Scientists outline facts vs. fiction: 'Marianas Trench does not make Guam and CNMI tsunami-proof’

By Jayvee Vallejera 

Fresh from the July 30 experience of an 8.8-magnitude earthquake in Russia, which brought less than a foot of sea rise in Guam and the Northern Mariana Islands, meteorologists are redoubling efforts to make sure residents do not become jaded with tsunami warnings.

Meteorologist Landon Aydlett of the National Weather Service Guam, Laura Kong from the International Tsunami Information Center and physical oceanographer Marie Eble collaborated on a public service advisory that debunks six mistaken notions that both territories are tsunami-proof.

The advisory says several tsunami “myths” continue to circulate in the region, likely originating from many sources and widespread assumptions. However, the authors said, most of these are easily debunked with a quick look through the historical record.

One easily debunked myth is the idea that both Guam and the CNMI have never had a tsunami.

Citing a book on the tsunami history of Guam and an entry from The Tsunami Society, the authors said Guam has had three tsunamis—in 1849, 1892 and 1993—which caused damage at more than one location and an additional two to six other locally generated tsunamis in the last 200 years.

In 2011, a 9.1-magnitude earthquake in Tohoku, Japan generated a Pacific-wide tsunami.

Both Guam and Saipan experienced tsunami waves with amplitudes up to 2 feet in Saipan and 18 inches on Guam, both happening during low tides, the authors said.

The last one was in July, when an 8.8-magnitude earthquake hit Kamchatka, Russia. It triggered a Pacific-wide alert, and tsunami waves of more than 5 meters high struck near the quake’s epicenter. Tsunami waves were measured at both Guam (0.4 feet) and Saipan (0.7 feet).

Another common belief is that Guam and the CNMI have never been hit by large waves like those that hit Japan in 2011 and Indonesia in 2004.

The word “large” is subjective, the authors said, pointing out that any tsunami capable of reaching Guam has the potential to threaten lives, infrastructure and property. It will depend on the tide level, the strength of the earthquake itself, and other earthquake parameters that will determine the amount of energy that goes into the tsunami waves.

One of the most widely accepted myths is that the Mariana Trench protects Guam and the CNMI. As the deepest trench in the world, it will reportedly dampen and soften the tsunami wave’s energy.

That is not correct, the authors said. On the contrary, they said, the Mariana Trench is a local source for tsunamis.

While distant earthquakes could trigger tsunamis that may reach Guam and the CNMI within two to three hours, any tsunami triggered by the Marianas Trench will hit Guam and the CNMI within just 10 to 45 minutes.

“Earthquake-generated tsunamis in the western North Pacific could reach the shores of Guam and the CNMI within two to four hours (or less if in the Mariana Trench),” they said.

The three authors said both American Samoa and Samoa had the same belief that they were protected by the Tonga Trench (the world’s second deepest, behind the Mariana Trench), but that myth was shattered when an 8.1-magnitude earthquake hit in September 2009, followed by two severe aftershocks.

“These earthquakes generated a tsunami [that] reached more than 20 meters high in some areas, resulting in widespread destruction and killing nearly 200 people,” they said.

Both Guam and the CNMI also take pride in their offshore reefs that many believe make the two territories tsunami-proof. Yes, the authors said, fringing reefs could disperse wave energy, “but dissipation is not complete elimination.”

They pointed out that waves, whether from storm surge or tsunamis, are still able to penetrate reefs and reach the coast.

The authors cited a 2012 tsunami hazard assessment that seems to indicate that reefs offshore of Garapan in Saipan appear to dampen tsunami waves, but not for fringing reefs offshore of Rota, Tinian, or along other coastal areas of Saipan.

The authors also torpedoed the mistaken notion that all tsunamis are the same and affect coastal areas similarly. Many people have cited the lack of waves or impacts similar to what happened in Japan, Russia and Hawaii last July as proof of Guam and the CNMI being tsunami-proof.

The authors clarified that the direction of the waves and the distribution of their strength depend on the path of the tsunami waves, as well as the characteristics of the earthquake’s epicenter and the lay of the land under the sea.

“Tsunami impact can, and does, vary significantly along coastlines, even though [they are] near one another,” they added.

People must also learn that a tsunami is not just one wave, they said. Tsunamis are a series of waves. The first, and maybe the second, wave may not be the largest, they said.

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