Infrasound from ocean waves observed in Tahiti

Abstract

[1] The dominant source of infrasonic waves detected in Tahiti is associated with ocean swells generating permanent signals from 0.1 to several hertz. At thousands of kilometers from Tahiti, microbaroms are mainly produced by standing ocean waves near low-pressure systems in the south Pacific. Their monitoring over one year exhibits clear seasonal trends correlated with changes in the prevailing stratospheric wind direction. More locally, ocean waves propagating towards Tahiti also generate surf noise when impacting against the reefs. Analyzing the infrasonic recordings, the main source regions of surf signals are identified and their locations and seasonal variations are discussed.

The radiation of atmospheric microbaroms by ocean waves

“…In these cases there is a severe reduction in predicted levels as a consequence of the increased attenuation in the thermosphere. While this is consistent with observations 1, 9,11,12,37 the numerical values for the propagation as predicted by such linear models should not be taken literally since, in the thermosphere, the density a decreases dramatically so that the accuracy of the linear approximation to the atmospheric response is doubtful. 38 For downwind propagation, however, the sound gets trapped in the duct formed by the stratosphere ͑below 60 km͒ so that received levels are not greatly influenced by the thermosphere.…”

“…Their propagation is highly dependent on the direction of the atmospheric winds. [9][10][11][12] Atmospheric microbarom spectra measured downwind from the wave system have typical peak levels of about 0.1 Pa/ ͱ Hz. 13 Note that oceanic microbarom signals are a thousand times greater than atmospheric signals.…”

Distinguishing high surf from volcanic long‐period earthquakes

“…In 1992, a glacial outburst flood on Mount Spurr, Alaska, generated seismic signals having both LP and tremor characteristics that were initially thought to be an eruption precursor [Nye et al, 1995]. Infrasound recordings near coastlines show that breaking ocean waves are also capable of producing repeating events whose energy spectra overlap the LP band and have similar durations [Aucan et al, 2006;Garcés et al, 2003;Le Pichon et al, 2004]. For volcano monitoring and research purposes, it is critical to be able to quickly distinguish whether an increase in LP activity is due to volcanic unrest or to nonvolcanic sources such as wave action or a surging glacier.…”

Ambient infrasound noise

“…Due to their pervasiveness, they routinely set the noise levels and thus determine the detection thresholds in that band. Their generation depends upon the amplitudes of ocean waves produced by storms [e.g., Arendt and Fritts, 2000], and their propagation depends strongly on stratospheric winds [Garcés et al, 2004;Le Pichon et al, 2004].…”