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This technical report explains details on the results of the investigation to build a subsurface structural model and a selection of earthquake data at the target site of the blind prediction exercise for "The 6th International Association of Seismology and Physics of the Earth's Interior/International Association of Earthquake Engineering International Symposium on the Effect of Surface Geology on Seismic Motion (ESG6)." The selection process of the target site in the Kumamoto Plain, Japan, in ESG6 was explained with a historical review of the blind prediction tests in the previous ESG conferences. We have collected existing subsurface structural and earthquake data and conducted geophysical and geotechnical surveys in and around the target site to generate important velocity structure and the earthquake data used in the blind test. Microtremor data were obtained in triangular arrays ranging in side lengths from 1 to 962 m, and active surface wave data were derived along a 36-m line at the site. These data were provided for the prediction of a subsurface structural model in the first step of the blind prediction exercise. We also conducted a velocity logging in a borehole to a bottom depth of 39 m at the site and laboratory tests of soil samples from the borehole. We constructed a velocity profile of the shallow and deep sedimentary layers from a combination of the geophysical and geotechnical data at the site, and validated it by comparing the characteristics of the ground motion data from the moderate event. This "preferred velocity model" was provided as a standard model to the participants in the second and third steps of the blind prediction test to predict the earthquake ground motions of a moderate event and the mainshock of the 2016 Kumamoto earthquake. [ABSTRACT FROM AUTHOR]

In this study, we compared observations and predictions submitted by participants for blind prediction experiments for ground motions using aftershocks, foreshock, and mainshock of the 2016 Kumamoto earthquake sequence, Japan, to improve our understanding of the quality of state-of-the-art methods on the reproducibility of the effects of surface geology on seismic motions. In the blind predictions, 1D, 2D, and 3D methods, Green's function methods, spectral ratio approaches, and other approaches were applied. As for PGA/PGV, acceleration/velocity duration, Fourier spectrum, pseudo-velocity response spectrum, and site amplification factors, the observed values are mostly within the range of average ± σ of all the predictions in the case of weak and strong ground motions. The results of the mean absolute percentage errors for these indices show that the applied methods can predict weak and strong ground motions for the three components in the range of one-half to twice the observations. The average goodness-of-fit (GOF) scores for weak and strong ground motions indicate either a very good fit (6.5–8) or a good fit (4.5–6.5) for the three components. Finally, examples of the categorized methods are quite limited; however, results indicate that the predictions by all the categorized methods can adequately reproduce weak and strong ground motions within either a very good or good fit. Although we could not find a significant difference in the results from the categorized methods, scores by the 2D and 3D methods in the frequency range of 0.5–1 and 1–2 Hz for all the blind predictions are higher than the scores by the other methods. The GOF score for the part after the S-wave by the 2D and 3D methods is higher than that by the 1D method. This supports that the predictions by the 2D and 3D methods due to the accounting of the proper geometry could reproduce the basin-induced and/or basin-transduced surface waves excited by the basin-edge effect more than the 1D method using the earthquake record observed at the reference site. [ABSTRACT FROM AUTHOR]

SUMMARY: The local ground motion amplification related to the geology at a specific site (i.e. the so-called site effects) may be classically quantified through the SSR (standard spectral ratio) technique applied on earthquake recordings. However, such a quantification might be challenging to carry out in low-to-moderate seismicity regions. Methods based on background ambient noise, such as noise-based standard spectral ratio (SSRn), might be of great interest in these areas. But noise-derived amplification is particularly sensitive to local anthropogenic sources, which may introduce biases in the evaluation of site effects by dominating the geological effects, especially for frequencies higher than 1 Hz. A hybrid approach (SSRh), developed to reduce biases in noise-based spectral ratios by combining classical earthquake-based spectral ratio (SSR) and SSRn, was recently introduced and relies on a site reference. We here investigate the applicability of SSRn and SSRh in a heavily industrialized environment in the Tricastin Valley (south-east France), where critical facilities are located. We continuously recorded ambient noise from 2020 February to March on a 400-sensor seismic array covering an area of about 10 km by 10 km. We demonstrate that SSRn and SSRh computed below 1 Hz are able to reproduce amplification factors provided by SSR. By contrast, at frequencies higher than 1 Hz, SSRn strongly deviates from SSR. SSRh shows closer results to SSR but presents a dependence to the choice of the local site reference, thereby questioning the possibility to use SSRh blindly to estimate the local amplification in our context. These discrepancies reflect the impact of local anthropogenic sources. We therefore introduced a two-step workflow to mitigate the influence of local sources. The first step is to define a characteristic time window to optimally isolate significant transient signals. The second step consists in selecting the time segments that do not contain these transients with a clustering-based approach. By applying this workflow, we were able to remove some strong anthropogenic transient signals likely to be generated by local sources at some sites and therefore to locally improve the amplification assessment through noise-based spectral ratios. However, stationary sources, whose impact cannot be removed through our procedure, remain a major issue. This study provides some insights into the application of SSRn and SSRh in noisy industrialized areas, especially regarding the impact of local noise sources. It illustrates the difficulty of having a procedure for mitigating the impact of these sources that is efficient everywhere inside such a complex anthropized environment, where different types of sources (including stationary sources) cohabit. [ABSTRACT FROM AUTHOR]

