Abstract

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 (ML <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.

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