Abstract
In order to evaluate the detection and localisation improvement of
a small aperture array in the Northern Apennines, we installed an
irregularly spaced test configuration in the vicinity of Città
di Castello (CDC) for a period of two weeks. The experimental array
consisted of nine 3-component stations with inter-station distances
between 150 m and 2200 m. Seismic data were digitised at 125 Hz and
telemetered to a mobile acquisition, processing and storage centre.
The data could only be recorded in trigger mode. The peculiarity
of the test array installation was the exclusive use of 3-component
sensors at all array sites, which also allowed beamforming for S-phases
on the horizontal components. Since the altitudes of the single array
sites differed considerably among each other, for f-k analysis and
beamforming an elevation correction was included. During the two
weeks of operation about 20 local earthquakes with magnitudes ML<2.6,
1 regional, and several teleseismic events were recorded. In addition
to these events, the array occasionally triggered on coherent noise-signals
generated by local industrial activity. The data analysis was performed
by means of f-k analysis and beamforming, providing wavenumber characteristics
of the incident plane wave. Typical apparent velocities were determined
to be 4.8 km/s and 6 km/s for Pg-phases and \~10 km/s for Pn-phases.
We observed local seismic events, which occurred just beneath the
array. In these cases wavefronts with unusual high apparent velocities,
similar to those found for the Pn-phase, were observed. Since no
continuously recorded array data were available, we extrapolated
the lower detection magnitude threshold as a result of the SNR improvement
due to array beamforming. Compared to the actual detection threshold
of MT \~1.6 reached by the national seismic network in this area,
a nine element array would improve this value up to MT \~0.8.
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