ALBERT

Description: Common ambient noise tomography studies use straight ray tomography for deriving group or phase velocity maps from long-term cross correlation of station pairs. Being suitable for low frequencies and large inter-station distances, this simplification can be problematic in near-surface studies where higher frequencies are considered and stronger lateral velocity variations are encountered. The eikonal tomographic approach avoids the assumption of straight rays by computing local group or phase velocities from the gradient of the traveltimes measured between the virtual sources and receivers, therefore approximating the true propagation trajectories of surface waves. In this work, we applied the eikonal approach to data from a dense, one-month long seismic deployment consisting in 400 3-component nodes installed over a 10x10 km zone in the heavily industrialized area of the Tricastin Nuclear Site (Rhône valley). This area is located above a sediment-filled, deeply incised canyon dug during the Messinian Salinity Crisis. The extreme subsurface topography of this canyon makes this area an interesting target for an eikonal tomographic study. We implemented an automated method that measures the phase traveltimes of the fundamental mode of Rayleigh and Love waves from the ambient noise cross correlations’ phase spectra. We then applied the eikonal approach to the measured traveltimes, resulting in a set of phase velocity maps for Rayleigh waves that are in good agreement with the existing geological knowledge and cover a frequency range from 0.4 to more than 5 Hz.

Description: Wide-angle seismic reflection/refraction (WA) surveys provide data that can be modeled to obtain lithospheric-scale P-wave velocity (VP) models. The interpretation of these datasets is often performed as a laborious and time-consuming trial-and-error procedure, in which the relevant model parameters (layer thickness and VP) are manually adjusted until the forward modeling matches the observed travel-times. In this work, we present a fully automatic iterative nonlinear approach to invert WA datasets based on the simulated annealing technique. We test our proposed approach with data from the MARCONI-3 WA profile (southern Bay of Biscay) and compare the outcome with an existing detailed interpretation, discussing the similarities between the two models and the agreement between our model and the observed travel-times.

Description: Assessing the potential and extent of earthquake-induced liquefaction is paramount for seismic hazard assessment, for the large ground deformations it causes can result in severe damage to infrastructure and pose a threat to human lives, as evidenced by many contemporary and historical case studies in various tectonic settings. In that regard, numerical modeling of case studies, using state-of-the-art soil constitutive models and numerical frameworks, has proven to be a tailored methodology for liquefaction assessment. Indeed, these simulations allow for the dynamic response of liquefiable soils in terms of effective stresses, large strains, and ground displacements to be captured in a consistent manner with experimental and in-situ observations. Additionally, the impact of soil properties spatial variability in liquefaction response can be assessed, because the system response to waves propagating are naturally incorporated within the model. Considering that, we highlight that the effect of shear-wave velocity Vs spatial variability has not been thoroughly assessed. In a case study in Metropolitan Concepción, Chile, our research addresses the influence of Vs spatial variability on the dynamic response to liquefaction. At the study site, the 2010 Maule Mw 8.8 megathrust Earthquake triggered liquefaction-induced damage in the form of ground cracking, soil ejecta, and building settlements. Using simulated 2D Vs profiles generated from real 1D profiles retrieved with ambient noise methods, along with a PressureDependentMultiYield03 sand constitutive model, we studied the effect of Vs spatial variability on pore pressure generation, vertical settlements, and shear and volumetric strains by performing effective stress site response analyses. Our findings indicate that increased Vs variability reduces the median settlements and strains for soil units that exhibit liquefaction-like responses. On the other hand, no significant changes in the dynamic response are observed in soil units that exhibit non-liquefaction behavior, implying that the triggering of liquefaction is not influenced by spatial variability in Vs. We infer that when liquefaction-like behavior is triggered, an increase of the damping at the shallowest part of the soil domain might be the explanation for the decrease in the amplitude of the strains and settlements as the degree of Vs variability increases.

Description: Superficial geological layers can strongly modify the surface ground motion induced by an earthquake. These so‐called site effects are highly variable from one site to another and still difficult to quantify for complex geological configurations. That is why site‐specific studies can greatly contribute to improve the hazard prediction at a specific site. However, site‐specific studies have historically been considered difficult to carry out in low‐to‐moderate seismicity regions. We present here seismological datasets acquired in the framework of the French–German dense array for seismic site effect estimation project in the heavily industrialized area surrounding the French Tricastin Nuclear Site (TNS). TNS is located above an ancient canyon dug by the Rhône River during the Messinian period. The strong lithological contrast between the sedimentary fill of the canyon and the substratum, as well as its expected confined geometry make this canyon a good candidate for generating site effects that are variable on short spatial scales. To investigate the impact of this geological structure on the seismic motion, we conducted complementary seismic campaigns in the area. The first main campaign consisted of deploying 400 nodes over a 10 × 10 km area for one month and aimed at recording the seismic ambient noise. A second seismic campaign involved the deployment of 49 broadband stations over the same area for more than eight months. This complementary campaign aimed at recording the seismicity (including local, regional, and teleseismic events). These different designs allowed us to target a variety of seismic data at different spatial and temporal scales. Beyond the interest for local operational seismic hazard applications, these datasets may be valuable for studying seismic wave propagation within complex kilometer‐scale sedimentary structures. In this article, we present the deployment designs as well as initial analyses to provide information on the characteristics and the overall quality of the data acquired to future users.