Giving new life to old data: 2010s seismic data reveal striking evidence of a hurricane’s landfall

Imagine you could observe a hurricane as it makes landfall by using the same methods as seismologists — by listening to waves rippling through the environment around you.

This is exactly what a team led by Dr. Qing Ji, a postdoctoral researcher at the UT Austin Jackson School of Geosciences, has done. In a Science paper released in August, Ji and his collaborators leveraged archival data from US Transportable Array seismoacoustic stations in Louisiana to detect the structure and passage of a landfalling hurricane.

One array, many uses

When the US Transportable Array project (TA) was operated from 2003 – 2021 to help image the interior of North America with seismometers, hurricane monitoring wasn’t on the list of scientific applications. The project was a massive effort to move broadband seismometers piecemeal across the United States and parts of Canada and Mexico. Like laying out strips of cloth for a quilt, portable seismometers were installed temporarily in moving rectangular arrays with 75-km spacing, providing dense coverage for high-resolution sensing. The goal was to measure earthquakes, and use the information gleaned from seismic energy to sculpt a subterranean image of the continent.

Mid-project, more capability was added to these stations — pressure, infrasound, and meteorological sensors were installed, helping stations measure phenomena from atmosphere to earth’s interior.

USArray deployments over time. (Credit: ANF)

Hurricane formation

It might seem far-fetched to use seismic stations to monitor hurricanes, an occurrence solidly in the realm of atmospheric science, but it’s quite ingenious. Hurricanes (or typhoons in the western Pacific) are partially the product of intense and sustained atmospheric pressure lows. These swirling monsters form in warm waters during the end of meteorological summer, when ocean temperatures are at their highest. As the water temperature surpasses 80 F, the air above begins to rise, creating a low pressure pocket underneath. This unlocks a runaway effect: air from above then rushes inward to fill this pocket, creating a vortex of moisture, wind, and energy.

The structure of a hurricane is also continuously shaped by pressure differences. As these furious storms barrel across the ocean toward land, their centers — referred to as “the eye of the storm” — remain the lowest pressure zone relative to their outer stormy rain bands. And this pressure difference can be detected by seismic equipment.

Waves are everywhere

That’s because the types of mechanical waves that are detected by seismometers — typically elastic waves — can also be caused by sound waves. Sound reaches our ears as a series of pressure waves, which stretch and squeeze the air molecules. This same stretching and squeezing exerts force on the earth when pressure waves meet the ground and propagate through, creating seismoacoustic waves. They can be detected using seismometers or infrasound sensors, depending on the frequency. Infrasound refers to very long-period sound waves that are below the threshold of human hearing. They’re generated by a variety of natural events, from earthquakes to severe, stormy weather — like hurricanes. In the case of a hurricane, low-energy pressure waves can be recorded as surface pressure fluctuations by infrasound sensors, while the force transferred by these pressure waves to the ground can be detected by seismometers.

A cross-section diagram of a hurricane. The hurricane is depicted as a spiral structure sliced in half to reveal white columns and rows of clouds to show the structure of thunderstorms, and the innermost column is noted as the "eyewall." A spiral red arrow aligned with the columns indicates direction of circulating warm & moist air that rises upwards out of the hurricane. An annotation for "low pressure" points to the center of the hurricane structure, which is called the "eye." Blue arrows fall inward to the eye and are noted as "descending air." The background is a deep sky blue. — A cross-sectional view of the structure of a hurricane and the processes that sustain it. (Credit: NOAA)

Listening to the environment

Ji’s team used data from TA stations along the Gulf Coast that recorded data during Hurricane Isaac’s landfall in August 2012. Isaac was a tropical storm that evolved into a Category 1 storm as it neared the coast. Several states — Texas, Louisiana, Mississippi, Alabama, and Florida — all experienced several inches of rain, hurricane-force winds, and ultimately just over three billion dollars of damage as a result of Isaac. While not the most destructive storm of the 2010s, it occurred at just the right moment for TA stations, equipped with seismometers, infrasound sensors, and meteorological monitors, to detect several strands of evidence that marked Isaac’s landfall.

