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

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.