Earthquake early warning detection is more effective for minor quakes than major ones. This is according to a new study from the United States Geological Survey.
Seismologists modelled ground shaking along California’s San Andreas Fault, where an earthquake of magnitude 6.5 or more is expected within 30 years.
They found that warning time could be increased for residents if they were willing to tolerate a number of “false alarms” for smaller events.
This would mean issuing alerts early in an earthquake’s lifespan, before its full magnitude is determined. Those living far from the epicentre would occasionally receive warnings for ground shaking they could not feel.
“Alternatively, we could warn you every time there was an earthquake that might produce weak ground shaking at your location… A lot of baby earthquakes don’t grow up to become big earthquakes,” she added.
Earthquake early warning systems have been in place in Mexico and Japan for years. Now, a system called ShakeAlert is being developed for the west coast of North America.
So how can it predict the unpredictable? In short, it can’t.
“It’s a misnomer… because it’s not earthquake early warning, the earthquake has already happened… It’s ground motion early warning,” said Dr Minson.
Earthquakes occur along tectonic plate boundaries and faults in the Earth’s crust – long fractures where friction has built up over time.
Dr Lucy Jones, a seismologist who was not involved in the study, explained: “The whole fault doesn’t move at once. It starts at an epicentre and the rupture moves down the fault like how you would rip a piece of paper.”
Life on the line
The physics of earthquakes is one of the reasons why a single, universal warning system hasn’t been rolled out across all quake prone countries.
California and Japan have populations living directly alongside fault lines, and cannot waste precious seconds before warning their citizens.
In both countries, the p-waves and some very rapid algorithms determine the potential magnitude and dispatch an alert.
But in Mexico, the capital city is about 300km from the nearest tectonic plate boundary.
This allows geologists to use a system that can take some more time to issue a warning. They wait to detect the s-waves.
Sirens blare in the streets of Mexico City whenever ground shaking above M5 is detected.
Better living through algorithms
However, the country’s usually robust SASMEX early warning system didn’t have sufficient time to respond to last September’s second earthquake.
The big one
Back in California, work continues on ShakeAlert.
It is due to have a limited public rollout later this year, supported by federal funding which was approved on Friday 23 March.
The system will use both ground-based seismometers and satellite observations to provide the maximum warning time to populations on the west coast.
Dr Jones agrees: “I do not believe that is a great shortcoming of early warning systems as long as it is understood… we need to invest in appropriate education for the general public about early warning for the information to be most effectively used.”