A local earthquake tomography model of the Fucino fault-controlled basin (central Apennines, Italy) obtained through a very dense temporary network

Abstract

The Fucino basin (central Italy) is the largest Plio-Quaternary tectonic depression of the Apennines extensional belt. The basin is bounded to the north and east by two main normal fault systems striking WSW-ENE and NW-SE, respectively. These fault systems controlled the syntectonic depositions of lacustrine and coarse clastic sequences that reach a total thickness of 1.5 km. The NW-SE fault system is the source of the Mw 7.0, 1915 central Italy earthquake and of previous M6-7 earthquakes recognized through paleoseismic trenching. On the other hand, current activity and seismogenic potential of the WSW-ENE structures are uncertain. The shallow architecture of both fault systems (< 2 km depth) is well defined by surface data and seismic reflection profiles, but the fault’s deep geometry is poorly known. Large uncertainties also regard the crustal structure underneath the basin at seismogenic depths (i.e.; 5-15 km depth) despite a close deep seismic reflection profile (i.e., CROP11 line). The instrumental seismicity occurring beneath the Fucino basin is scarce. On the contrary, an intense activity concentrated to the north (2009, Mw 6.3, L’Aquila sequence) and 25-30 km to the south, where both low-to-moderate sequences and diffuse swarm-like seismicity were recorded in recent years. In 2008-2009, a dense passive seismic survey was carried out in the Fucino area to investigate the basin seismic response and local site effects. The temporary network included 18 stations, with an average spacing of 2-3 km, operating in continuous mode with a sampling rate of 125 Hz and equipped with 5 second seismometers. In this study, we re-processed the data recorded by the Fucino temporary network, integrated by the permanent stations of the Italian seismic network and Abruzzo regional network installed on the surrounding ridges, to construct a new earthquake catalog and perform a local-scale passive tomographic survey. We used a standard (STA/LTA) algorithm to detect very local weak events in addition to those used in the previous site-effects study. P- and S-wave arrival times of the detected seismic events were hand picked and weighted according to a standard scheme. Seismograms for stations deployed in the Fucino basin show strong complexities especially for P-waves onsets that are often masked by background noise. We used the final dataset in terms of P- and S-waves arrival times as input for a Local Earthquake Tomography targeting the upper crustal velocity structure and active faults underneath the Fucino basin and surrounding ridges. The tomographic model, presented in terms of Vp and Vp/Vs, aimed at recovering the crustal heterogeneities with a spatial resolution finer with respect to previous tomographic surveys of central Apennines. The 3D distribution of Vp and Vp/Vs and of relocated events helped us to identify the velocity contrasts related to the main faults and to improve our knowledge on their geometry at depth.