Adjusting an active shallow crustal ground motion model to regions with scarce data: application to France.

The objective of this work is to test whether an empirical Ground Motion Model (GMM) developed for high-seismicity regions can be effectively adapted to a neighbouring region with lower seismic activity. We select the ITA18 suite of GMMs (Lanzano et al. in Bull Seismol Soc Am 109(2): 525-540, 2019a), developed for Italy, which is a region dominated by moderate-to-strong shallow crustal earthquakes, and assess their applicability to Continental France, where the seismic activity is less frequent and characterised by lower magnitudes. Based on a dataset of more than 2300 records of events with 3.0 ≤ MW ≤ 5.2, occurred in France in the time interval 1996–2019 (named FR20), we perform a residual analysis and calibrate an adjustment factor for both horizontal and vertical-to-horizontal (VH) components of Peak Ground Acceleration, Peak Ground Velocity, and 5% damped Spectral Acceleration (SA). Apart from the median correction, no modification of the scaling with magnitude, focal mechanism, and VS,30 is introduced, while the distance scaling is adjusted to capture the lower anelastic attenuation of the French data. In addition, to overcome the underestimation of the ITA18 model for the short period VH spectral amplitudes in the near-source region (Repi < 15 km), an additional empirical corrective factor is introduced. In spite of the good agreement of the adjusted model with respect to the median trends of the FR20 dataset, a regionalization of the source effects is introduced to reduce the relatively high between-event variability of the proposed model. The proposed model provides predictions similar to ITA18 in the most seismically active regions (Alps or Pyrenees), while, in the other zones, the predicted amplitudes are richer at high frequencies. Given the paucity of seismic records in these zones, this behavior should be confirmed on the basis of additional data (e.g. physics-based simulations, geologic and tectonic features). The use of the proposed model for hazard applications is recommended within the validity limits of the data (3.0 ≤ MW ≤ 5.2). However, the similarity of the ground motion in the Alps and Pyrenees with the predictions of ITA18 suggests that the adjusted model could be also employed for higher magnitudes, upon suitable checks. [ABSTRACT FROM AUTHOR]