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When a tsunami is barreling towards a coastline, the only way to stay safe is to flee to higher ground. But even when people are far enough away from the start of the tsunami to have hours of warning, no one really knows how big a tsunami will be or the damage it will inflict until it comes ashore.
Now, Chiang C. Mei at the Massachusetts Institute of Technology and Usama Kadri at Cardiff University hope to give people more warning time by detecting acoustic waves for earthquake-triggered tsunamis. Sound travels substantially quicker than the pressure wave of the tsunami currently used for warnings.
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Mei and Kadri calculate that high frequency sound waves can be detected far enough in advance to extend tsunami warnings, but Emile Okal at Northwestern University in Evanston, Illinois, cautions that their theoretical approach has limitations.
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“What I fear in a study like this is that you are measuring the wrong frequency,” says Okal. Both sound and tsunami are pressure waves. Mei and Kadri use the magnitude of high-frequency sound waves made by earthquakes to predict the distribution of resulting low-frequency tsunami waves. But it’s not a straightforward conversion because earthquakes aren’t so simple.
How a fault moves during an earthquake and in turn trigger a tsunami can be complicated. The fault may move faster in some places, have more displacement, or even unzip from one end to another. All of this could impact the high frequency sound waves produced, confusing Mei and Kadri’s model.
Good for small quakes
This is going to be valid only for reasonably small earthquakes,” says Okal. “When you get to ones that generate tsunami, the higher frequency of big earthquakes are going to be controlled by the [tsunami] source not being homogenous.”
The new model also doesn’t help with the other major limitation in current tsunami early warning: predicting wave height before it comes on shore. This, too, has its roots in the difficulty of quickly analyzing how a fault’s movement is displacing water to create a tsunami.
We often have a difficult time determining if a tsunami will be a devastating series of waves several metres tall, or a meagre few centimetres like the event that swept down the coast of western North America after an earthquake in Alaska last week. For now, the best response for coastal communities is to head to higher ground immediately if the ground starts shaking.
In the future, this model could be tested by modifying equipment already in place. The current network of early tsunami detection buoys – called Deep-ocean Assessment and Reporting of Tsunami, or DART buoys — could be used to detect any acoustic waves. Hydrophones, extremely sensitive underwater directional microphones used to monitor for violations of the nuclear test ban treaty, could also be put to use.
Journal reference: Journal of Fluid Mechanics, DOI: 10.1017/jfm.2017.811
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