Using GPS (global positioning system) data to measure how points on land move following an undersea earthquake could help geologists decide if the tremor will cause an ocean-wide tsunami. Combined with existing tsunami warning systems, the data could speed up decisions about whether to issue an alert and avoid false alarms, say US researchers.
Tsunami warning systems use a combination of seismometers to measure tremors and ocean buoys to spot pressure waves. But it is difficult to pinpoint the exact strength of an undersea quake. This can cause problems for those deciding whether to issue a warning, says Seth Stein, a geophysicist at Northwestern University in Illinois, US.
“The hardest job is to distinguish quakes that are big from those that are dangerously big,” he told New Scientist. “Richter scale 8 is quite a big earthquake, but about 8.5 is the magic number. Above that, ocean-wide tsunamis start to happen.”
GPS measurements of points around a quake could determine more quickly than current methods whether this threshold has been exceeded, he says.
The 2004 Sumatran quake – which caused the Asian tsunami – was eventually measured at between 9.2 and 9.3, but seismometers can initially only determine whether a quake is larger than about 7.
“It normally takes a couple of hours to know whether it was over 8.5 or not,” says Stein. “Limits on how much energy can be stored in rock mean the first body waves of a quake don’t get bigger, but just ring for longer.”
Taking GPS measurements of points on land around a quake can answer this crucial question within 15 minutes, according to a study by Stein and co-workers from the University of Nevada, US.
To prove the technique can work, they used GPS data recorded during the first 15 minutes of the 2004 Sumatran quake, which made it clear the tremor would go on to cause a devastating tsunami in the Indian Ocean.
Software developed at NASA’s Jet Propulsion Laboratory was used to measure the position of 38 GPS stations between 300 and 7500 kilometres from the quake’s epicentre northwest of Sumatra. Knowing how these stations moved to within 7 millimetres makes it possible to measure long-frequency waves from the quake and estimate its size.
“Using that 15 minutes of data it was possible to say the quake was a 9.” This was very close to the 9.2 or 9.3 figure eventually determined for the quake, says Stein. “We think this would be a useful third component to the existing tsunami warning system. By taking out the guesswork it could make it more accurate and avoid false alarms.”
Geophysicist Paul Burton at the University of East Anglia, UK, agrees, but cautions that the system’s effectiveness will depend on the location of the earthquake. “It’s feasible but might not help in all circumstances.”
If the GPS stations available are not located in the right place relative to the epicentre, satellite measurements may not be that helpful, he says.
“Another consideration is whether high-tech systems for tsunami warnings will be here in three to four hundred years time,” says Burton. “Educating people right now and making sure the knowledge about what to do is kept alive can make a huge difference. For example, a 10-year-old British girl saved many lives during the Indian Ocean tsunami when she remembered the early signs of a tsunami from her geography lessons.”
Geophysical Research Letters (DOI: 10.1029/2006GL026145)