the observed level of ground motion on the soft soils of Christchurch was very strong and highly variable. Many studies are now emerging that analyse the amplification effect of these soft soils, usually by estimating a frequency-dependent amplification function relative to a rock outcrop station, or ‘reference site’. If the rock outcrop has its own amplification due to weathering or topographic effects, then the calculated amplification for the soil sites can be compromised. This study examines ten seismic stations in Canterbury to determine the best reference site for Christchurch, using the horizontal-to-vertical spectral ratio (HVSR) method for S-wave shaking. More broadly, this study uses HVSR to expand existing knowledge of the dynamic characteristics of seismic stations in the Canterbury area. Most rock stations show their own local amplification effects that reduce their individual ability to be used as reference stations. The recently installed Huntsbury station (HUNS) appears to be the best reference site for Christchurch, but this will need to be verified when more records become available. In the meantime, the D13C temporary station is currently the best reference station for site effect studies in both Christchurch and Lyttelton

Derivation of consistent hard rock (1000<Vs<3000 m/s) GMPEs from surface and down-hole recordings: Analysis of KiK-net data

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 VS30 is the basis to account for site conditions. As current GMPEs are poorly constrained for VS30 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 VS30 and κ0 values. The present study investigates alternative methods on the basis of a KiK-net dataset corresponding to stiff and rocky sites with 500 < VS30 < 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 VS in the range [1000, 3000] m/s with two independent corrections: 1/down-hole recordings modified from within 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 VS30 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

Understanding single-station ground motion variability and uncertainty (sigma) – Lessons learnt from EUROSEISTEST

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

Empirical ground-motion models adapted to the intensity measure ASA 40

Relative average-spectral-acceleration (ASA40), a recently developed intensity measure, is defined as the average spectral pseudo-acceleration on the probable interval of evolution of the fundamental frequency of a structure. This article presents two ground motion prediction equations (GMPEs) appropriate for the prediction of ASA40, using a pan-European strong motion database. Taking advantage of the strong correlation between the new intensity measure ASA40 and the spectral pseudo-acceleration (SA), existing GMPEs predicting SA can be adapted to predict ASA40. The first GMPE used in this study is the modified version of a new generation ground motion model, ASB13. In order to decrease the high aleatory uncertainty (sigma) that accompanies predictions when using this modified model, a new model is developed for the prediction of ASA40. Its range of applicability is for magnitudes Mw from 5.5 to 7.6 and distances out to 200 km, it includes site amplification and it is applicable for a range of periods between 0.01 and 4 s. The proposed model decreases the aleatory uncertainty by almost 15 % with respect to the uncertainty of the modified ground motion model