21 April 2022–A device that could detect minute, transient fluctuations in gravity caused by an earthquake could be a useful tool in earthquake early warning systems, researchers said at the Seismological Society of America’s Annual Meeting.
The Superconducting Earthquake Early-warning Device (SEED), proposed by Ho Jung Paik of the University of Maryland and colleagues, would take advantage of the fact that these gravity signals travel at the speed of light.
Earthquake early warning systems like the U.S. Pacific Coast system called ShakeAlert, which uses seismic wave data to notify people about potential ground shaking in their area, can deliver warnings within tens of seconds. Adding devices like SEED to the system has the potential to speed up earthquake detection over a broader region, Paik said.
At the moment, regular broadband seismometers can detect gravity signals related to earthquakes about a minute after the rupture of a magnitude 8 or larger earthquake. The SEED device proposed by Paik and colleagues is still being designed, but the researchers hope it will allow them to detect gravity signals from large earthquakes within 5 to 10 seconds.
Shifting rocks around a fault or seismic waves that dilate and compress the Earth as they move through it are some of the reasons earthquakes produce gravity signals, explained Jean-Paul Ampuero of Université Côte d’Azur. “Any movement of mass will change the gravity field around it, and an earthquake does move mass around.”
The SEED instrument proposed by the researchers is based on research by Paik and colleagues to develop a superconducting gravity gradiometer (SGG) that was originally meant to travel on spacecraft in Earth or planetary orbits. The design consists of six magnetically levitated and cryogenically cooled cylinders that serve as accelerometers capable of measuring the tiny vibrations caused by a change in the gravitational field.
To be sensitive enough to be used for earthquake early warning, the researchers are scaling up the size of the original SGG design and will use a commercially available superconducting quantum interfering device or SQUID that can help reduce some of the extraneous “noise” in the system. “Right now our target sensitivity requires another two and half orders of magnitude of improvement [over the original],” Paik said.
In a 2018 study, Ampuero and his colleagues suggested that a device like SEED should be able to detect a magnitude 6.5 or larger earthquake at a distance of about 100 kilometers away within 5 to 10 seconds after the start of the earthquake.
With this kind of sensitivity, he said, SEED might also be useful for tracking water flow underground, or magma movement beneath volcanoes. “Anything having to do with things moving fast under our feet—these could be targets for these very sensitive instruments.”
SEED would not be portable, since it needs to operate in a cryogenic laboratory, but Paik said the devices could be installed at major university labs on the U.S. West Coast to provide complementary data to existing earthquake early warning systems.
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