Physical Sources of High‐Frequency Seismic Noise on Cascadia Initiative Ocean Bottom Seismometers

TitlePhysical Sources of High‐Frequency Seismic Noise on Cascadia Initiative Ocean Bottom Seismometers
Publication TypeJournal Article
Year of Publication2020
AuthorsHilmo, R, Wilcock, WSD
JournalGeochemistry Geophysics Geosystems
Type of ArticleJournal Article

Physical sources of high‐frequency seismic noise in the ocean are investigated using data from the Cascadia Initiative (CI) ocean bottom seismometer (OBS) network, hindcasts of wind speed, waves, and the bottom currents predicted by a regional ocean circulation model and observed at sites on cabled observatories. Seismic data in the 5–12 Hz band are considered because it is best for detecting regional earthquakes and lies between the frequencies of local microseisms and the seasonal whale calls. Median noise levels in this range vary by ~20 dB between sites at a given depth but on average decrease with increasing depth. On the continental shelf, the orbital motions of ocean waves are a major source of noise while at the quietest sites in the deep ocean, noise increases when wind speeds exceed ~10 m/s. On the continental slope and abyssal plain within about 100 km of the slope, seismic noise is not predicted at specific sites by the bottom currents in the ocean circulation model. In these regions, ocean currents are inferred to be the primary source of noise, because noise varies on tidal periods, is low on buried seismometers, and has spatial variations broadly consistent with those of median absolute currents. Comparisons between OBSs suggest that high‐frequency noise is reduced by low‐profile hydrodynamic designs but not by shielding. Many OBSs also record numerous short duration events on and near the continental shelf that have been attributed elsewhere to animals bumping into the sensor or gas bubbles moving through sediments. Environmental noise generated by waves, wind, and ocean currents limits the ability of seafloor seismometers to detect the ground motions from earthquakes. This research uses data from a large deployment of seafloor seismometers off the coast of the Pacific Northwest of America to understand the seismic noise at frequencies of 5–12 Hz which coincides with the frequency band used to detect regional earthquakes. Instruments deployed in shallow water on the continental shelf record high noise levels caused by motions from ocean waves that reach the seafloor. The quietest sites are found in the deep ocean far from the coast, and here the noise is created by wind blowing spray from whitecaps. At intermediate sites on the continental slope and deep ocean just seaward of the continental slope, the noise results from ocean currents that are related to ocean circulation and tides. Some shallow instruments also record large numbers of short signals that are not earthquakes and may be either a result of animals bumping into the seismic sensor or gas bubbles moving through sediments. The best way to reduce seismic noise is to bury the seismic sensors, but when this is infeasible, the instruments should have a short hydrodynamic design. Seismic noise is high on the shelf due to wave orbital currents, and noise at the quietest abyssal plain sites is correlated with wind speed Ocean currents are the primary source of high‐frequency noise on the continental slope and abyssal plain within ~100 km of the slope High‐frequency current‐generated seismic noise is reduced on OBSs with squat hydrodynamic designs and is not clearly reduced by shielding.


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