Derivation of consistent hard rock (1000<Vs<3000 m/s) GMPEs from surface and down-hole recordings: Analysis of KiK-net data

A key component in seismic hazard assessment is the estimation of ground motion for hard rock sites, either for applications to installations built on this site category, or as an input motion for site response computation. Empirical ground motion prediction equations (GMPEs) are the traditional basis for estimating ground motion while VS30 is the basis to account for site conditions. As current GMPEs are poorly constrained for VS30 larger than 1000 m/s, the presently used approach for estimating hazard on hard rock sites consists of “host-to-target” adjustment techniques based on VS30 and κ0 values. The present study investigates alternative methods on the basis of a KiK-net dataset corresponding to stiff and rocky sites with 500 < VS30 < 1350 m/s. The existence of sensor pairs (one at the surface and one in depth) and the availability of P- and S-wave velocity profiles allow deriving two “virtual” datasets associated to outcropping hard rock sites with VS in the range [1000, 3000] m/s with two independent corrections: 1/down-hole recordings modified from within motion to outcropping motion with a depth correction factor, 2/surface recordings deconvolved from their specific site response derived through 1D simulation. GMPEs with simple functional forms are then developed, including a VS30 site term. They lead to consistent and robust hard-rock motion estimates, which prove to be significantly lower than host-to-target adjustment predictions. The difference can reach a factor up to 3–4 beyond 5 Hz for very hard-rock, but decreases for decreasing frequency until vanishing below 2 Hz

Mantle structure under Gibraltar constrained by seismic waveform complexity

We study the Africa-Iberia plate boundary in the vicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-mantle structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper mantle structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the Alboran Sea regio

Non-conventional seismological constraints on subduction zone structure: Preliminary results from the Alboran Sea between Spain and Marocco

International audienc

A Robust Method for Assessing 3-D Topographic Site Effects: A Case Study at the LSBB Underground Laboratory, France

International audienceBy means of three-dimensional (3-D) numerical simulations, including the Laboratoire Souterrain à Bas-Bruit (LSBB) topography, we carefully analyze site effects assessments yielded by two approaches: the classical site to reference spectral-ratio method (SRM) and the statistical median reference method (MRM). We show for both isotropic and double-couple point sources that a 94% reduction in the number of stations of a regularly spaced array yields MRM site-effect estimates within 5% of those obtained from the absolute regional median, and within 20% using a 98% station reduction with irregularly located sites. In contrast, the SRM yielded site-effect overestimates greater than 50% in some areas and up to 100% in specific sites, which makes the MRM much more robust than the SRM. We determined a 33% probability to exceed an amplification factor of 2, and an 8% probability to exceed a factor of 3 due to topography in the surroundings of the sharpest summit of the LSBB area

Frequency‐Scaled Curvature as a Proxy for Topographic Site‐Effect Amplification and Ground‐Motion Variability,

International audienceWe introduce a new methodology to predict the topographic site‐effect amplification. Ground motions obtained from a large database of 3D earthquake simulations show that the curvature of the Earth’s surface, defined as the second spatial derivative of the elevation map, is correlated with the topographic site amplification. The highest correlation between the frequency‐dependent topographic amplification and the topographic curvature is reached when the curvature is smoothed over a characteristic length equal to the S wavelength divided by two (i.e., frequency‐scaled curvature [FSC]). This implies the amplification is caused by topographic features for which horizontal dimensions are similar to half of the S wavelength. The largest ground‐motion variabilities are found at sites located on slopes and on the largest summits, whereas intermediate variabilities occur over narrow ridges and a stable behavior in the bottom valleys. The FSC proxy allows the identification of topographic features with similar characteristic dimensions and probabilistic estimates of amplification values accounting on the variability of ground motions due to source–site interactions. Amplification estimates using the FSC proxy are robust and easily computed from digital elevation maps provided that reasonable values of S‐wave velocities are available in the area of interest

