Activity Pattern of the Sea-Star, Marthasterias glacial is, in Port-Cros Bay (France, Mediterranean Coast)

Abstract. Tagged individuals of Marthasterias giacialis (L.) followed in situ for 24 h cycles or for up to 4 consecutive days show that sea-star activity is not only nocturnal; we observed a non-synchronized (between individuals) movement, with 1 or 2 (ocassionally 3) peaks; movement, occurs mainly between 6:00 p.m. and 9:00 a.m., without a clear peak at the population level, independent the prevailing light conditions (for 6:00–9:00p. m. and 6:00–9:00a. m. periods). M. giacialis shows periods of inactivity ranging from 1 to 3 days. It is hypothesized that the study site is visited for short term feeding raids by individuals coming from the deep water and then moving back.     

Characterization of site conditions (soil class,V<Subscript>S30</Subscript>, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, V_S30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of V_S30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated V_S30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.     

Array performances for ambient vibrations on a shallow structure and consequences over <Emphasis Type="Italic">V</Emphasis><Subscript><Emphasis Type="Italic">s</Emphasis></Subscript> inversion

Valuable information about one-dimensional soil structures can be obtained by recording ambient vibrations at the surface, in which the energy contribution of surface waves predominates over the one of other types of waves. The dispersion characteristics of surface waves allow the retrieval of the shear-wave velocity as a function of depth. Microtremor studies are usually divided in two stages: deriving the dispersion (or auto-correlation) curve from the recorded signals and inverting it to obtain the site velocity profile. The possibility to determine the dispersion curve over the adequate frequency range at one site depends on the array aperture and on the wavefield spectra amplitude that can be altered by filtering effects due to the ground structure. Microtremors are usually recorded with several arrays of various apertures to get the spectral curves over a wide frequency band, and different methods also exist for processing the raw signals. With the objective of defining a strategy to achieve reliable results for microtremor on a shallow structure, we analyse synthetic ambient vibrations (vertical component) simulated with 333 broadband sources for a 25-m deep soil layer overlying a bedrock. The first part of our study is focused on the determination of the reliable frequency range of the spectral curves (dispersion or auto-correlation) for a given array geometry. We find that the wavenumber limits deduced from the theoretical array re sponse are good estimates of the valid spectral curve range. In the second part, the spectral curves are calculated with the three most popular noise-processing techniques (frequency–wavenumber, high-resolution frequency–wavenumber and spa tial auto-correlation methods) and inverted indi vidually in each case. The inversions are performed with a tool based on the neighbour hood algorithm that offers a better estimation of the global uncertainties than classical linearised methods, especially if the solution is not unique. Several array apertures are necessary to construct the dispersion (auto-correlation) curves in the appropriate frequency range. Considering the final velocity profiles, the three tested methods are almost equivalent, and no significant advantage was found for one particular method. With the chosen model, all methods exhibit a penetration limited to the bedrock depth, as a consequence of the filtering effect of the ground structure on the vertical component, which was observed in numerous shallow sites.