The underwater sound of an earthquake at the Main Endeavour Hydrothermal Vent Field

Brendan Smith – brendan.smith@dal.ca
Twitter: @bsmithacoustics
Instagram: @brendanthehuman
Dalhousie University, Department of Oceanography, Halifax, Nova Scotia, B3H 4R2, Canada

Additional author:
Dr. David Barclay

Popular version of 1aAO4 – Passive acoustic monitoring of a major seismic event at the Main Endeavour Hydrothermal Vent Field
Presented at the 187th ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0034918

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

The Main Endeavour Hydrothermal Vent Field (MEF) is located on the Juan de Fuca Ridge in the Northeast Pacific Ocean. This ridge is a seafloor spreading center, where tectonic plates pull apart and new oceanic crust is formed as magma upwells from beneath the earth’s surface. This movement of the earth’s crust causes cracks to form, allowing seawater to penetrate downwards towards the magma below, where it circulates and eventually resurfaces into the ocean at temperatures over 300 degrees Celsius. Uniquely adapted organisms thrive at these sites, surviving from energy provided not by the sun, but by the heat and chemical composition of the vent fluid.

Figure 1: Black-smoker hydrothermal vent chimney at the Main Endeavour Hydrothermal Vent Field (Image courtesy of Ocean Networks Canada)

Long term measurements of hydrothermal vent activity are of scientific interest. However, the high temperatures and caustic chemical characteristics make it challenging to place probes directly in the vent flow. For this reason, passive acoustics (listening) can be a useful tool for hydrothermal vent monitoring, because the hydrophones (underwater microphones) can be located a safe distance from the vent fluid. Ocean Networks Canada have had a hydrophone at MEF continuously recording for over 5 years, and for the past year, a 4-element hydrophone array has been recording at this location.

The motion of the tectonic plates in these regions causes a lot of seismic activity, such as earthquakes. On March 6, 2024, a large ~4.1 magnitude earthquake was recorded at MEF, and earthquake rates were the highest observed since 2005. This earthquake was recorded on the hydrophone array and can be seen in the spectrogram in Figure 2.

Figure 2: Spectrogram of ~4.1 magnitude earthquake at MEF

Figure 3 shows differences in the soundscape at Endeavour before, during, and after the earthquake. The changes after the earthquake persist more than 1-week following the event. The duration and higher frequency components of the changes in the soundscape suggest sources other than seismicity.

Figure 3: Acoustic spectra before, during, and after the earthquake at MEF

The hydrophone array also provides us with the opportunity to gain further insights. For example, surface wind/wave-generated noise is a predominant source of ambient sound in the ocean, and the coherence, or spatial relationship between multiple hydrophone elements in the presence of this sound source, is well known. We can compare the measured coherence with the expected (modeled) coherence to explore any deviations, which could be attributed to hydrothermal vent activity. In Figure 4 we see differences between the measurements and model below 1 kHz (outlined by black boxes), suggesting the influence of hydrothermal vent sounds on the local soundscape.

Figure 4: Measured and modeled acoustic vertical coherence at MEF

In conclusion, passive acoustic monitoring can be used to monitor changes in hydrothermal vent fields in response to seismic activity. This earthquake provided a test case to prepare for a more major seismic event, which is expected to occur at Endeavour in the coming years. Passive acoustic monitoring will be an important tool to document vent field activity during this future event.

Noise reduction for low frequency sound measurements from balloons on Venus

Taylor Swaim – tswaim@okstate.edu

Oklahoma State University
Stillwater, Oklahoma 74078
United States

Kate Spillman
Emalee Hough
Zach Yap
Jamey D. Jacob
Brian R. Elbing (twitter: @ElbingProf)

Popular version of 2pCA6 – Infrasound noise mitigation on high altitude balloons
Presented at the 184 ASA Meeting
Read the article in Proceedings of Meetings on Acoustics

While there is great interest in studying the structure of Venus because it is believed to be similar to Earth, there are no direct seismic measurements on Venus. This is because the Venus surface temperature is too hot for electronics, but conditions are milder in the middle of the Venus atmosphere. This has motivated interest in studying seismic activity using low frequency sound measurements on high altitude balloons. Recently, this method was demonstrated on Earth with weak earthquakes being detected from balloons flying at twice the altitude of commercial airplanes. Video 1 shows a balloon launch for these test flights. Due to the denser atmosphere on Venus, the coupling between the Venus-quake and the sound waves should be much greater, which will make the sound louder on Venus. However, the higher density atmosphere combined with vertical changes in wind speed is also likely to increase the amount of wind noise on these sensor. Thus development of a new technology to reduce wind noise on a high altitude balloon is needed.

Video 1. Video of a balloon launch during the summer of 2021. Video courtesy of Jamey Jacob.

Several different designs were proposed and ground tested to identify potential materials for compact windscreens. The testing included a long-term deployment outdoors so that the sensors would be exposed to a wide range of wind speeds and conditions. Separately, the sensors were exposed to controlled low-frequency sounds to test if the windscreens were also reducing the loudness of the signals of interest. All of the designs showed significant reduction in wind noise with minimal reduction in the controlled sounds, but one design in particular outperformed the others. This design uses a canvas fabric on the outside of a box as shown in the Figure 1 combined with a dense foam material on the inside.

Figure 1. Picture of balloon carrying the low frequency sound sensors. Compared an early design to no windscreen with this flight. Image courtesy of Brian Elbing.

The next step is to fly this windscreen on a high altitude balloon, especially on windier days and with a long flight line to increase the amount of wind that the sensors will experience. The wind direction at the float altitude of these balloons will change in May and then rapidly increase, which this will be the target window to test this new design.