Seismic Detection of Internal Gravity Waves at the Dongsha Atoll, South China Sea

Date: 4/26/2019

Time: 04:30 PM

Room: Pike

Oceanic internal waves are gravity waves localized on density stratifications in the water column and are ubiquitous. They can propagate thousands of kilometers before breaking and the ensuing turbulent mixing affects coastal processes and climate feedbacks. 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. Here we use data from the South China Sea where the world’s largest internal waves occur to identify internal wave signals in an onshore passive seismic dataset for the first time. The key data comes from the Dongsha Atoll where temporary (October 15, 2013 – May 31, 2014) 15-minute temperature mooring measurements in the water column at the eastern forereef directly detect the passage of internal waves by a sudden drop in temperature, and are complemented by a long-term seismic observatory on the western side of the atoll. Our goal is to correlate the established internal wave signal in the oceanic (temperature) data with a signal in the seismic. We produce a time series of internal waves in the oceanic data by taking the 2-hour smoothed envelope squared of a 50-minute zero phase high-pass. In the seismic, we take the 4-hour smoothed envelope of a 200-1000 second (expected period of internal waves) zero phase band-pass. We find a high-power signal in the seismic that consistently lags behind the oceanic internal wave signal. We find that the daily activity rates (fraction of day with detections) are similarly high at times we expect internal waves (fall, spring) and low at times we do not expect internal waves (winter). The seismic and oceanic activity rates are correlated at a 99.5% significance. It appears that we have successfully developed a technique to detect internal waves that opens up the possibility of utilizing the global seismic network to construct broad spatial maps and long-term timeseries of oceanic internal waves.

Presenting Author: Heather R. Shaddox

Authors