Source characteristic of 2000 small earthquakes nucleating on the Alto Tiberina fault system (Central Italy)

San Francisco

The Alto Tiberina Fault (ATF) is a 60 km long east-dipping low-angle normal fault, located in a sector of the
Northern Apennines (Italy) undergoing active extension since the Quaternary (Chiaraluce et al. 2007). The
ATF has been imaged by analyzing the active source seismic reflection profiles, and the instrumentally
recorded persistent background seismicity. The present study is an attempt to separate the contributions of
source, site, and crustal attenuation, in order to focus on the mechanics of the ATF, as well as the syn- and
antithetic structrure related on the ATF hanging-wall (i.e. Gubbio fault and Umbria Valley fault). In order to
compute source spectra, we perform a set of regression over the seismograms of ~ 400 small earthquakes
(0.5 < ML < 3.0) recorded between 2010 and 2014 at 50 permanent seismic stations deployed in the
framework of the Alto Tiberina Near Fault Observatory project (TABOO; Chiaraluce et al., 2014), three of
which located in shallow boreholes.
Because we deal with some very small earthquakes, we maximize the signal to noise ratio (SNR) with a
technique based on the analysis of peak values of bandpass-filtered time histories, in addition to the same
processing performed on Fourier amplitudes. We rely on Random Vibration Theory (RVT, Cartwright and
Longuet-Higgins, 1956) to completely switch from peak values in the time domain to Fourier spectral
amplitudes. So far, highly accurate, stable source spectra have been used to compute moment magnitudes
(Mw) of all the events in the present data set, whereas in future developments the same data will be used to
gain insights into the underlying mechanics of faulting and the earthquake processes.