Results from the temporary installation of a small aperture seismic arrayin the Central Apenninesand its merits for local event detectionand location capabilities

T. Braun, J. Schweitzer, R. M. Azzara, D. Piccinini, M. Cocco, E. Boschi

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.

Keywords


seismic array;beamforming;f-kanalysis;Central Apennines

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References


DOI: https://doi.org/10.4401/ag-3359
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X