LAUSR

The Earth's hum is the continuous oscillations of the Earth at frequencies between 2 and 20mHz in the absence of earthquakes. The hum strongest signal consists mainly of surface waves. These seismic waves can be generated by infragravity waves propagating over a sloping ocean bottom close to the coast. So far, this theory has only been tested quantitatively using European seismic stations. We use seismic data recorded all around the Indian Ocean together with an ocean wave model that provides time-frequency varying hum sources. We show that seasonal variations of the hum sources are smaller in the southern hemisphere than the northern hemisphere. Using these sources, we model Rayleigh wave root-mean-square amplitudes in the frequency band 3.5-20mHz, and the good agreement with seismic data on the vertical component confirms the theory of hum generation. Because the Indian Ocean is uniquely connected to the southern hemisphere oceans but lies partly in northern hemisphere latitudes, the seasonal pattern of the hum recorded there is particular and shows no significant seasonal variations. At similar to 10mHz the hum is strongly influenced by local events, such as the passage of a cyclone close to a seismic station. In the absence of earthquake, the solid Earth is continuously vibrating at very low frequencies (2 to 20mHz). These vibrations, called seismic hum, were discovered in the 1990s and can be recorded by seismic stations everywhere on Earth. These vibrations have frequencies least a thousand times lower than audible sound, and should not be confused with the persistent sounds - that are also called hum - that can be heard in different regions. They are generated by ocean infragravity waves propagating over a sloping ocean bottom close to the coast. So far, this theory has only been tested quantitatively using European seismic stations. We analyze seismic data recorded all around the Indian Ocean, and we model them using hum sources derived from an ocean wave model. The good fit between data and model confirms the theory of hum generation. We further show that the hum recorded in the Indian Ocean is very specific and displays no significant seasonal variations. Finally, we demonstrate that when a cyclone arrives at a coast, it creates hum sources that increase the very low frequency seismic signal recorded by the nearby stations.