Françoise Courboulex

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models.

An unknown active fault revealed by microseismicity in the south‐east of France

[1]xa0A seismic swarm occurred in the south-east of France in December 2000, about 15 km north of the densely populated cities of the French-Riviera. More than 300 events were located by a dense temporary seismic network that had been installed in the epicentral region one month before. We evidence an alignment of the seismicity that we interpret as an unknown active fault. On the basis of regional tectonics, we propose that this fault results from basement faulting cutting through the detached overlying sedimentary cover.

A site effect study in the Verchiano valley during the 1997 Umbria-Marche (Central Italy) earthquakes

Strong site effects were observed during the two MW 5.7 and MW 6.0 main shocks of the Colfiorito seismic crisis which occured on September 26, 1997 in Umbria-Marche (Central Italy).The most obvious indications of these effects are the dramatic differences in damage shown by buildings of similar construction in neighboring villages.Such observations were specifically made in the Verchiano valley in the fault area, 15 km south of Colfiorito where the Verchiano village and the Colle and Camino hamlets were heavily damaged (MCS intensity IX-X) since the first main shock of 1997/09/26,while in contrast, the Curasci village located 2 km eastwards remains almost intact.In order to study the anomalous ground motion amplifications in this area, an array of 11, 3-components seismo- and accelero-meters was set up during the 1997/10/20-24 period, extending from the western side of the valley, up to the top of Mount San Salvatore, going accross the Colle and Curasci hamlets.During the experiment, 67 aftershocks enlightened the valley from the Colfiorito (10 km north) and the Sellano (6 km south) active swarms.Seismic refraction experiments were conducted at the same time in the 500 m wide, 1500 m long Verchiano valley in order to determine the thickness and main characteristics of the alluvial infilling.The main results are: (i) compared to the valley side ground motion, and for all the events, recordings in the central part of the valley (piana di Verchiano) show relative amplification of ∼10 with a clear lengthening of the seismogram duration by a factor of ∼2 – (ii) broad band relative amplification of ∼6–8 is also clearly identified at the top of the Mount San Salvatore overhanging the valley – (iii) any of the site effect measurements done explains by itself the strongly contrasted damage observed at Colle and Curasci: i.e. the modification of the near-field radiation pattern by interaction with the free heterogeneous surface may have induced local shadow zones that saved Curasci.

Ground-Motion Variability and Implementation of a Probabilistic–Deterministic Hazard Method

Abstract A key step in probabilistic seismic-hazard assessment is the prediction of expected ground motions produced by the seismic sources. Most probabilistic studies use a ground-motion prediction model to perform this estimation. The present study aims at testing the use of simulations in the probabilistic analysis instead of ground-motion models. The method used is the empirical Green’s function method of Kohrs-Sansorny etxa0al. (2005), which takes into account the characteristics of the source, propagation paths, and site effects. The recording of only one small event is needed for simulating a larger event. The small events considered here consist of aftershocks from the M xa06.4 Les Saintes earthquake, which struck the Guadeloupe archipelago (French Antilles) in 2004. The variability of the simulated ground motions is studied in detail at the sites of the French Permanent Accelerometric Array. Intrinsic variability is quantified: ground motions follow lognormal distributions with standard deviations between 0.05 and 0.18 (log units) depending on the spectral frequency. One input parameter bearing large uncertainties is the ratio of the stress drop of the target event to the small event. Therefore, overall sigma values (and medians) are recomputed, varying stress drop ratio values between 1 and 15. Sigma values increase but remain in general lower or equal to the sigma values of current ground-motion prediction models. A simple application of this hybrid deterministic–probabilistic method is carried out at several sites in Guadeloupe for the estimation of the hazard posed by an M xa06.4 occurring in the rupture zone of the Les Saintes event.

