Localizing ground motion models in volcanic terranes: Shallow events at Mt. Etna, Italy, revisited.

Abstract

We present a set of revised ground motion models (GMMs) for shallow events at Mt. Etna volcano. The recent occurrence of damaging events, in particular two of the strongest earthquakes ever instrumentally recorded in the area, has required revising previous GMMs as these failed to match the observations made for events with local magnitude ML >4.3, above all for sites situated close to the epicenter. The dataset now includes 49 seismic events, with a total of 1600 time histories recorded at distances of up to 100 km, and ML ranging from 3.0 and 4.8. The model gives estimates of peak ground acceleration (PGA, both horizontal and vertical), peak ground velocity (PGV, both horizontal and vertical) and 5%-damped horizontal pseudoacceleration response spectral ordinates (PSA) up to a period of 4 s. GMMs were developed by using the functional form proposed by Boore and Atkinson (2008). Furthermore, with a slightly modified approach, we also considered a regression model using a pseudo-depth (h) depending on magnitude according to the scaling law by Azzaro et al . (2017). Both models were applied to hypocentral distance ranges of up to 60 km, and up to 100 km, respectively. From the statistical analysis, we found that reducing the maximum distance from the event up to 60 km and introducing a magnitude-dependent pseudo-depth, improved the model in terms of total error. We compared our results to those derived with the GMMs for shallow events at Mt. Etna found by Tusa and Langer (2016) and for volcanic areas by Lanzano and Luzi (2019). The main differences are observed at short epicentral distances and for higher magnitude events. The use of variable pseudo-depth avoids sharp peaks of predicted ground motion parameters around the epicenter, preventing instabilities when using a GMM in probabilistic seismic hazard analysis.