Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

Quantifying Uncertainties for earthquakes' Magnitude and Depth

International audienceMagnitude estimates of earthquakes from the intensity macroseismic field are marred by uncertainties. Two main types of uncertainties can be identified: one linked to the quality of the intensity data and the epistemic uncertainties of the intensity prediction equations (IPE) used to estimate magnitude from the macroseismic field. Quality of the intensity data depends on how detailed the testimonies of the earthquake are and on their reliability. In some macroseismic databases, a quality factor is associated to each intensity data point. IPE are calibrated on earthquakes for which macroseismic data and instrumental data, i.e. magnitude and depth, are available. The coefficients of IPEs depend then on the quality of the instrumental data, the quality of the intensity data and the calibration dataset. We present here the QUake-MD methodology, acronym for Quantifying Uncertainties for earthquakes' Magnitude and Depth. QUake-MD quantifies uncertainties in magnitude/depth estimates for earthquakes known only by their macroseismic fields by taking into account the quality of intensity data and the IPE epistemic uncertainties. Intensity data quality is used to weight the inversion process of intensity data in the application of the IPEs and to associate uncertainties to the inverted depth and magnitude. IPE epistemic uncertainties are taken into account by the use of several IPEs. Uncertainties associated to the inverted depth and magnitude combined to the use of different IPEs can be used to build a probability density function of the plausible depth, magnitude and epicentral intensity associated to the considered earthquake. To illustrate the strength of the methodology we use the intensity data collected by the BCSF (Bureau Central Sismologique Français) following recent earthquakes and compare macroseismic and instrumental estimates of Mw, depth and associated uncertainties

QUake-MD: open source code to Quantify Uncertainties in Magnitude -Depth estimates of earthquakes from macroseismic intensities

International audienceThis paper presents a tool to quantify uncertainties in magnitude/depth/epicentral intensity (M/H/Io) estimates for earthquakes associated with macroseismic intensity data. The tool is an open-source code written in python and named QUake-MD (Quantifying Uncertainties in earthquakes' Magnitude and Depth). In QUake-MD uncertainties are propagated from the individual intensity data point (IDP) to the final M/H/Io solution. It also accounts for epistemic uncertainties associated to the use of different intensity prediction equations (IPE). For each IPE, QUake-MD performs a sequential least square inversion process to estimate the central value of the M/H/Io triplet. QUake-MD then explores the uncertainties around the M/H solution by constructing a probability density function of possible M/H solutions constrained to be consistent with the range of plausible Io, a plausible depth range and IDP uncertainties. The resulting probability density functions of all IPEs provided to QUake-MD are then stacked to obtain a final probability density function of possible M/H/Io solutions representative of both data quality and IPE epistemic uncertainties. This tool thus provides end-users with a more complete understanding of the uncertainties associated with historical earthquake parameters, beyond the classical standard deviation values proposed today in parametric earthquake catalogues

How to deal with macroseismic magnitudes at borders: a methodological comparison of their estimates along the French-Italian border

International audienceThe estimation of magnitudes of earthquakes that occurred before the instrumental period is a key issue in seismic hazard assessment. Such estimates are based on information provided by historical sources, which are translated into macroseismic intensity by means of intensity scales, which are then used, through different methodologies, to compute earthquake parameters. Within the AHEAD initiative we are beginning to analyze differences between methodologies regarding both magnitude and uncertainty estimates. For this initial exercise we relied on the information available in the SISFRANCE macroseismic database for a set of events located along the French-Italian border and compared the Boxer (Gasperini et al 2010) and QUake-MD (Provost and Scotti, 2017) methods.Although the two methods provide consistent results, we find a systematic magnitude bias between these two methods when using the epicentral information (location and epicentral intensity) provided by Boxer. However, within the limits of applicability of each methodology this bias disappears when using for the QUake-MD methodology, the epicentral location and intensity value of the SISFRANCE database. Furthermore, uncertainties estimates provided by the two methodologies overlap underlining the importance of quantifying uncertainties in parametric earthquake catalogues.In this presentation, we will discuss the preliminary results obtained when calibrating both methodologies with the same dataset. In this framework, requirements and challenges of calibration dataset selection will also be presented

Methodological comparison of macroseismic magnitude estimates for events along the French-Italian border

International audienceMagnitude estimates of earthquakes occurred before the instrumental period are a key issue in seismic hazard assessment. Such estimates are based on information provided by historical sources, which is translated into macroseismic intensity by means of intensity scales. Methods for computing earthquake parameters from intensity data differ within and between countries. We compare magnitude estimated from the application of the Boxer (Gasperini et al 2010) and QUake-MD (Provost and Scotti, 2017) methods to a set events located along the French-Italian border. Using the same macroseismic data and epicentral location we find a systematic magnitude (+-/ 0.5) bias for M>5.0 and for M5.0.However when we consider the uncertainties reported in the two methodologies, both magnitude estimates intersect, underlining the importance of quantifying uncertainties in parametric earthquake catalogues and the need to propagate such uncertainties in seismic hazard assessment

Reducing differences in earthquake activity rate estimates across borders in Europe.

International audienceEarthquake activity rates estimated in cross-border regions can differ between countries. For example, considering the FCAT or CPTI15 catalogues in the Italian and French Alps, earthquake activity rates can differ from 20 to 80% depending on the magnitude bin. This study aims at answering the following question: how much of the difference in the annual seismicity rates in the Alps cross-border region is due to different methodologies used to compute historical earthquake magnitudes? To answer this question, we built two new historical (pre-1980) parametric earthquake catalogues for this region considering a common post-1980 earthquake catalogue based on the CPTI15. To focus on methodological differences, it was necessary to first build a common macroseismic and instrumental magnitude dataset to calibrate the two methodologies considered, namely Boxer and QUake-MD. We then applied the two methodologies to the same macroseismic dataset to build the two new historical (pre-1980) parametric earthquake catalogues. Finally, we computed earthquake rates for the two catalogues and found them to be statistically similar. This exercise underlines that reducing existing differences in seismic activity rate estimates across border regions will necessarily require a common definition of instrumental magnitudes and a common macroseismic dataset

Comparison between two methodologies for assessing historical earthquake parameters and their impact on seismicity rates In the Western Alps

International audienceWe investigate the differences in seismicity rate estimates from two historical earthquake catalogues obtained with two methodologies (Boxer and QUake-MD) calibrated on a common dataset of macroseismic intensities and calibration events. The two methodologies were then applied to a test data set of historical earthquakes covering the France, Italy and Switzerland Alpine region. Differences between the resulting magnitude estimates and instrumental magnitudes show a standard deviation of 0.4 for both methodologies, with a mean residual of 0.01 for Boxer and -0.04 for Quake-MD. A systematic difference in magnitude estimates between the two methodologies that correlates with the depth estimated by Quake-MD has been observed. This is attributed to the difference in the treatment of the depth parameter between Boxer and QUake-MD. Nevertheless, differences in magnitude estimates between the two methodologies show a mean residual of 0.006 and a standard deviation of 0.35 resulting in seismicity rates that are not significantly different considering the associated uncertainties. Such results made us believe that the European community could gain in the reduction of epistemic uncertainties associated with the estimate of historical earthquake parameters by agreeing on a common macroseismic and calibration dataset across borders. These efforts should be strongly encouraged. On the other hand, we show that even in the ideal conditions of this benchmark (same calibration events and same macroseismic intensity dataset), methodological differences can lead to systematic differences in magnitude estimates. It is therefore paramount to explore different methodologies for a more realistic quantification of the epistemic uncertainties in estimates of maximum magnitudes and seismic activity rate