Following the 1923 Kanto earthquake in Japan, Japanese researchers noticed the strong effects of surface geology on seismic motion (ESG) and began to investigate these effects to quantify the site amplification factors (SAFs) associated with soft surface sediments. On the other side of the Pacific Ocean, ESG received limited attention until the 1985 Michoacan, Mexico earthquake revealed significant long-period amplification inside Mexico City that manifested as the source of devastating damage to high-rise buildings. Since then, seismologists and earthquake engineers have performed a lot of studies on various ESG issues worldwide. We have not yet reached common conclusions on how to quantitatively predict SAFs over a broad frequency band of engineering interest, 0.1 to 20 Hz, for moderate to strong input from different types of earthquakes in different tectonic settings. However, we found here several basic guidelines useful for successfully modeling ground motions as a common approach to ESG studies. First, in this letter, we briefly review our history of understanding ESG, which is closely related to the key settings required for reliable quantifications of SAFs, and then introduce various emerging techniques for broadband quantitative evaluations of SAFs based on the vast amount of observed ground motions primarily from dense Japanese strong-motion networks. Based on the findings of our investigation and the physical relationships behind the parameters, the authors would like to recommend that researchers on ESG and related topics would refer to the five basic guidelines proposed in the conclusions for the successful implementation of techniques to delineate SAFs in a specific region of interest, such as the use of Fourier spectra instead of response spectra. We have started applying the proposed techniques to regions outside Japan. The implementation of the statistical validation exercises will follow. [ABSTRACT FROM AUTHOR]

1D velocity profiles at a strong motion station in the northern part of the Kumamoto plain, Japan, were submitted in Step-1 of the blind prediction exercise of strong ground motions in the sixth international symposium on effects of surface geology on seismic motion (ESG6). Individual participants were requested to estimate a 1D S-wave velocity profile of sedimentary layers from the given data obtained by microtremor array explorations and surface-wave explorations at the site. This paper reports the target site, methods used by the individual participants, and the submitted results. More than half of the participants estimated the phase velocities of the Rayleigh-wave in the frequency range from 0.53 to 29.8 Hz. The statistical analysis of the phase velocity dispersion curves indicates that the standard deviation was below 40 m/s at the frequencies above 3.4 Hz, and it was below 20 m/s above 20 Hz. The S-wave velocity profiles are also similar to a depth of 20 m. The standard deviation was below 45 m/s. The average S-wave velocity in the top 30 m from the surface is 207.3 ± 60.7 m/s for the submitted profiles. The large variation is related to the introduction of the near-surface low velocity layers. The large variation of the S-wave velocities was found in the deep part. The average S-wave velocity at a depth of 1500 m was 2674 m/s with the standard deviation of 786 m/s. We compared 1D amplifications for the submitted profiles. Common peaks can be identified at 0.3–0.4 Hz and 1–2 Hz, excluding two teams. However, the amplifications vary much in the frequency range higher than 4 Hz. Through the experiment, it was found that the dispersion curves and the shallow S-wave velocity structures are estimated with a low standard deviation among the participants. Further development of the techniques for deep S-wave velocity profiling was found to be required. [ABSTRACT FROM AUTHOR]

