Thursday, January 27, 2005

Tsunami threats in North America:

Excerpts from a recent Press Release on expert witness testimony before the House Committee on Science:

"Brigadier General David L. Johnson (ret.), Assistant Administrator of the National Ocean and Atmospheric Administration (NOAA) and Director of the National Weather Service (NWS), said that between 1900 and 2004, 923 tsunamis struck the Pacific Ocean, 120 of which caused casualties and damage. 'Furthermore, there was no single year during this period that was free of tsunamis,' he said.

Dr. Charles 'Chip' Groat, Director of the U.S. Geological Survey (USGS), told the Committee that there is a 10-14 percent chance that the Oregon coast will be hit by a tsunami comparable in size to the one that recently hit South Asia within the next 50 years."

While the current focus on tsunami threats is mostly in the Pacific because of the amazingly high frequency of tsunamis there (923 tsunamis in 104 years!), the threat to the Atlantic coast is just as real - although less frequent. Most of this difference in tsunami frequency has to do with how plate tectonics controls what happens at the edges of the two ocean basins.

Here's why: first, remember that the total surface area of the Earth does not change. This means that when seafloor is created somewhere (for example, at a mid-ocean ridge), then an equivalent area of surface has to be destroyed elsewhere. Surface is in fact destroyed in subduction zones (see the post below on the Asian tsunami for a diagram of subduction).

The Atlantic has relatively gentle tectonic activity on the mid-Atlantic Ridge where the seafloor is spreading, pushing the Americas away from Europe and Africa. The Pacific, on the other hand, is bounded by subduction zones, where a lot of seafloor is being destroyed. Subduction is a generally more violent process that generates the great earthquakes - 1960 Chile, Alaska 1964. Aside: subduction also creates lots of volcanoes, and this leads to the "Ring of Fire" that has been so often mentioned, and confusingly associated with tsunamis.

So - one can expect that each ocean's tsunami frequency from subduction earthquakes varies with the amount of subduction going on at the boundaries of that ocean. And that is the case: There are more of these types of tsunamis in the Pacific Ocean, less in the Indian, and even fewer in the Atlantic.

However, there are other sources for tsunamis. You could think of subduction-generated tsunamis as what happens when you tip a tray of water - but you could also dump something into the tray and cause a wave, set off a firecracker underwater, or hurl a ball into the tray. Many of you have probably seen video of the glacier disintegrating in a Greenland harbour that causes a massive wave, or the calving glacier that causes a wave that drowns several unfortunate spectators. Landslides fall into this category of sources, and in fact a landslide was the source of the highest tsunami wave witnessed, about 500 meters (!), on July 10 1958 in Lituya Bay, Alaska.

Landslide-generated tsunamis are the main threat in the Atlantic. There are several places being looked at as possible sources - the main one being the Cumbre Vieja volcano in the Canary Islands. It is quite common for volcanoes to split off large parts of their flanks, which then slide off, either very gradually, or very suddenly. If Cumbre were to let go suddenly, models indicate that a 10 to 25 meter high wave would reach the Eastern Coast of North America within a few hours. So much for Miami, Charleston, Washington, New York, Boston, Halifax, etc. etc.

The other landslide threat is the large pile of sediments that build up at the mouths of rivers, or at the edge of the continental shelf. There are large sediment piles long most of the East coast of the US, built up by the erosion of the Appalachian mountains - sonar surveys over the deeps just outboard of the shelf show several places where enormous landslides have spread out many miles onto the seafloor. The Albemarle-Currituck slide is thought to have occurred about 18,000 years ago, and almost certainly caused a devastating tsunami along much of the mid-Atlantic coast. The tsunami waves probably carried several miles inland, and certainly penetrated along the larger rivers and bays of the region.

Another source for tsunamis is an underwater explosion - the firecracker I referred to above. Just about everybody has heard about the August 1883 Krakatoa eruption, which generated a considerable wave in the Sunda Strait. The closest large town, Merak, faced a catastrophic 35 meter high wave. There is not much threat for this type of eruption for the volcanic islands in the Atlantic off the African coast. Wrong type of volcano.

The last threat (and this applies equally to all oceans) is from an asteroid impact. If something large does a belly-flop into the ocean, it causes a large wave. I have addressed the threats to Earth from these types of collisions recently, and they are minuscule.

The source of the next East-coast tsunami will be either a volcanic collapse or a submarine landslide. When? Maybe tomorrow. Maybe several thousand years from now. But it will occur someday.

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