International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

PREdiction of NOn‐LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong‐motion recordings obtained at well‐known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log_(10) scale) for a peak ground acceleration of 5  m/s^2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation

PRENOLIN project. Results of the validation phase at sendai site

One of the objectives of the PRENOLIN project is the assessment of uncertainties associated with non-linear simulation of 1D site effects. An international benchmark is underway to test several numerical codes, including various non-linear soil constitutive models, to compute the non-linear seismic site response. The preliminary verification phase (i.e. comparison between numerical codes on simple, idealistic cases) is now followed by the validation phase, which compares predictions of such numerical estimations with actual strong motion data recorded from well-known sites. The benchmark presently involves 21 teams and 21 different non-linear computations. Extensive site characterization was performed at three sites of the Japanese KiK-net and PARI networks. This paper focuses on SENDAI site. The first results indicate that a careful analysis of the data for the lab measurement is required. The linear site response is overestimated while the non-linear effects are underestimated in the first iteration. According to these observations, a first set of recommendations for defining the non-linear soil parameters from lab measurements is proposed. PRENOLIN is part of two larger projects: SINAPS@, funded by the ANR (French National Research Agency) and SIGMA, funded by a consortium of nuclear operators (EDF, CEA, AREVA, ENL)

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

NON-LINEAR SOIL RESPONSE AT STRONG MOTION OBSERVATION SITES WITH A FOCUS ON BOREHOLE ARRAY

International audiencebehaviour in seismological data. It focuses mainly on vertical arrays as an exceptional naturallaboratory to study the propagation of seismic waves through the subsurface soil layers. The reviewhighlights the breakthroughs that were performed such as the demonstration of the significance of thenon-linear soil behaviour or the proposal of methodologies to back calculate the non-linear soilproperties.The PGA is one of the most used parameters to describe the level of seismic solicitations. The reportedPGA threshold from which non-linear soil behaviour is observed, has been decreasing since the 80’sthanks to the increase of available data, the improvement of the data quality, of the computationalefficiency. The results of all studies lead to (1) new methodologies to invert data and provide in-situcharacterisation of soil behaviour and (2) the definition of proxy parameters that reflect the impact ofnon-linear soil behaviour on site responses. From these proxies it is now possible to take into accountnon-linear soil behaviour in the analysis of site responses without using numerical simulations. Futureperspectives in this field concern the densification of the instrumented sites and the characterisation ofsoil properties to provide natural laboratories that are well-constrained. For the assessment of the insitunon-linear soil properties, the quantification of the uncertainties and spatial variability alsorepresent a challenge. To do so we need to gather even more the data which have been made availablethrough international collaborations

NON-LINEAR SOIL RESPONSE AT STRONG MOTION OBSERVATION SITES WITH A FOCUS ON BOREHOLE ARRAY

International audiencebehaviour in seismological data. It focuses mainly on vertical arrays as an exceptional naturallaboratory to study the propagation of seismic waves through the subsurface soil layers. The reviewhighlights the breakthroughs that were performed such as the demonstration of the significance of thenon-linear soil behaviour or the proposal of methodologies to back calculate the non-linear soilproperties.The PGA is one of the most used parameters to describe the level of seismic solicitations. The reportedPGA threshold from which non-linear soil behaviour is observed, has been decreasing since the 80’sthanks to the increase of available data, the improvement of the data quality, of the computationalefficiency. The results of all studies lead to (1) new methodologies to invert data and provide in-situcharacterisation of soil behaviour and (2) the definition of proxy parameters that reflect the impact ofnon-linear soil behaviour on site responses. From these proxies it is now possible to take into accountnon-linear soil behaviour in the analysis of site responses without using numerical simulations. Futureperspectives in this field concern the densification of the instrumented sites and the characterisation ofsoil properties to provide natural laboratories that are well-constrained. For the assessment of the insitunon-linear soil properties, the quantification of the uncertainties and spatial variability alsorepresent a challenge. To do so we need to gather even more the data which have been made availablethrough international collaborations

PRENOLIN: International Benchmark on 1D Nonlinear Site‐Response Analysis—Validation Phase Exercise

This article presents the main results of the validation phase of the PRENOLIN project. PRENOLIN is an international benchmark on 1D nonlinear (NL) site‐response analysis. This project involved 19 teams with 23 different codes tested. It was divided into two phases; with the first phase verifying the numerical solution of these codes on idealized soil profiles using simple signals and real seismic records. The second phase described in this article referred to code validation for the analysis of real instrumented sites. This validation phase was performed on two sites (KSRH10 and Sendai) of the Japanese strong‐motion networks KiK‐net and Port and Airport Research Institute (PARI), respectively, with a pair of accelerometers at surface and depth. Extensive additional site characterizations were performed at both sites involving in situ and laboratory measurements of the soil properties. At each site, sets of input motions were selected to represent different peak ground acceleration (PGA) and frequency content. It was found that the code‐to‐code variability given by the standard deviation of the computed surface‐response spectra is around 0.1 (in log10 scale) regardless of the site and input motions. This indicates a quite large influence of the numerical methods on site‐effect assessment and more generally on seismic hazard. Besides, it was observed that site‐specific measurements are of primary importance for defining the input data in site‐response analysis. The NL parameters obtained from the laboratory measurements should be compared with curves coming from the literature. Finally, the lessons learned from this exercise are synthesized, resulting also in a few recommendations for future benchmarking studies, and the use of 1D NL, total stress site‐response analysis