Smooth moves

But what does the Transportable Array add to hurricane monitoring? Atmospheric scientists in the business of hurricane tracking rely on complex methods to measure the hurricane boundary layer — possibly the most important component for understanding how a hurricane will evolve. The HBL is the lowermost 1 – 2 km of the atmosphere where turbulence starts to influence a hurricane’s development. Hurricanes thrive when roughness from terrain is minimized, such as over the ocean surface, and will weaken substantially when terrain like land, buildings, and mountains are encountered. But warm waters and high convergence of air will sustain a hurricane — causing wind speeds to increase, the eye’s pressure to fall further, and ultimately the hurricane to intensify. To measure the HBL’s pressure and wind speed, either portable towers, radar, or aircraft must be flown into the storm to take readings. These methods introduce considerable problems, such as bodily harm to personnel tasked with piloting a stormfaring plane, to availability and longevity of monitoring instruments.

So with TA stations already in place and equipped with sensors applicable to hurricane monitoring, archival TA data presents a new, safer opportunity to complement these existing methods.

Landfall

In 2012, TA stations had been installed along the Gulf Coast and were reliably taking data. As Isaac approached the coast on August 29, station TA.645A began to record evidence of the hurricane as long-period pressure and broadband infrasound signals.

A multi-paneled image showing the location of hurricane Isaac and Transportable Array stations, a diagram of each station, and the data. Panel A shows a satellite image of the hurricane with it's path and development from tropical storm to hurricane marked. A dark blue line crossing from the lower right to upper left shows the path. Blue dots are steps where Isaac was graded as a tropical storm, while yellow dots are when it was upgraded to a category 1 hurricane. The yellow dots occur just offshore to the coast of Louisiana. Station TA.645A is marked in pink and the location is shown as a pink triangle overlapping the hurricane's path near the coast. Red annotations for the timing of hurricane progressions are included (8/28 is in the Gulf, 8/29 is just offshore, and 8/30 is interior Louisiana). Transportable Array stations are marked as orange triangles in a grid-shaped pattern over land. Legends are along the bottom. Panel B is an inset that shows plots the northing and easting in km of key monitoring stations, including TA.645A relative to the storm. Panel C is a contextual map of the US, the hurricane path, and a red square that shows the study area. Panel D is a conceptual diagram of station TA.645A's layout. A wind diffuser sits at the surface, weighed down by a cylindrical brown pumice rock bag. An inlet tube flows out the bottom and connects to a pressure sensor encased in a vault between two foam layers, 1.5m from the wind diffuser. The vault extends 2.1 m below the surface. Below the pressure sensor is a seismometer. Panel E shows the pressure and infrasound data that depict the passage of the eyewalls and eye in mbar and Pa, respectively. The infrasound data, which show 20 - 100 s period waves, show more noise coresponding to eyewall occurrence, and almost no noise in the presence of the eye. The low pressure center is marked at 968 mbar for atmospheric pressure. Panel F shows the seismic data for short-period and long-period waves. The short-period signals reflect microseismic activity. The long-period signals match the infrasound signals. A teleseismic earthquake is seen in both data and annotated in yellow. — Hurricane Isaac made landfall in Louisiana in late August 2012. It passed by several Transportable Array stations (panels A, B, and C). TA stations were equipped with sensors, shown in Panel D, such as a pressure sensor and seismometer, below the surface. Atmospheric pressure, infrasound, and seismic signals were successfully recorded by station TA.645A (panels E & F), clearly showing the passage of Isaac’s eyewall and eye nearby. (Credit: Ji et al., Science)

As Isaac inched closer to the coast near TA.645A, the infrasound sensor detected surface pressure fluctuations up to 40 Pa, a clear sign of the eyewall’s approach. The eyewall is the strongest part of a hurricane, where wind speeds are at their highest and heavy rain is fueled by towering thunderstorms. This is where the most damage upon landfall is typically accrued.

