Super-sensitive gravity sensors under development for Earth- and Mars-orbiting satellites may also be able to provide up to 30 seconds advance warning of large earthquakes, scientists say, even if those earthquakes occur under the ocean, where existing early warning systems can’t immediately detect them.
Traditional warning systems use seismometers located as close as possible to fault zones, where they can intercept seismic waves traveling toward more populated areas. But those waves travel at limited speed (usually 4–6 kilometres per second), and can’t be detected until they reach the nearest seismometers.
For offshore quakes, such as the truly enormous ones that can occur in marine trenches, that can waste tens of seconds, vastly reducing the possible warning time. In coastal communities, the most destructive part of these seismic waves may be practically upon them before the first such warning can possibly be issued.
The new system, in development at the University of Maryland, US, takes advantage of the fact that powerful vibrations from earthquakes don’t just shake the ground. They shift rocks around as they move – in the process creating minute changes in the Earth’s gravity field.
“Any movement of mass will change the gravity field around it, and an earthquake does move mass around,” says Jean-Paul Ampuero, a seismologist at the Université Côte d’Azur, France.
For more on earthquake warning systems read: Tracking earthquakes deep underground
These changes are tiny, but with the new hyper-sensitive instruments being developed for NASA spacecraft, Ampuero’s colleague Ho Jung Paik said last week in Bellevue, Washington, at a meeting of the Seismological Society of America, it’s possible to detect them almost instantly from a range of about 120 km, even if they are far offshore.
That’s enough to give up to 30 seconds’ extra warning. That may not sound like a lot, but most injuries occur when people are moving around when the shock first hits, Sara McBride, a social scientist with the US Geological Survey in Los Altos, California, said at the same meeting. Thirty seconds’ warning is plenty of time to drop to the floor, seek cover from falling objects, and brace yourself for what’s to come, she said.
The new instrument, still in development, is called SEED (Superconducting Earthquake Early-warning Device). It will use an array of magnetically levitated niobium cylinders, each weighing about 10 kilograms, separated by about one metre, and cooled to 4.2°K (-268.95°C).
By using currents flowing through superconducting coils surrounding these cylinders, Paik says, these cylinders can be used as accelerometers that can quickly detect minute changes created by the passage of gravity waves created by an earthquake. They can also detect the direction from which these gravitational waves originated.
To be clear, the instrument won’t be used on orbiting satellites. It’s simply derived from something Paik’s team is helping develop for NASA. Nor can it be deployed all over the globe. Not only is it expensive, but it requires a cryogenic lab capable of maintaining the 4.2°K temperatures that allow the superconducting coils to work properly.
That means that SEED, once fully developed, won’t replace traditional seismometer-based early warning systems. But it could supplement them, particularly in high-risk, heavily populated areas close enough to dangerous faults for the added warning to be worth the expense. In the US, Paik says, he’s looking to collaborate with four major universities: CalTech, UCLA, UC-Berkley, and the University of Washington, all of which have cryogenic laboratories that could easily use the instrument, and all of which are in population zones close to dangerous faults. After that – who knows?