Abstract

An original database of regional bulletins is analyzed to derive first-order relationships that link the sounds generated by small earthquakes to the amplitude of ground motions and to local magnitude scales. The database is a collection of about 1000 macroseismic observations collected during the 1984–1999 period in the French Pyrenees from a staff of regular observers. For each observation, the main parameters are the qualitative perception of the sound (faint or loud), the hypocentral distance, and the earthquake local magnitude expressed on the scale used by the French Laboratoire de Détection et de Géophysique (ldg) network. A local relationship between the magnitude, the hypocentral distance, and the amplitude of the P-wave vertical ground displacement recorded in local seismological stations is established. Audibility thresholds of small earthquakes are then expressed as a function of Richter (ML) magnitude and hypocentral distance. The values obtained provide lower bounds for human perception of earthquakes.

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