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Seismoacoustic Recordings of Small Earthquakes in the Pyrenees: Experimental Results

¹ CNRS, Université Paul Sabatier
Observatoire Midi-Pyrénées
14, Avenue Edouard Belin
31400 Toulouse, France
 (M.S., S.B., J.-F.F.)
² Observatoire Midi-Pyrénées
57, avenue d’Azereix
65008 Tarbes, France
 (C.P.)

An experiment was conducted in the Lesponne Valley (central French Pyrenees) to record the sounds that frequently accompany earthquakes in this region. Seismic and acoustic signals of a short-period seismometer and a condenser microphone were continuously monitored for ten months. During this period, four small (M_L <2.8) earthquakes provided clear audio recordings at short epicentral distances (<20 km). Sounds associated not only to the direct P transmission, but also SV-to- P conversion into the atmosphere are identified, thus confirming former hypotheses formulated by Hill et al. (1976). Unambiguous quantitative constraints on the seismic-to-acoustic conversion are provided by the computation of transmission coefficients. The dominant audio frequencies are recorded in the 5- to 60-Hz band, that is, at the boundary between sounds and infrasounds, and at the lower bound of human audibility. High-frequency infrasounds (about 8 Hz) are interpreted as the transmission to the atmosphere of Rayleigh waves induced locally by a thin soil layer.