Observations of infrasound from surf in southern California



[1] We observe 10,000's of infrasound signals annually, in the frequency range from 1–5 Hz, at the I57US infrasound array in Southern California. 75% of these signals arrive at I57US at azimuths between 260 and 320 degrees. There is a good correlation between the amplitudes of signals from 260–320° and wave height measurements offshore Southern California, providing firm evidence that the signals from these azimuths are caused by surf action. By modeling the propagation of infrasound for specific time periods (using 3D ray-tracing and up-to-date atmospheric models), we show that amplitudes of signals recorded at I57US are also dependent upon atmospheric winds. The modeled rays that return in the stratosphere fit observations more closely than modeled rays that return in the troposphere, suggesting the signals are from stratospheric returns. Our findings are the first observations of surf infrasound at a long-range from the source region. This suggests that surf signals might serve as a probe of the atmosphere, much like microbarom signals are used to probe the atmosphere at greater range.