The Earth's Hum Variations From a Global Model and Seismic Recordings Around the Indian Ocean

Abstract

Abstract The Earth's hum is the continuous oscillations of the Earth at frequencies between 2 and 20 mHz 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 3.5–20 mHz, 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 ~10 mHz the hum is strongly influenced by local events, such as the passage of a cyclone close to a seismic station.