Ground-Motion Simulations of the 2004 Mw 6.4 Les Saintes, Guadeloupe, Earthquake Using Ten Smaller Events

The validity and the stability of a ground-motion simulation method based on the recordings of a single small event as an empirical Green’s function (EGF) is tested on a seismic crisis that occurred 25xa0km offshore of the Guadeloupe Islands (Caribbean arc). We aim to determine if (1)xa0the method enables us to reproduce the observed ground motion, (2)xa0the choice of the small event taken as an EGF is crucial for the simulations, and (3)xa0the method provides valuable results compared with ground-motion prediction equations (GMPEs). We have successively used the recordings of 10 small earthquakes ( M wxa04.2–5.1) to simulate the ground motions generated by the mainshock ( M wxa06.4), at 12 accelerometric stations. We first determined the moment and focal mechanisms of the 10 events chosen as an EGF, as well as the stress-drop ratio C between each of these events and the mainshock. Then, we simulated 500 accelerograms for each EGF and each station. A good reproduction of the mainshock response spectra, the peak ground acceleration, and the duration of the signal was obtained using 9 out of 10 EGFs. For stations with site effects, the results obtained are much closer to the real data than values given by the GMPEs on sediment sites. In the case of blind predictive simulation, we propose to calibrate the stress-drop ratio C through a comparison between the simulated response spectra on rock site stations and the values predicted by GMPEs.

Site Effects in Port‐au‐Prince (Haiti) from the Analysis of Spectral Ratio and Numerical Simulations

Abstract To provide better insight into seismic ground motion in the Port‐au‐Prince metropolitan area, we investigate site effects at 12 seismological stations by analyzing 78 earthquakes with magnitude smaller than 5 that occurred between 2010 and 2013. Horizontal‐to‐vertical spectral ratio on earthquake recordings and a standard spectral ratio were applied to the seismic data. We also propose a simplified lithostratigraphic map and use available geotechnical and geophysical data to construct representative soil columns in the vicinity of each station that allow us to compute numerical transfer functions using 1D simulations. At most of the studied sites, spectral ratios are characterized by weak‐motion amplification at frequencies above 5xa0Hz, in good agreement with the numerical transfer functions. A mismatch between the observed amplifications and simulated response at lower frequencies shows that the considered soil columns could be missing a deeper velocity contrast. Furthermore, strong amplification between 2 and 10xa0Hz linked to local topographic features is found at one station located in the south of the city, and substantial amplification below 5xa0Hz is detected near the coastline, which we attribute to deep and soft sediments as well as the presence of surface waves. We conclude that for most investigated sites in Port‐au‐Prince, seismic amplifications due to site effects are highly variable but seem not to be important at high frequencies. At some specific locations, however, they could strongly enhance the low‐frequency content of the seismic ground shaking. Although our analysis does not consider nonlinear effects, we thus conclude that, apart from sites close to the coast, sediment‐induced amplification probably had only a minor impact on the level of strong ground motion, and was not the main reason for the high level of damage in Port‐au‐Prince.

Les enseignements du petit séisme de Peille (Alpes-Maritimes, France)

All that the small Peille earthquake (South-East of France) can teach us. The Peille earthquake (Mw = 3.4) occurred on 1st November 1999, 15 km to the north of the cities of Nice, Monaco and Menton. This event was very well recorded by 20 seismological stations located between 6 and 50 km from the epicenter. We used a non-linear location method and showed that the earthquake occurred at shallow depth (3 ± 1.5 km) close to the Peille-Laghet fault trace. Moreover the computed focal mechanism is consistent with the left lateral strike slip movement of this fault. An empirical Greens function deconvolution showed that the rupture occurred on a fault portion smaller than 600 m long. This earthquake was also the occasion to point out remarkable site effect amplifications in some areas of the city of Nice. uf6d9 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS seismic risk / seismic hazard / earthquakes / site effects / seismotectonics / Alps / France

Numerical and Empirical Simulation of Linear Elastic Seismic Response of a Building: The Case of Nice Prefecture

The structural motion of a tall reinforced concrete (RC) building on alluvial soil in Nice (France) is continuously recorded using accelerometers. The structural behavior of the building is studied using operational modal analysis (OMA) to identify its dynamic properties, a finite element (FE) model to reproduce the building response, and empirical Greens functions (EGFs) to generate the structural response to ground motions stronger than those registered in the analyzed seismic area. These different approaches are applied for the analysis of seismic response of the instrumented building and results are consistent. The FE model is calibrated by comparing natural frequencies and mode shapes with those obtained using OMA. Numerically-simulated time histories are qualitatively and quantitatively compared with recordings showing good agreement. Based on regional earthquakes, linear seismic response of the building is simulated for a stronger scenario earthquake using EGF. This approach allows for structural deformation analysis of existing buildings without the need of structural plans and mechanical parameter calibration in the case where the seismic response is within linear elastic regime.