The Sounds of Small Earthquakes: Quantitative Results from a Study of Regional Macroseismic Bulletins

Abstract

An original database of regional bulletins is analyzed to derive firstorder 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 (M L ) magnitude and hypocentral distance. The values obtained provide lower bounds for human perception of earthquakes.

Earthquake sound perception

“…However, it has the advantage of potentially yielding a great quantity of data. The distribution of reports mainly indicates the following: a decrease in the percentage of respondents hearing the sound at increasing distances from the epicenter [ Davison , 1938], a correlation between the size of the area of interest with respect to an audible sound and the event magnitude [ Sylvander and Mogos , 2005], and the specific shape of the area where sound is heard, related to the geometry of the seismic source [ Tosi et al , 2000].…”

“…Although our results are limited to the Italian region, the reader should note that Italy contains a large variety of tectonic settings and outcropping layers, and is not limited by particular cases. Our results were also in qualitative agreement with a study on the sound of small earthquakes in the French Pyrenees [ Sylvander and Mogos , 2005].…”

“…Much depends on an organism's sensitivity to low pitches. On the other hand, the discrepancy between some observations and ground movement has led to the hypothesis that earthquake sound has a higher frequency content with respect to the seismic signal [ Souriau , 2006; Sylvander and Mogos , 2005], likely due to the influence of non‐linear effects that create higher frequencies from lower frequency movements of the surface.…”

Earthquake Sounds

“…Most of his data came from Britain, and he classified about 20,000 descriptions of sounds into 7 classifications that are largely consistent with low rumbling sounds but also include some descriptions of impulsive sounds which are heard more often at short epicentral distances. A more recent intensity catalog with special emphasis on sounds was collected by Pierre Stahl in the French Pyrenees (Sylvander and Mogos 2005) and the US Geological Survey "Did You Feel It?" catalog of Internet-collected intensity data includes information on sounds in its 1.25 million observations (Wald et al 1999).…”

“…More recently, Sylvander and Mogos (2005) and Tosi et al (2012) developed relationships between the locations where sounds are heard, earthquake magnitude, and epicentral distance and considered sound as a factor in human detection of small earthquakes. Souriau (2006) also explored relationships between heard sounds and shaking and points out that sound may be responsible for waking people and thus could contribute to intensity values at the low end of the scales.…”