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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.

Nepal is one of the most seismically active regions in the world, as highlighted by the recent devastating 2015, Mw~7.8 Gorkha earthquake, and a robust assessment of seismic hazard is paramount for the design of earthquake-resistant structures. In this study, we present a new probabilistic seismic hazard assessment (PSHA) for Nepal. We considered data and findings from recent scientific publications, which allowed us to develop a unified magnitude homogenized seismicity catalog and propose alternative seismic source characterization (SSC) models including up-to-date parameters of major thrust faults like main frontal thrust (MFT) and main boundary thrust (MBT), while also considering existing SSC models and various seismic hazard modeling strategies within a logic tree framework. The sensitivity analyses show the seismic hazard levels are generally higher for SSC models integrating the major thrust faults, followed by homogenous volume sources and smoothed seismicity approach. The seismic hazard maps covering the entirety of Nepal are presented as well as the uniform hazard spectra (UHS) for five selected locations (Kathmandu, Pokhara, Biratnagar, Nepalganj, and Dipayal) at return periods of 475- and 2475-years considering Vs,30 = 760 m/s. The results obtained are generally consistent with most recent studies. However, a notable variability in seismic hazard levels and several discrepancies with respect to the Nepal Building Building Code NBC105: 2020 and global hazard model, GEM are noted, and possible causes are discussed.

Estimating earthquake ground motions at reference bedrock is a major issue in site-specific seismic hazard assessment. Deriving or adjusting empirical ground motion models (GMMs) for reference bedrock is challenging and affected by large epistemic uncertainties. We propose a methodology to simulate region-specific reference bedrock time histories by combining spectral decompositions of ground motions with Empirical Green’s Functions (EGFs) simulation technique. First, we adopt the nonparametric spectral decomposition approach to separate the contribution of source, path, and site. We remove the average source and site effects from observed small-magnitude recordings in the target region through deconvolution in the Fourier domain. This way, the obtained deconvolved EGFs represent path term only. Then, we couple the EGFs with k− 2 kinematic rupture models for target scenario events. For each target magnitude, a set of rupture models following a ω-squared source spectrum are generated sampling the uncertainties in kinematic source parameters (e.g., slip distribution, rupture velocity, hypocentral location, stress drop, and rupture dimensions). The proposed approach is validated using recorded ground motions at reference sites from multiple earthquakes in Central Italy. The power of this approach lies in its ability to map the path-specific effects into the ground-motion field, providing 3-component time histories covering a wide frequency range, without the need for computationally expensive approaches to simulate 3D wave propagation. The region-specific, site-effects-free dataset produced by this approach can be used alone or in combination with existing empirical datasets to adjust existing GMMs, derive new GMMs, or select hazard-consistent time histories to be used in soil and structural response analyses.

The quantification of earthquake-generated site-specific ground motions is necessary for accurate and reliable seismic hazard assessment for critical structures. To this aim, empirical and numerical approaches are typically used in order to characterize the site response at the target site. This is not straightforward in low-seismicity regions and even more challenging when the target structure is located at great depth. This article, focusing on a practical application for an underground radioactive waste repository project to be located in the eastern part of the Paris Basin (France), presents and discusses the results and the faced challenges in the characterization of the site transfer function (TF) from surface to several depths as well as when assessing the applicability of the ergodic site terms from empirical ground motion models to the target site. We first present the data collected in the last years at the project site and the empirical TFs between the surface and two locations at depth (–490 m and –445 m). The empirical transfer function is robust in the frequency range 0.5–10 Hz and less reliable above 10 Hz due to the reduced number of usable recordings. A good consistency is found with the numerical transfer function based on a 1D soil model derived from local boreholes data. A transfer function based on noise recordings is also computed and compared to the earthquake-based one showing a good agreement of the horizontal components. These results clearly open several perspectives on the potential use of ambient noise data and numerical modelling in order to quantify the site response at any point of the underground project layout. Then, we investigate the site-specific high-frequency decay parameter κ 0 using the available earthquake recordings at the surface. The estimates of κ 0 obtained combining both acceleration- and displacement-based approaches are affected by large uncertainties owing mainly to the lack of data at short distances (< than 100 km). Nonetheless, a range of κ 0 values having a central κ 0 =0.042 s and lower and upper bounds of 0.02 s and 0.058 s is proposed and V s - κ 0 adjustment factors are calculated using the IRVT approach (Al Atik 2014 BSSA 104:336–346 ) for few example cases. Such adjustment factors together with the empirical TFs and related uncertainties can be used to remove the ergodic assumption on the site term of GMMs in order to improve classical ergodic seismic hazard assessment for structures at surface and at depth.