Site amplification is an important component of strong ground motion prediction as it differs among sites, reflecting its specific local subsurface geology. Here, we confirm that site amplifications are similar in a neighborhood area over a long period. However, few studies have investigated the spatial properties in a wide region (i.e., the whole of Japan). In this study, we explored the spatial properties of site amplifications based on the generalized inversion technique (GIT) using Fourier amplitude spectra (FAS) as well as pseudo-velocity response spectra (pSv) as the latter is an important index for engineering purposes and the most similar type of response spectra to FAS. The spatial distributions of S-wave site amplifications (SA-S), especially within large sediment basins (e.g., the Kanto and Osaka Basins in Japan), were found to be relatively similar in proximate areas for a long period ranging from 2 to 8 s. This suggests that we could easily predict the site amplifications using an empirical approach through spatial interpolation based on the properties obtained by the GIT. Furthermore, we propose a prediction procedure for site amplification for the whole duration from the SA-S at an arbitrary site. We used the correction function, which converts the SA-S to the site amplification for the whole duration (SA-W), including an S-wave portion and a subsequent portion. This function is called the whole-duration to S-wave spectral ratio (WSR) and is stable in terms of spatial properties. As we could estimate the SA-S either by theoretical transfer functions or observed microtremors, we can easily predict the SA-W based on the proposed WSR concept. We found that SA-S in pSv is more or less similar to SA-S in FAS, however, SA-W in pSv fails to capture the effects of the long duration of ground motions inside a large basin so that we cannot recommend to use pSv for the prediction of whole duration of ground motion. [ABSTRACT FROM AUTHOR]

Quantitative estimation of seismic risk over a region requires both an underlying probabilistic seismic hazard model and a means to characterise shallow site response over a large scale. The 2020 European Seismic Risk Model (ESRM20) builds on the 2020 European Seismic Hazard Model (ESHM20), requiring additional information to firstly parameterise the local site condition across all of Europe, and subsequently determine its influence on the prediction of seismic ground motion. Initially, a harmonised digital geological database for Europe is compiled, alongside a model of topographic/bathymetric elevation and a database of 30 m averaged shearwave velocity measurements ( V S 30 ), in order to produce separate 30 arc-second maps of inferred V S 30 based on topography and on geology. We then capitalise on a large database of seismic recording stations in Europe for which site-to-site ground motion residuals ( δ S 2 S S ) have been determined with respect to the shallow crustal ground motion model used in the ESHM20. These residuals allow us to incorporate site amplification functions into the European GMM calibrated upon either observed or inferred V S 30 , or on the European geology and topography models. We present the resulting pan-European seismic site amplification model and assess its impact on seismic hazard and risk compared against other approaches. The new site amplification model fulfils the requirements of the ESRM20 and, providing uncertainty is fully propagated, yields estimates of seismic hazard and risk at a large space scale that may be comparable to other methods often applied at local/urban scale where better-constrained site information is available. [ABSTRACT FROM AUTHOR]

This article introduces the extended timing annotation dataset (ETAD) product for Sentinel-1 (S-1) which was developed in a joint effort of German Aerospace Center (DLR) and European Space Agency (ESA). It allows to correct range and azimuth timing of S-1 images for geophysical effects and for inaccuracies in synthetic aperture radar (SAR) image focusing. In combination with the precise orbit solution, these effects determine the absolute geolocation accuracy of S-1 SAR images and the relative collocation accuracy of repeat pass image stacks. ETAD contains the gridded timing corrections for the tropospheric and ionospheric path delays, the tidal-based surface displacements, and the SAR processing effects, all of which are computed for each data taken using standard models from geodesy and auxiliary atmospheric data. The ETAD product helps S-1 users to significantly improve the geolocation accuracy of the S-1 SAR products to better than 0.2 m and offers a potential solution for correcting large-scale interferometric phase variations. The product layout and product generation are described schematically. This article also reports first the results for different SAR techniques: first, the improvement in geolocation accuracy down to a few centimeters by verification of accurately surveyed corner reflector positions in the range–azimuth plane; second, the well-established offset-tracking technique, which is used for systematic ice velocity monitoring of ice sheets and glaciers, where ETAD can reduce velocity biases down to subcentimetric values; and third, the correction of atmospheric phase contributions in wide-area interferograms used for national and European ground motion services. These early results proof the added value of the ETAD corrections and that the product design is well-suited to be integrated into the processing flows of established SAR applications such as absolute ranging of targets, speckle/feature tracking, and interferometry. [ABSTRACT FROM AUTHOR]

