6th Colloquium on historical earthquakes & paleoseismology studies

Macroseismic field mission in Mayotte after the seismic sequence in May and June 2018

24 Oct 2018, 12:10

20m

Han-sur-Lesse

Oral Preference Session 1

Speaker

Christophe Sira (EOST (Ecole et Observatoire des sciences de la Terre - Strasbourg)/UMS830-CNRS)

Description

The 2018 seismic sequence of Mayotte began the 10 May at 50-60 km east from the island and Mamoudzou (most important city of Mayotte with approximately 70000 inhabitants) with earthquakes of magnitude 3 – 4. The major event with a magnitude of 5.7 (BRGM) took place 5 days after (Mw 5,8-5,9 USGS), preceded the day before by a magnitude 5.2. Until the 15 June (period considered in the study of BCSF-RENASS), the few stations of the local seismological network recorded more than 1000 earthquakes with 38 of magnitude > 4.9 (BRGM, 2018). The seismicity remained steady until mid-June, with almost daily earthquake of magnitude between 5 and 5,5 and on average about 20 of magnitude > 3.5 with a peak of more than 80 M>3.5 the 1st June. If the intensities (severity of the ground shaking) seemed, in view of the available preliminary information, to not exceed V (EMS-98 scale) for every event in particular because of the epicentral distance (50 to 70 km), the damage reported by the municipalities during the following days, both in level of damage as in number of affected buildings, brought us to activate a field survey with our Macroseismic intervention group (GIM) to analyze the real impact. Considering the impossible distinction of the effects for each event, the objective of this field survey was to establish the level of damage of buildings, according to their vulnerability, produced by the seismic swarm until the date of our investigation in each municipality. This assessment of cumulated damage versus vulnerability at the city scale was then associated to an “equivalent intensity”. These values, despite they cannot be taken into account for scientific studies or seismic scenarios as real EMS98 (severity of ground shaking), they reflect the impact of a seismic sequence on macroseismic observations and allow to help the interpretation of macroseismic data for similar cases in the historical period. We will underline in this presentation the difficulties we had for this specific macroseismic study related to a seismic swarm with several events inducing damages and the difference with a classical estimation of the macroseismic intensities for an event only.