After powerful earthquakes rumbled in Ridgecrest, California, NASA and Caltech researchers released four ballons into the air to detect the aftershock’s sound waves. (Image Credit: NASA/JPL-Caltech)
Sometime between July 4th and July 6th, 2019, powerful earthquakes struck near Ridgecrest, California, setting off over 10,000 aftershocks. Taking advantage of this event, researchers from NASA’s Jet Propulsion Laboratory and Caltech flew high-altitude balloons mounted with instruments to test their earthquake-detection capabilities over that region. Then, on July 22nd, ultra-sensitive barometers attached to a balloon detected low-frequency sound waves triggered by an aftershock. They say that this technology could study quakes on Venus.
Development for this balloon-based seismology technique started in 2016. Since sound waves are generated from seismic waves, information is transferred from the subsurface and into the atmosphere. This can be collected by studying sound waves from the air using the same approach that seismologists use to study seismic waves on the ground.
During the aftershocks, the team released two heliotrope balloons, ascending to altitudes of 11 to 15 miles as the sun heated them, returning to the ground at dusk. While drifting, the balloons’ barometers measured air pressure changes over the area as the aftershocks’ faint acoustic vibrations moved through the air.
The sun’s heat caused the balloons to rise into the air. During a balloon’s flight on July 22nd, 2019, a balloon detected low-frequency acoustic waves produced by an aftershock. (Image Credit: NASA/JPL-Caltech)
“Trying to detect naturally occurring earthquakes from balloons is a challenge, and when you first look at the data, you can feel disappointed, as most low-magnitude quakes don’t produce strong sound waves in the atmosphere,” said Quentin Brissaud, a seismologist at Caltech’s Seismological Laboratory and the Norwegian Seismic Array (NORSAR) in Oslo, Norway. “All kinds of environmental noise is detected; even the balloons themselves generate noise.”
The previously conducted tests involved detecting explosives on the ground just below tethered balloons and acoustic signals from seismic waves produced by a seismic hammer. The researchers then wondered if they could use this same technique to detect earthquakes. However, this presented a challenge because earthquakes weren’t guaranteed to strike while the balloons float.
On July 22nd, ground-based seismometers recorded a magnitude 4.2 aftershock approximately 50 miles away. After 32 seconds passed, a balloon detected a low-frequency vibration while rising to an altitude of three miles. Computer-model and simulation-based analysis revealed that this was the first time an earthquake was detected from a balloon mounted with an instrument floating in the sky.
“Because there is such a dense network of seismometer ground stations in Southern California, we were able to get the ‘ground truth’ as to timing of the quake and its location,” said Brissaud, the study’s lead author. “The wave we detected was strongly correlated with nearby ground stations, and when compared to modeled data, that convinced us – we had heard an earthquake.”
The team plans on flying the balloons over seismically active areas, allowing them to understand the infrasound signatures that occur from these events—integrating more barometers to one balloon and flying more than one would allow them to determine where an earthquake occurs without relying on ground-based stations.
The same technology could be used on Venus, floating over regions that should be seismically active based on satellite imaging. “If we drift over a hotspot, or what looks like a volcano from orbit, the balloon would be able to listen for acoustic clues to work out if it’s indeed acting like a terrestrial volcano,” said Krishnamoorthy, who was also the technical lead for the Ridgecrest balloon campaign. “In this way, balloons could provide the ground truth for satellite measurements.”
Read more about this effort at JPL.
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