A study of the relation between ocean storms and the Earth's hum

Abstract

We previously showed that the Earth's “hum” is generated primarily in the northern oceans during the northern hemisphere winter and in the southern oceans during the summer. To gain further insight into the process that converts ocean storm energy into elastic energy through coupling of ocean waves with the seafloor, we here investigate a 4‐day‐long time window in the year 2000 that is free of large earthquakes but contains two large “hum” events. From a comparison of the time functions of two events and their relative arrival times at the two arrays in California and Japan, we infer that the generation of the “hum” events occurs close to shore and comprises three elements: (1) short‐period ocean waves interact nonlinearly to produce infragravity waves as the storm reaches the coast of North America; (2) infragravity waves interact with the seafloor locally to generate long‐period Rayleigh waves; and (3) some free infragravity wave energy radiates out into the open ocean, propagates across the north Pacific basin, and couples to the seafloor when it reaches distant coasts northeast of Japan. We also compare the yearly fluctuations in the amplitudes observed on the two arrays in the low‐frequency “hum” band (specifically at 240 s) and in the microseismic band (2–25 s). During the winter, strong correlation between the amplitude fluctuations in the “hum” and microseismic bands at BDSN is consistent with a common generation mechanism of both types of seismic noise from nonlinear interaction of ocean waves near the west coast of North America.