Comparison of the ranges of uncertainty captured in different seismic-hazard studies

The inclusion of epistemic uncertainties, generally via logic trees (Kulkarni et al., 1984), within probabilistic seismic‐hazard assessments (PSHAs) is becoming standard for all types of studies (commercial, governmental, or research; site specific, national, regional, or global). Consequently many studies publish expected ground motions for a given annual frequency of exceedance (AFE) or return period derived from the hazard curves for the mean, median, and various fractiles (percentiles)

Site effects observations and mapping on the weathered volcanic formations of Mayotte Island

Since the 2000s, local seismic hazard studies have shown that Mayotte Island presented superficial geological formations prone to lithological site effects. The seismic sequence initiated in May 2018 confirmed the importance of such effects, both in terms of intensity and spatial extension. The analysis of the recorded strong motions showed that weathered volcanic formations are prone to significant site effects with mean amplification factors for peak ground acceleration (PGA) between 1.4 and 4.9 and that a complex combination of lithological and topographic site effects is in action. We thus implement a regional scale map of site effects for the fast calculation of strong motion and damage maps for crisis management purposes. We also provide a first estimate of key site parameters for eight stations: surface geology, resonance frequency, an amplification factor proxy for PGA, a ${V}_{{S},30}$ value, if available, and an estimated EC8 soil class

Seismic damage scenarios for Mayotte: a tool for disaster management

A new marine volcano is erupting offshore Mayotte since May 2018, generating numerous earthquakes. The population felt many of them and the stronger shaking of the ongoing sequence caused slight damage to buildings. Historical records also confirm that damaging earthquakes had occurred in the past in this region. Seismic damage scenarios are a key tool for supporting the decision-making process, the preparedness, and for designing appropriate emergency responses. This paper provides the outcomes of a work consisting in improving the seismic risk assessment as a part of disaster risk governance and exposes the scientific background of this workflow. It illustrates its use with two earthquakes. Related post-seismic surveys provide observations that are useful to check the validity of the reference dataset. The paper also discusses the main characteristics of the rapid loss assessment tool that has been developed to provide operational information for crisis management

Onset of a submarine eruption east of Mayotte, Comoros archipelago: the first ten months seismicity of the seismo-volcanic sequence (2018–2019)

International audienceFrom 10 May 2018 to 1 November 2022 (time of writing), an unprecedented seismic activity is observed east of Mayotte Island (France), related to the largest submarine eruption ever recorded with offshore geophysical studies. Using signals from regional and local seismic stations, we build a comprehensive catalog of the local seismicity for the first ten months of the sequence. This catalog includes a total of 2874 events of magnitude (Mlv) ranging from 2.4 to 6.0, with 77% of them relocated using a double difference location procedure. The hypocentral locations over this period are highly dependent on the small seismic network available. Therefore we compare the locations of later events using a similar network and those estimated from a local ocean bottom seismometer (OBS) network installed since March 2019. Based on the time space evolution and characteristics of the seismicity, five distinct phases can be identified, corresponding to the successive activation of two deep seismic swarms, related to the lithospheric-scale magma ascent up to the seafloor, along with progressive deepening of the seismicity interpreted as decompression of a 40 km deep reservoir

Major ions, δ18O, δ13C and 87Sr/86Sr compositions of water and precipitates from springs along the Cameroon Volcanic Line (Cameroon, West Africa): Implications for provenance and volcanic hazards

A combined study of major ions, δ18O, δD, 13C, 87Sr/86Sr isotopes, X-ray diffraction, scanning electron microscopy, and electron probe microanalyses on springs and spring mineral precipitates along the Cameroon Volcanic Line (CVL) was undertaken to understand water chemistry, and infer the type and origin of the precipitates. The waters are of evaporated Na + KCl and non-evaporated Ca + MgHCO3 types, with the more mineralized (electrical conductivity-EC of 13130 μS/cm) Lobe spring inferred to result from interaction of circulating 49 °C waters with magmatic volatiles of the active Mt. Cameroon. Water mineralization in the other springs follows the order: Sabga A > Sabga B > Bambui B > Bambui A > Nyos Cave. But for the Nyos Cave spring, all other springs contain fluoride (up to 0.5–35.6 mg/l above WHO potable water upper limit). The Sabga spring contains arsenic (up to 1.3 mg/l above the WHO limits). The springs show low fractionation temperatures in the range of 19–43 ᵒC. The Lobe and Sabga A springs are precipitating dolomite (CaMg(CO3)2), while the Nyos Cave, Bambui A, Bambui B and Sabga B springs precipitate trona ((Na3H(CO3)2.H2O). Our data suggest a marine provenance for the carbonates, and point to a volcanic input for the Lobe, Nyos, Sabga A, and Bambui A springs. The latter springs are therefore proposed as proxies for monitoring volcanic activity for hazard mitigation along the CVL

Séisme des Saintes. 21 novembre 2004. Saint-Claude. Endommagement de l'église.

Parish church of Saint-Claude (19th century), near the town hall. The church and its bell tower were damaged. The bell tower in concrete structure (column/beam), was sheared in its upper third. In the commune, it is the sector of Matouba that has suffered the most damage, such as the Louis Chalcol school, with a reinforced concrete framework filled with fill, which shows serious damage.Image taken during the Macroseismic Response Group (MERG) field mission that took place from November 25 to December 1, 2004, following the Saintes earthquake. This earthquake was of magnitude 6.3 (MW), its epicentre being located in the south of Guadeloupe, between the archipelago of the Saintes and Dominica. The city of Saint-Claude is 35km from the epicentre and the intensity of the tremors felt is between VI and VII on the European intensity scale EMS-98.Eglise paroissiale de Saint-Claude (XIXe siècle), proche de la mairie. L'église et son clocher ont été endommagés. Le clocher en structure béton (poteau/poutre), a été cisaillé dans son tiers supérieur. Sur la commune, c'est le secteur de Matouba qui a subi le plus de dommages, comme par exemple l'école Louis Chalcol, avec une ossature en béton armé à remplissage, qui présente de graves dommages.Image prise lors de la mission de terrain du Groupe d'intervention macrosismique (GIM) qui s'est déroulée du 25 novembre au 1er décembre 2004 suite au séisme des Saintes. Ce séisme était de magnitude 6.3 (MW), son épicentre se situant au sud de la Guadeloupe, entre l’archipel des Saintes et la Dominique. La ville de Saint-Claude se situe à 35km de l’épicentre et l’intensité des secousses ressenties se situe entre VI et VII sur l’échelle d’intensité européenne EMS-98