Source scaling of microearthquakes at Mt. Etna Volcano and in the Calabrian Arc (southern Italy)

Abstract

Microearthquake data recorded at Mt. Etna volcano and in the Calabrian Arc (Southern Italy) have been used to study the scaling of the main source parameters. After correction of the P‐wave displacement spectra for both path attenuation and the site amplification effects, an inverse method has been applied to compute spectral parameters (low frequency spectral level, corner frequency and high frequency roll‐off). Results show that seismic moments (M0) range from ca. 1016 to 1021 dyne‐cm at Mt. Etna and from ca 1017 to 1020 dyne‐cm in southern Calabria. Corner frequencies range from ca. 3 to 15 Hz. In both areas we found that the stress drop decreases (from ca. 100 to few bars) with decreasing seismic moment. A simple Brune source model is not able to explain the source scaling, because the typical fall‐off observed at higher frequencies (ω−3 as an average) is larger than that of Brune's theory. Furthermore, the relation between the seismic moment and the corner frequency points to a scaling law behaving as M0∝ωc−4.5 and M0∝ωc−3.6 on average, at Mt. Etna and in southern Calabria respectively, and higher for smaller events in both areas. Therefore, the cube law (M0 ∝ ω−3) generally accepted for large earthquakes does not hold true for the analysed microearthquakes, neither for those which occur along the tectonic structures of southern Calabria, or along those hosting the roots of the feeding system of the largest European volcano.