Isaac’s eye passed just 4 km away from TA.645A midday on August 29, and infrasound pressure dropped to 0 Pa. The station’s barometer recorded a low pressure zone of ~968 mbar, which roughly matched other empirical barometric readings of the eye, strongly indicating that the infrasound data accurately captured surface pressure changes caused by the eye’s passage.

TA stations, obviously, are equipped with seismometers. So what did the seismic readings indicate? Both short period and long period signals were detected, but the long period showed the clear fingerprints of Isaac. These signals indicated vertical ground displacements on the order of ± 15 microns as the eyewall passed near TA.645A — movement comparable to the width of human hair. Moreover, the data displayed high coherence between pressure and displacement. In other words, that the pressure fluctuations likely directly caused the ground displacement.

A multi-panel figure showing the surface pressure, vertical seismic displacement, and coherence between the two measurements. Panel A shows a colorful log PSD plot of surface pressure and annotates areas corresponding to the eyewall and the eye in red and blue, respectively. This falls to the center of the panel. Panel B shows vertical seismic displacement, also annotating eyewall and eye, and points out microseisms and teleseismic earthquakes. Panel C shows the coherence in dark red to yellow, where areas of yellow show coherence between the surface pressure and vertical seismic displacement. The area of yellow indicates that surface pressure and seismic displacement mirror one another. — TA station data resolves the passage of Hurricane Isaac. Distance from the station is shown as a white curve. In panel A, the surface pressure spectrogram is shown with colors denoting the logarithmic values of wavelet power spectral density (PSD). The height of these lines corresponds to noise intensity. Panel B shows the vertical seismic displacement, and shows distant seismic signals across the top, noted as secondary and primary microseisms. Panel C notes the coherence between the pressure and displacement signals, with values of 1 indicating that pressure and displacement follow one another. (Credit: Ji et al., Science)

The final leap

But the team could go one step further and use the infrasound pressure data to illuminate conditions within the hurricane boundary layer. As it turns out, infrasound pressure readings are correlated with independent measures of 10-meter wind speed, which in turn can help characterize turbulence. So where direct measurements or reanalysis products of wind speeds are sparse, missing, or of poor resolution, infrasound pressure fluctuations can act as a stand-in.

Altogether, the serendipitous location of seismoacoustic stations revealed the ability to capture hard-to-resolve characteristics of hurricanes. Ji’s team contends that placing these typically geophysical instruments at other scientific monitoring network stations with meteorological capabilities, or vice versa, can help a variety of earth scientists creatively measure phenomena that would otherwise be difficult to capture.

Two panel plot showing 10-meter wind speed and turbulent dissipation inferred from seismoacoustic data collected in this study. The top panel shows calculated 10-meter windspeed as a red line over the duration of the hurricane, with 10-m wind (m/s) on the y-axis and date along the x-axis. The data is compared to other measures and estimates for the event and aligns well with the trend of the data, which shows increased wind speed in the eyewall and a decrease in the eye. The bottom panel computes turbulent dissipation as discrete values across August 29 and 30, which correspond to the passage of the eyewall and eye near the station. The trend of the data follows the same as the wind speed during this time, with a distinct dip in values indicating presence of the eye. — Infrasound pressure data used to trace out 10-meter wind speed and turbulent dissipation. In panel A, infrasound pressure is denoted by the red line and scaled by the righthand axis. Compared to in situ stations nearby and a reanalysis weather product, ERA5, the pressure data follows the same trend and magnitude. In panel B, the conversion of movement of air to heat (turbulent dissipation), calculated from the infrasound pressure, also follows the same behavior. (Credit: Ji et al., Science)

While the Transportable Array is no longer active, data from the project remains archived by the NSF National Geophysical Facility, allowing researchers to return to data with new insights and new scientific questions. Infrasound sensors and seismometers are, of course, located at many geophysical network station sites to listen for tremors from earthquakes, tsunamis, and volcanoes, and this new work demonstrates their use to go beyond the traditional uses. But this does not mean traditional methods of hurricane monitoring — radar, eyewall flights, and portable radio towers — are now defunct. Rather, this presents the possibility of enhancing the available toolbox with networks that already exist.