Seismic Detection of Oceanic Internal Gravity Waves From Subaerial Seismometers

Abstract

Oceanic internal gravity waves propagate along density stratification within the water column and are ubiquitous. They can propagate thousands of kilometers before breaking in shoaling bathymetry and the ensuing turbulent mixing affects coastal processes and climate feedbacks. Despite their importance, internal waves are intrinsically difficult to detect as they result in only minor amplitude deflection of the sea surface; the need for global detection and long time series of internal waves motivates a search for geophysical detection methods. The pressure coupling of a propagating internal wave with the sloping seafloor provides a potential mechanism to generate seismically observable signals. We use data from the South China Sea where exceptional oceanographic and satellite time series are available for comparison to identify internal wave signals in an onshore passive seismic data set for the first time. We analyze potential seismic signals on broadband seismometers in the context of corroborating oceanographic and satellite data available near Dongsha Atoll in May–June 2019 and find a promising correlation between transient seismic tilt signals and internal wave arrivals and collisions in oceanic and satellite data. It appears that we have successfully detected oceanic internal waves using a subaerial seismometer. This initial detection suggests that the onshore seismic detection and amplitude determination of oceanic internal waves is possible and can potentially be used to expand the historical record by capitalizing on existing island and coastal seismic stations.Oceanic internal gravity waves are similar to the more familiar surface gravity waves that travel along the air‐water density boundary at the surface of the ocean, but instead travel along density boundaries within the water column. Internal waves are important for coastal processes, climate feedbacks, and general oceanic dynamics and are therefore important to detect and track on a global scale over time. However, since internal waves are buried within the water column, they are difficult to detect. Seismology may be able to aid in detecting and measuring the size of internal waves since a traveling internal wave will deform the underlying seafloor and generate a local tilt signal that should be observable on coastal broadband seismometers. Here we perform an initial evaluation of the seismic detectability of internal waves by using Dongsha Atoll in the South China Sea where we compare an onshore broadband seismometer to internal waves identified in corroborating oceanic and satellite data. We find correlations between the timing of transient seismic tilt signals and internal waves identified in oceanic and satellite data. This is promising evidence of the first onshore seismic detection of internal waves. Internal waves can generate local tilt potentially observable on subaerial broadband seismometers We find promising evidence of the first onshore seismic detection of internal waves Seismic coupling preferentially selects waves that collide nearshore