Tsunamis can keep pummelling the same patch of coast for days at a time if conditions are right. The finding means the initial big wave may not be the only source of danger, and authorities might need to rethink their evacuation responses following a tsunami alert.
On 8 September 2017 a deadly struck Mexico – the first of two major tremors in the area that month. The quake caused a tsunami in the Pacific Ocean. It reached a maximum height of three metres when it hit the Mexican coast – relatively modest for tsunami standards.
However, the tsunami was unusually long-lived. Local tide gauges show that weaker tsunami waves kept lashing the coast for three days after the initial wave.
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Long-lasting tsunamis have been seen before. Waves from the 2011 Tohoku tsunami that devastated parts of Japan , but that tsunami was caused by a bigger earthquake and the initial wave reached run-up heights of 30m. The Mexico tsunami’s persistence is surprising. “It’s 100 times smaller in terms of energy,” says Diego Melgar at the University of Oregon in Eugene – which makes it odd that its effects lasted almost as long as the effects of the Tohoku tsunami.
To work out why the tsunami had such staying power, Melgar and Angel Ruiz-Angulo of the National Autonomous University of Mexico created a computer model of the tsunami.
Echoing waves
The key factor seems to be that the tsunami happened not far offshore and within the Tehuantepec continental shelf. This region of the ocean has a very flat and shallow seafloor that stretches over 100 kilometres offshore. A tsunami trapped in one of those shallow shelves “just knocks about for a long time”, says Melgar. It reflects off the shore and bounces out to sea – and then, when it hits the large body of very deep water at the edge of the continental shelf it reflects back towards the coast. “It reverberates for a very long time,” like a sound echoing in a concert hall with excellent acoustics.
“Our concern is what’s going to happen when this part of the coast gets a bigger earthquake or a more tsunamigenic earthquake,” says Melgar. A really big tsunami there might reach 15 metres in height. “After that first big wave it’s possible to then see four to seven later arrivals, which won’t necessarily be as big as the first one, but they might be 5-7 metres – and this could go on for 12 hours, 18 hours, even a day.”
To protect people from these secondary waves, evacuations in the area need to last longer, Melgar says. “You will likely have to evacuate the low-lying areas for at least a day, if not longer. That obviously necessitates more planning by the civil authorities. You need to house people for at least a day, to have water and supplies and evacuation routes.”
Similar shallow continental shelves are found elsewhere, and might trap tsunamis in the same way. They include the Pacific coasts of Guatemala and Nicaragua, and on the Caribbean side along the coasts of Guatemala and Honduras. Alaska and northern Japan also have shallow continental shelves. “Fortunately these things are not the norm,” says Melgar.
Tsunami researchers have tended to focus on the threat from the first, biggest wave, says Melgar. “Of course, that’s important. But now there seems to be this secondary hazard that comes right behind it.”
Geophysical Research Letters
