Probabilistic seismic hazard assessment for South-Eastern France

The accurate evaluation and appropriate treatment of uncertainties is of primary importance in modern probabilistic seismic hazard assessment (PSHA). One of the objectives of the SIGMA project was to establish a framework to improve knowledge and data on two target regions characterized by low-to-moderate seismic activity. In this paper, for South-Eastern France, we present the final PSHA performed within the SIGMA project. A new earthquake catalogue for France covering instrumental and historical periods was used for the calculation of the magnitude-frequency distributions. The hazard model incorporates area sources, smoothed seismicity and a 3D faults model. A set of recently developed ground motion prediction equations (GMPEs) from global and regional data, evaluated as adequately representing the ground motion characteristics in the region, was used to calculate the hazard. The magnitude-frequency distributions, maximum magnitude, faults slip rate and style-of-faulting are considered as additional source of epistemic uncertainties. The hazard results for generic rock condition (Vs30 = 800 m/s) are displayed for 20 sites in terms of uniform hazard spectra at two return periods (475 years and 10,000 years). The contributions of the epistemic uncertainties in the ground motion characterizations and in the seismic source characterization to the total hazard uncertainties are analyzed. Finally, we compare the results with existing models developed at national scale in the framework of the first generation of models supporting the Eurocode 8 enforcement, (MEDD 2002 and AFPS06) and at the European scale (within the SHARE project), highlighting significant discrepancies at short return periods.     

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.     

Soft X-Ray Pre-Flare Emission Studied in Yohkoh-SXT Images

The goal of this study is to improve our knowledge of the spatial relation between pre-flare and flare X-ray sources, to find other connections between the two phenomena (if they exist) and to study the role of pre-flare heating in flare build-up. We selected all flares with available preflare data observed by Yohkoh during the period October 1993–October 1994 and thus created a data base of 32 flares. When studying the spatial relation we discovered that our events can be classified into 3 categories: Co-spatial, Adjacent/Overlapping and Distant according to the spatial separation between the pre-flare and flare source(s) in the same field of view. The 'Co-spatial class of events, of which we found 8 cases, refers to flares that had a visible pre-flare soft X-ray structure with the same size, shape, and orientation as the main flare loops at the flare site at least 5 min before the start of the impulsive phase. We suggest that this is strong evidence that for a significant number of flares the flare structure is active in soft X-rays several minutes or more before the flare begins. However, an analysis of the physical properties of the flare sites, including temperature and intensity variation found no consistent feature distinguishable from other non-flaring active region emission and hence no definite evidence of a special 'pre-flare or 'precursor phase in solar flares.