Investigating the Role of Fluids in the Source Parameters of the 2013–2014 Mw 5 Matese Seismic Sequence, Southern Italy

Abstract We investigate the variability of Brune stress drop (Δσ), apparent stress (τa), and Savage–Wood radiation efficiency (ηsw=τa/Δσ), in the 2013–2014 Mw 5.0 earthquake sequence that struck the Matese area in the southern Apennines range of Italy. The sequence is clustered in a relatively small crustal volume in the 13–22 km depth range, which is greater than that of background seismicity and normal-faulting sequences that occurred under the range axis, usually located in the first 15 km of the crust. We find high Savage–Wood radiation efficiency values for most of the analyzed earthquakes located in a narrow crustal volume, with values ranging from well above the self-similarity value to very high values as high as 0.55. In addition, a large variability in radiation efficiency (up to 90%) is observed for two similar magnitude events at different depths. Previous studies reported seismic evidence of fluid involvement in the nucleation process of the Matese earthquakes. By integrating our results with crustal geophysical data published recently, we propose that most of the earthquakes characterized by high values of ηsw are nucleated within high pore pressure zones located in the crystalline midcrust of Adria. We reckon that high pore pressure fluids of deep origin played a role in the rupture process and were responsible for the mixed shear-tensile sources inferred from the analysis of the S-wave/P-wave spectral amplitude ratio for most of 2013–2014 earthquakes.