Comparison Between Two Methodologies for Assessing Historical Earthquake Parameters and Their Impact on Seismicity Rates in the Western Alps

Abstract

We 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 rates.

The European Preinstrumental Earthquake Catalogue EPICA, the 1000–1899 catalogue for the European Seismic Hazard Model 2020

“…For the latter earthquakes it is usually very difficult, and sometimes impossible, to trace back the historical information they rely upon. In addition, although the methodologies for assessing earthquake parameters from intensity data, each with its pros and cons, are nowadays robust, they rarely provide a unique solution from the same intensity dataset, and the selection of a single best method is not straightforward (Cecić et al, 1996;Bakun et al, 2011;Stucchi et al, 2013;Provost et al, 2022). This fragmentation and variety of both data and methods strongly affect the consistency of earthquake catalogues across country borders.…”

“…This fragmentation and variety of both data and methods strongly affect the consistency of earthquake catalogues across country borders. For example, a very good representation of the longterm seismicity at the national scale is provided by the catalogues of Switzerland (ECOS-09; Fäh et al, 2011), France (F-CAT17; Manchuel et al, 2018) and Italy (CPTI15; Rovida et al, 2020bRovida et al, , 2022. These three catalogues provide robust parameters derived with advanced, well-calibrated and welldocumented methodologies exploiting the richest historical macroseismic databases in the world.…”

“…Such discrepancies of course have important consequences on the elaborations based on them such as, in the case of seismic hazard assessment, the computations of seismic activity rates, which may result in inconsistencies across country borders (e.g. Rong et al, 2011;Beauval et al, 2020;Provost et al, 2022).…”