The near‐coastal microseism spectrum: Spatial and temporal wave climate relationships

Comparison of the ambient noise data recorded at near‐coastal ocean bottom and inland seismic stations at the Oregon coast with both offshore and nearshore buoy data shows that the near‐coastal microseism spectrum results primarily from nearshore gravity wave activity. Low double‐frequency (DF), microseism energy is observed at near‐coastal locations when seas nearby are calm, even when very energetic seas are present at buoys 500 km offshore. At wave periods >8 s, shore reflection is the dominant source of opposing wave components for near‐coastal DF microseism generation, with the variation of DF microseism levels poorly correlated with local wind speed. Near‐coastal ocean bottom DF levels are consistently ∼20 dB higher than nearby DF levels on land, suggesting that Rayleigh/Stoneley waves with much of the mode energy propagating in the water column dominate the near‐coastal ocean bottom microseism spectrum. Monitoring the southward propagation of swell from an extreme storm concentrated at the Oregon coast shows that near‐coastal DF microseism levels are dominated by wave activity at the shoreline closest to the seismic station. Microseism attenuation estimates between on‐land near‐coastal stations and seismic stations ∼150 km inland indicate a zone of higher attenuation along the California coast between San Francisco and the Oregon border.