Preferred habitat of tropical tuna species in the Eastern Atlantic and Western Indian Oceans: a comparative analysis between FAD-associated and free-swimming schools

Abstract An ecological niche modelling (ENM) approach was developed to describe the suitable habitat of skipjack (SKJ) and juvenile yellowfin (YFT) tuna in the Tropical Atlantic and West Indian Oceans. The environmental envelop of the potential habitat in each ocean was defined using occurrence data independently of the fishing mode and derived from purse seine fishing sets of the French fleet during 1997-2014. Daily satellite-derived chlorophyll-a content (CHL) and fronts (CHL gradient) were used as a proxy for food availability while circulation model derived-sea surface temperature, salinity, height anomaly, current and oxygen as well as the mixed layer depth contributed to identify the physical suitable conditions of each species. Only the cluster that showed no CHL front was excluded for the parameterization in order to enhance the favourable feeding habitat. In a second step, the distances of both the free swimming schools (FSC) and schools associated with drifting Fishing Aggregating Devices (FADs) to the closest potential habitat were computed and compared. The results highlighted (i) high spatial seasonality of both the simulated feeding habitat and tuna populations in the Indian Ocean compared to the tropical Atlantic, (ii) major differences between both oceans regarding the distance of FAD catches to the potential habitat with median values above 200 km in the Atlantic and below 16 km in the Indian Ocean, while equivalent distances for FSC were observed for both species and areas (below 2 km and 43 km respectively) in agreement with stomach content analysis, (iii) an increased rate of FAD fishing operations in the decade from 2003 to 2013 (up to about 70% in the Atlantic and 96% in the Indian Ocean) occurring mostly in poor environments in the tropical Atlantic while frequently in relatively productive waters in the Indian Ocean (except east of 58°N) as well as an overall 300% increase of juvenile YFT presence in both ocean sets and (iv) a recent intensification of fishing effort from March to May in the Mozambique Channel in agreement with an increase of favourable habitat, while no effort of that fleet occurred in the open waters off the Gabon upwelling (from 1°N to 5°S and East of 17°W) from May to September where favourable habitat was enhanced by the model. In all cases the seasonal maximum number of fishing sets corresponded to the minimum extent of potential habitat, which commonly varied by 30% from year to year. Overall, this comparative analysis emphasizes the strong attraction of tropical tuna species to floating objects although feeding opportunities may vary considerably depending on hydrographic regimes and on the dynamics of productive habitats

Can we trust high-frequency content in strong-motion database signals ? Impact of housing, coupling, and installation depth of seismic sensors

Seismic hazard studies provide indicators of seismic motion that are expressed for "free-field," that is, representative of the ground motion exactly at the free surface, without disturbances due to interactions between soil and buildings or other structures. Most of these studies are based on ground-motion prediction equations, which are, themselves, formulated to predict free-field motion, as they are derived from similarly free data. However, is this really the case? In this study, we use several examples to illustrate how small structures hosting permanent strong-motion stations (often anchored on small concrete slabs) generate soilstructure interaction effects that can amplify the high-frequency part of the earthquake signal (10 Hz) by up to a factor of 2-3 for stations on soils. We also show that the installation depth of a station, even if very shallow (i.e., a few meters), can change the recorded response, mainly by deamplifying the signal in high frequencies (> 10 Hz) by a factor up to 0.3. Such effects imply that there are actual differences between recorded and true free-field signals. Depending on the housing conditions, these effects can have significant impact on response spectra at high frequencies, and on measurements of the kappa parameter. It is, thus, becoming clear that such effects should be taken into account in studies involving high-frequency seismic motion. To do so, scientists need a detailed description of the conditions of installation and housing of seismological and accelerometric stations, which often lacks from the metadata distributed through the various, commonly used web services. Increasing such information and facilitating the access to it would allow the identification of stations that are problematic and of those that are truly close to free-field recording conditions. In a subsequent step, it would be important to quantify the modification curve of the response of stations that experience such effects