Plots of power spectral density (a measure of how much energy is present at different frequencies) at ocean bottom seismometers AXCC1 (top panel), located offshore Cascadia, and MOBB (bottom panel), located in Monterey Bay, California. The x axis represents time, labeled as days of the year 2015, while the y axis represents wave periods, in seconds (labeled at left). Colors indicate the power spectral density in decibels (dB), with red colors indicating more energy and blue colors indicating less energy. The dashed white line indicates the first appearance of strong energy at relatively long periods due to a storm impacting the Cascadia region of the west coast of the United States. Small gray dots indicate the timing of relatively large earthquakes globally, with the size of the earthquakes (on the moment magnitude scale, Mw) shown on the right side of the plot. Credit: Maurya et al. [2019], Figure 5 lower right panels
Source: Geochemistry, Geophysics, Geosystems

The Earth’s “hum” is an enigmatic, low-frequency signal that has been known to seismologists for some time, but whose source has remained poorly understood. It is generally thought that the hum is generated in ocean basins through the interaction of infragravity waves with the seafloor. However, the hum is not evenly distributed in space or time, and the exact mechanisms through which it is generated remain unclear.

Maurya et al. [2019] combine seismic observations with ocean wave height data to track two large storms in the Pacific Ocean and delineate how each of them generates hum. Their work shows that the distribution of Earth’s hum sources strongly depends on the propagation characteristics of the corresponding storms and how the storms interact with the coastline. This study has important implications not only for how we understand the generation of infragravity waves in the ocean, but also for studies of deep Earth structure that take advantage of the hum signal.

Citation: Maurya, S., Taira, T., & Romanowicz, B. [2019]. Location of seismic “hum” sources following storms in the North Pacific Ocean. Geochemistry, Geophysics, Geosystems, 20.

—Maureen Long, Editor, Geochemistry, Geophysics, Geosystems

Text © 2019. The authors. CC BY-NC-ND 3.0
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