On February 6, 2023, two strong earthquakes, magnitude (M w) 7.7 and 7.6, occurred 9 h apart on the East Anatolian Fault Zone in Türkiye. These devastating earthquakes directly affected a region of southeastern Türkiye, where approximately 15 million people live and caused severe destruction of structures. Unfortunately, more than 50.000 people lost their lives as a result. The study presented the tectonic structure of Elazığ, one of the provinces affected by these earthquakes, in detail. Then, the general seismic characteristics of the main shock. Additionally, the geological structure, existing building stock, and seismicity of Elazığ province are given. Furthermore, the damages, deficiencies, misapplications and defects, and lack of engineering services and supervision seen in the building stocks due to the field investigation are presented and discussed in detail. Finally, some suggestions were made to prevent all these damages, to observe deficiencies/misapplications, and to take precautions for existing damaged buildings. • Past and Present Seismological Activity of East Anatolian Fault zone and Elazığ city. • Properties of seismic characteristic of Kahramanmaraş twin earthquakes are presented. • Structural performance of structures of Elazığ city were assessed. • Deficiencies, misapplications, design errors, etc. that can be found in existing building stocks were revealed. • Lessons learned from these unique earthquakes were explained. [ABSTRACT FROM AUTHOR]

In order to examine the shallow geology under the site of the Skopje Seismological Observatory (SKO), Naka- mura technique was used on a set of selected seismic records from its strong motion instruments. The assessment of site conditions and amplification behavior was performed under different input levels of seismic ground motions. The datasets for analysis consisted of three types of seismic registrations: the first type were the microtremor measure- ments, the second type was a nearby man-made explosion, and the third type was a moderate size local earthquake with a magnitude of ML5.3 which occurred on 11th of September 2016. Horizontal vs. vertical ratio of the components recorded for all three types of source vibration were calculated. Similar predominant peaks and frequency distribution were observed when records from the local earthquake, the explosion and the ambient noise were analyzed. The results of the analysis with Nakamura method were compared with interpretations from the geological and geotechnical maps at the station location. Our findings support the idea that such geophysical methods can detect the underlying structural profile to a first approximation and can be used to develop consistent velocity models for large areas without expensive drilling deployments. [ABSTRACT FROM AUTHOR]

A fundamental modeling assumption in engineering seismology is that seismic ground motion is affected by source, propagation path and site effects. Contrary to the first two, comprehensive studies on site effects, the localized amplification effects on seismic ground motion due to the uppermost geological structure, have been limited in Iceland. As a result, site amplification has been modeled in ground motion models in a simplistic and qualitative way. In this study, we address this limitation by using the new Bayesian hybrid GMM proposed by Kowsari et al. (2020) to model the recorded ground motions at regional strong-motion stations in southwest Iceland, specifically the peak ground acceleration and pseudo-acceleration response spectra as a function of oscillator period (i.e., PGA and PSA at T = 0.01 - 3.0 s), earthquake magnitude, distance and hypocentral depth, and at first, we omit a site-term. Then, we incorporate the GMM in a Bayesian Hierarchical Model (BHM) that allows prior distributions of model parameters to be used and we partition the model residuals into various terms associated with the source, propagation path and station terms. Through Bayesian inference of the model parameters the results show that the station term residuals reveal a frequency-dependent site amplification at the recording sites. By inspecting this systematic behaviour, we propose four groups of station terms by their characteristics. We then carefully inspect the station locations in terms of their associated geological unit and observe that the geological units and station term groups are qualitatively correlated. By combining the two, we propose the first frequency-dependent average site amplification functions associated with key geological units in Iceland based on age and formation. Provisionally we refer to them as hard rock, rock, (Holocene) lava rock, and (unspecified) soil site classes, respectively. We then update the GMM by incorporating these amplification functions so that it now enables the prediction of PGA and PSA at these four site classes. As a result, the new model standard deviations are considerably lower, with those associated with the event term being τ = 0.05, station terms φ S 2 S = 0.10, intra-event φ = 0.17 and total σ = 0.18 in common logarithm units for PGA. The new site class-specific GMM now enables the site-specific ground motion for separate geological units which in turn improves site-specific seismic hazard assessment and informed site-specific engineering decision making in Iceland. • A new empirical ground motion model without a site term is embedded in a Bayesian Hierarchical Modeling (BHM) framework. • The model is calibrated to strong-motion data from mainshocks recorded in Southwest Iceland. • The BHM reveals the frequency-dependent station terms which are a proxy for localized site effects. • The station terms are correlated to key Icelandic surficial geological units, hard rock, rock, lava rock and soil. • The average station terms for each geological unit have been incorporated into the original ground motion model. [ABSTRACT FROM AUTHOR]

