How effective are earthquake early warning systems?

BBC | Published: March 29, 2018 13:51:22 | Updated: April 01, 2018 14:47:17

The logo of SkyAlert earthquake alert application is displayed on a computer screen in this illustration photo - Reuters/File

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.

"We can get [greater] warning times for weak ground motion levels, but we can't get long warning times for strong shaking," Sarah Minson, lead author of the study, told BBC News.

"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."

This release of energy moves in two parts.

Primary waves (p-waves) accordion out first and fastest.

They are followed by slower secondary, or s-waves, which cause the ground to ripple up and down - the shaking that people experience during a quake.

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