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 amplitudedeflection 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.

Instrument: Seabird Electronics SBE56 logging thermistor.  

Deployment details: We use 1-10 second sampling rate oceanic temperature measurements in the water column and on the ocean bottom during a temporary deployment in May/June 2019 around the fore reef of Dongsha Atoll (Figure 1c).  We consider a 20-meter mooring on the eastern side of Dongsha Atoll (FRE20) at 19.5 meters depth from May 13-June 11, 2019 and an ocean bottom temperature sensor at approximately 16.8 meters depth on the western side of Dongsha Atoll (FRW15) roughly 4.5 km southwest of a permanent seismic station onshore of Pratas Island from May 19-June 6, 2019.