The Xianshuihe fault zone consists of left-lateral, segmented strike-slip faults, which produce strong and relatively frequent earthquakes, and exposes millions of people to the risk of strong motion and earthquake-induced geologic hazards in Southwest China. Seismicity in this fault zone exhibits typical features of characteristic earthquakes. The stochastic finite-fault model was used to generate time-histories and peak values of strong ground motion at near-fault locations for the characteristic earthquakes. First, the source parameters were determined and verified by comparing them with the simulated time-histories and intensity distribution and observations from the 2014 Kangding earthquake (Mw 6.0). Then, scenario-based ground-motion hazard maps were produced from simulated ground motions of characteristic earthquakes. The scenario-based ground-motion hazard maps can be directly compared with historical and future earthquakes in the Xianshuihe fault zone. Scenario-based seismic hazard analysis also has other advantages. Our results show that current design ground motion for the Xianshuihe Fault area is not adequate. [ABSTRACT FROM AUTHOR]

Unique challenges arise when projecting dynamic site response in a deep, steep sided, irregularly shaped, sediment-filled basin. The influence of shallow sediments on site response was investigated for a 1-km-deep alluvial column in the Las Vegas Basin, subjected to weak ground motions. A one-dimensional equivalent-linear model was applied. Response analyses for deep deposits are complicated because dynamic material properties at depth are uncertain. To compensate, the model half space was placed well above the physical bottom of the sediment column. The depth to half space was selected by matching characteristics of projected surface response to measured data or expectations. Weak ground-motion datasets resulting from underground nuclear tests were considered. The best-match half space depth, 375 m, greatly exceeded the depth of the threshold shear wave velocity for engineering bedrock. The analysis captured site response over the period range 0.3–1.3 s. When the parameterization was tested for a weak-ground-motion earthquake dataset, projections were poorer but still instructive. [ABSTRACT FROM AUTHOR]

A spectacular band of precariously and semiprecariously balanced rocks extends from Riverside to near Borrego Valley, California, about halfway between the Elsinore and San Jacinto fault zones. The rocks are distributed in a band a few kilometers wide midway between the San Jacinto and Elsinore fault zones, an indication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones. These rocks have apparently been in place for thousands of years, and thus place important constraints on ground motions from earthquakes. On the basis of field tests and photographic analysis, the estimated quasi-static toppling accelerations for the precariously balanced rocks are ∼0.32 ± 0.10 g (g = gravitational acceleration). The dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the Izmit and Denali earthquakes. These constraints are roughly consistent with the median predicted value of ground motion for M7 earthquakes, but somewhat lower than +σ ground motion curves, and much lower than the values from the 2% in 50 yr hazard maps. The evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment, including median and standard deviation of ground motion attenuation curves (especially for hard rock, for which few instrumental data are available), the possible existence of random background earthquakes, and the smoothing distance for historical seismicity. [ABSTRACT FROM AUTHOR]

The relationship between the peak deformations of inelastic and corresponding linear single-degree-of-freedom (SDF) systems is investigated. Presented are the median of the inelastic deformation ratio for 214 ground motions organized into 11 ensembles of ground motions, representing large or small earthquake magnitude and distance, and National Earthquake Hazards Reduction Program (NEHRP) site classes B, C, and D; near-fault ground motions are also included. Two sets of results are presented for bilinear nondegrading systems over the complete range of elastic vibration period, Tn:Cμ for systems with known ductility factor, μ, and CR for systems with known yield-strength reduction factor, Ry. The influence of postyield stiffness on the inelastic deformation ratios Cμ and CR is investigated comprehensively. All data are interpreted in the context of acceleration-sensitive, velocity-sensitive, and displacement-sensitive regions of the spectrum for broad applications. The median Cμ versus Tn and CR versus Tn plots are demonstrated to be essentially independent of the earthquake magnitude and distance (over their ranges considered), and of site class. In the acceleration-sensitive spectral region, the median inelastic deformation ratio for near-fault ground motions is systematically different when plotted against Tn; however, when plotted against normalized period Tn/Tc (where Tc is the period separating the acceleration- and velocity-sensitive regions) they become very similar in all spectral regions. Determined by regression analysis of the data, two equations—one for Cμ and the other for CR—have been developed as a function of Tn/Tc, and μ or Ry, respectively, and are valid for all ground motion ensembles considered. These equations for Cμ and CR should be useful in estimating the inelastic deformation of new or rehabilitated structures—where the global ductility capacity can be estimated—and existing structures with known lateral strength. [ABSTRACT FROM AUTHOR]