Earthquake Sensors Can Hear a Hurricane’s Howl

Hurricanes are so turbulent that the crewed planes that penetrate them to collect data can plummet hundreds of meters with no warning. “Just one downdraft, and then everything went flying,” says Jun Zhang, a hurricane scientist at the University of Miami who flies into the storms with the National Oceanic and Atmospheric Administration (NOAA). He’s recalling a hurricane hunting flight in 2021 that suddenly plunged about 500 meters. (A typical patch of bad turbulence on a commercial flight won’t drop you more than 30 meters.)

Such aggressive storms literally shake the atmosphere to the point where earthquake sensors can record their signals. That got scientists at Stanford University thinking. Now, research published earlier this month in Science says seismic sensors reveal more detailed data about hurricanes than previously thought. In particular, they uncover information about a hard-to-study part of the hurricane called the boundary layer, which is critical for knowing if a hurricane will intensify.

The hurricane boundary layer is the lowest part of the storm’s atmosphere, typically reaching 1–2 kilometers above Earth’s surface. On one hurricane hunting flight in 2003, Zhang’s advisor flew into the layer, and the plane dove so low that the salty air took out one of the plane’s four engines.

“After that, we won’t really fly into the boundary layer,” says Zhang, who was not involved in the new study. He notes NOAA scientists still use drones, uncrewed surface vehicles, and parachuted sensors to study this chaotic zone.

The new research could mean one more tool for scientists like Zhang. The Stanford researchers parsed through data from earthquake sensors near the Louisiana coast that happened to be in the path of a 2012 hurricane. These sensors were seismoacoustic, picking up mechanical vibrations in addition to infrasound, which is inaudible to humans. Events like volcanic eruptions emit strong infrasound signals, and the researchers observed that turbulence from the hurricane also emitted such signals. They say this acoustic data could be used to improve knowledge of how the hurricane boundary layer works.

“The boundary layer is really important not just for forecasting in models… but it’s also the most underobserved part of the hurricane,” says atmospheric scientist Ipshita Dey, who worked on the project as a student at Stanford and is now at the National Centre for Atmospheric Science in the United Kingdom.

How seismic sensors can hear a hurricane

When the 2012 storm, called Hurricane Isaac, made landfall over the network of Louisiana sensors, the wind strength was classified as only a Category 1 out of the maximum Category 5. But that doesn’t mean the turbulence wasn’t intense. For what the researchers needed, it “was a really strong event,” says Qing Ji, who also worked on the project as a student at Stanford and is now a postdoctoral researcher at the University of Texas at Austin.

Hurricanes generate infrasound because of their turbulent winds. The chaotic air flows create pressure fluctuations that sweep across the ground, and seismoacoustic sensors pick up the cacophony.

At first, scientists thought that the Louisiana sensors would record a messy image of all the pressure fluctuations in a hurricane. After all, these storms are hundreds of kilometers wide, and as pressure changes throughout their layers, the acoustic waves push on the ground and create signals that might overlap. But the sensors actually picked up “very local” signals, says Stanford geophysics professor Eric Dunham, measuring pressure fluctuations only tens to hundreds of meters across.

“It really shifted our thinking from this very large scale, the whole hurricane scale, down to this few-kilometers-and-less scale, so then we focused on turbulence within that,” Dunham says.

Seismic signals could help existing atmospheric tools

Because crewed flights into the hurricane boundary layer are dangerous, the layer is commonly studied with sensors parachuted from planes from higher in the storm, remotely operated drones, or surface-level tools like buoys and wind measurement towers. Each has their pros and cons: the airborne tools can’t provide continuous measurements, and the surface tools are mainly picking up on changes very low in the atmosphere and often record only intermittently. To directly observe turbulence, measurements must be frequent, limiting scientists’ options.

Zhang says that while seismoacoustic sensors wouldn’t replace the existing tools his team uses, they could provide a helpful complement for the current methods of collecting data.

“If these relationships they presented in the paper prove robust across many hurricanes… those observations will be useful for model evaluation and physics improvement,” Zhang says. He adds that, for safety reasons, airborne observations of hurricanes over land are challenging, so seismoacoustic sensors have the potential to supply unique data once a hurricane makes landfall.

The researchers echo that the next step of their work is to study the same phenomenon in more storms to ensure that seismoacoustic signatures actually align with turbulence pressure signatures from the atmosphere. They also think that studying non-hurricane signals can help them answer that question.

“In the United States many seismic stations also have pressure sensors,” Ji notes. “I think one of our opportunities is to extend to not only hurricanes but also the general atmospheric condition, because [these seismic stations] are always working.”