People living in California will get more timely and accurate alerts about mega earthquakes with a new upgrade to the state's early warning system.
The U.S. Geological Survey (USGS) and EarthScope have started using GPS sensors in the ShakeAlert system, they announced on Wednesday. Up until this change, ShakeAlert has relied mainly on seismometers, which measure the ground's movement caused by seismic waves.
However, during the biggest and most destructive earthquakes, seismometers near the epicenter can saturate. EarthScope's Science Communication Associate Scott Johnson said that this was "like a highly sensitive microphone recording a very loud sound." This, in effect, has the potential to delay accurate information about earthquakes and the danger they pose.
However, GPS sensors do not saturate during big earthquakes, meaning they can provide an "immediate indication of the earthquake's magnitude," Johnson said.
This is particularly significant for anyone who lives in areas that would be affected by "Big Ones" on the San Andreas Fault, one of the most-active fault lines in the world.
The hope is that people will be told, in decent time, when earthquake threats are thought to be really dangerous, helping them to react appropriately and quickly.
For example, in 2019, when a 7.1-magnitude earthquake hit Ridgecrest, California, ShakeAlert reported a magnitude of 6.3 within 22 seconds.
A study carried out by the Seismological Society of America in 2020 found that GPS data would have reported a magnitude of 6.9 within 13 seconds.
A well-known case of underestimation took place in Japan in 2011, when an initial report of the earthquake's magnitude was put at 7.9 before it ended up reaching a magnitude of 9.1.
This led to a further underestimation of tsunami heights, with some of the first alerts predicting a wave lower than sea walls before communication was cut off. Around 18,000 people ended up dying in the disaster. Japan has since begun incorporating GPS sensors in its alert systems.
The older, seismometer-reliant system will still work "perfectly fine" for smaller earthquakes around California, Robert de Groot, operations team leader of the USGS' ShakeAlert System, told the Los Angeles Times.
The Network of the Americas (NOTA), an international geophysics sensor network that spans more than 20 countries, includes over 1,200 continuously operating GPS stations.
EarthScope maintains these stations and will use them "to deliver GPS data to the ShakeAlert system in real-time."
EarthScope said: "With that data stream now contributing to ShakeAlert, the system is even more capable of fulfilling its purpose—providing valuable seconds for protective actions that can help keep people safe."
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