Sensors designed to record earthquakes can reveal insights into a very different natural hazard – how hurricanes evolve and intensify as they strike land. The surprising connection has been made by researchers in the US, who examined data from seismic sensors installed in Louisiana for studying the interior of the Earth. They found that the sensors, which collected data on pressure fluctuations, provided information about how Hurricane Isaac grew and intensified as it struck and devastated the state in 2012.
Earthquakes and hurricanes are very different in how they form, move and impact the Earth. The parameters governing these natural hazards – and the way that we measure them – are also distinct given that one is a geophysical phenomenon and the other atmospheric.
Hurricanes, which form over warmer tropical and subtropical ocean waters, are rotating systems of cloud and thunderstorms. They consist of a calm, central “eye”, a surrounding layer of tall thunderstorms, and strong winds called the “eyewall”. Spiralling away from the eyewall are bands of clouds and thunderstorms.
Forecasters describe the structure and evolution of hurricanes using three main parameters: track, intensity and the radii of the wind layers around the eye. But when hurricanes strike land, particularly important is the turbulence in the lowermost layer. That’s where momentum, heat and moisture mix between the atmosphere and the ocean or land.
Eyeing up hurricane path and intensity
Measuring the turbulence in this boundary layer is critical both for forecasting wind intensity and for developing and validating weather models. Such data are usually collected using reconnaissance aircraft, ocean buoys and onshore towers and radar. But flying planes into hurricanes is dangerous, while onshore towers are sparse, making data-gathering tricky. Continuous monitoring of a storm’s passage is not easy either.
In the new study, Qing Ji, Ipshita Dey and Eric Dunham from Stanford University processed data from seismic stations in Louisiana that are part of a network of 1700 stations across the US. Equipped with both infrasound sensors and seismometers, these “seismoacoustic” stations are designed to record seismic activity.
The infrasound sensors measure pressure fluctuations at the Earth’s surface, with the seismometers recording the resulting elastic response. But when the hurricane passed through the equipment, the winds were strong enough to leave an additional signal. In fact, the Stanford team found that the data revealed the calm eye and the turbulent eyewall – as well as the circular layers of thunderstorms and winds as the hurricane passed through the sensors.
“The infrasound sensors provide direct measurements of turbulent pressure fluctuations,” says Dunham. “We also used the seismometer data together with data from other atmospheric sensors to calibrate a computer model of hurricane turbulence.” The model provided space-time correlations of pressure fluctuations around the station, while the simulated pressure spectra matched the recorded data at station.
The researchers then calculated the Earth’s elastic response using the simulated pressure field, which aligned with the observed vertical displacement. More importantly, the simulations showed that the relevant parameter for calculating the Earth’s elastic response to pressure fluctuations is the velocity at which those fluctuations are carried downstream — rather than the near-surface wind speed as had been assumed.
Exploiting existing sensor networks
Dunham told Physics World that he hopes the study will draw attention to the potential of seismoacoustic networks for atmospheric sciences. “Perhaps this will lead to deployments of seismometers and infrasound sensors in areas where coverage is currently sparse, like in the southeastern US,” he adds.
Those hopes are echoed by Ji – now based at the University of Texas, Austin – who says there is a growing interest in the use of these “noise” signals for environmental monitoring. As Ji points out, the beauty of seismic stations is that they record data continuously and have a high sampling rate compared to other sensors.
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Seismoacoustic stations have already been used to study various environmental and climatic phenomena, such as rising amounts of energy in ocean waves as waters warm and the evolution of tropical storms and accompanying winds. “The atmospheric imprints, however, are less studied partly due to the overall smaller amplitudes and the potentially more complicated nature,” Ji adds.
Dunham also sees an opportunity to add infrasound sensors and barometers, both of which measure atmospheric pressure, to existing seismometers. “Having multiple sensors at a single location opens up new opportunities for studying both the solid Earth and atmosphere,” he says.
The researchers report their findings in Science.
