Authors : Antoine Schlupp et Didier Bertil
Accurate mapping of earthquake-induced tremors is an indispensable input for damage assessment. It also makes it possible to apprehend the spatial variations of the tremors, which are in fact much more complex than those represented by the attenuation birds, mainly due to the source (mechanism, size of the rupture, directivity), propagation (spatial variability of attenuation) and site effects (geological or topographical). These parameters, which are difficult to characterise for a given earthquake, ipso facto make a theoretical modelling of the tremor unrepresentative, a situation aggravated in the event of uncertainties about the magnitude and hypocentric location. However, we have an asset in France, that of having real-time access to more than 200 seismological stations (soon 400 – Résif) and a network of 36,000 communes for the intensities deduced from testimonies and surveys (www.franceseisme.fr).
The ShakeMap V3.5 (USGS) program, distributed by the United States Geological Survey (USA), makes it possible to reconcile modeling and data, by correcting a priori modeling (based on the hypocenter, magnitude, relationships between magnitude and distance, accelerations and intensities, and site effects) with information from instrumental and macro-seismic observations during the earthquake. However, the quality and density of the data changes over time. Instrumental observations, available very rapidly, remain spatially limited and rare in urban areas, the main target for seismic hazard and risk. Moreover, the intensities deduced from macrocosmic data, which are spatially very dense, change from the preliminary intensity resulting from the first observations a few minutes after the earthquake, to the final intensity after several months after a full investigation. The ShakeMap is therefore also evolutionary over time, from a preliminary and rapid version to a final and late version.
Such a ShakeMap is made for the Pyrenees since 2012 with updates for 24 hours. In 2015, a national “ShakeMap” working group is created under the impetus of the Résif-RAP. It is coordinated by the BCSF-RéNaSS. The other members of the working group are from BRGM, CEA, CEREMA, IPGP/OVSG-OVSM, ISTerre, OMP and OSUNA. One of the objectives is to define a reference ShakeMap of the best possible quality. Since 2016, the national ShakeMap is operational on www.franceseisme.fr for any earthquake that is the subject of an alert (from the CEA in metropolitan France and the IPGP in the West Indies) with an update up to 7 days. It is called “Regional Estimation of the earthquake from instrumental and macro-seismic data”.
The maps and data used for each ShakeMap are made available on the website. Since 2017, the Résif-Seismicity transversal action is divided into 5 axes. Axis 4, animated and coordinated by the authors, focuses on this theme and its possibilities for improvement and valorisation.
The ShakeMap is a transverse product, drawing on Résif data from axes 1, 2 and 3 (bulletins, catalogues, macro-seismic data) and illuminating axis 5 (seismic hazard).
- Antoine Schlupp, EOST-IPG, Strasbourg : firstname.lastname@example.org
- Didier Bertil, BRGM, Orléans ; email@example.com
This article was published in the Newsletter Résif n°13, January 2018.