Tsunami awareness

The anniversary of the horrific 2011 Tohuku tsunami reminds us that something similar could occur here. What are the odds that it might?

  • Mar. 19, 2013 8:00 a.m.

The anniversary of the horrific 2011 Tohuku tsunami reminds us that something similar could occur here. What are the odds that it might?

Around the Pacific margins some off-shore areas have deep tectonically active trenches. These are the source of most tsunamis. Up-faulting produces mega-swells at the surface which may only be a few feet high, but may be many miles wide. These swells, in turn, race harmlessly across the ocean until they reach shallow water, where they act like a regular wave, but on an enormous scale.

There is no active trench off the B.C. coast in the present day, but there are abundant active faults on the continental shelf. These are part of a fault system which extends up the West Coast, and in which movements are lengthwise rather than vertical.

These factors might account for an apparent lack of tsunamis developing locally. There seems to be no cases since written records were kept. Areas with active trenches, such as Chile and Japan, have both minor tsunamis on occasion and major ones several times a century.

Mega-swells can travel for thousands of miles. The huge 1964 Chilean earthquake caused tsunami damage  and deaths over different areas of the Pacific. For example, eleven people died in Japan. Here in B.C. the event was marked by a relatively innocuous four foot swell.

The 1964 Alaskan quake at the eastern end of the Aleutian Trench generated tsunamis which killed over 100 people locally. Disturbances ran down the coast as far as California, with 12 deaths and severe damage at Crescent City. In the area between, damage was limited to areas  with poor geographic defences. In B.C., the swells measured at about four feet, and damage was largely limited to Port Alberni where a swell ran up the long narrow inlet and crested at the shallow end.

Crescent City is flat-lying and open to the ocean, much like Tohoku, and the city also suffered considerable damage from the Tohoku quake. The swells from it were barely measurable here.

A realist has to admit that for the last 150 or so years tsunamis have not been much of a problem here, and an optomist would hope that this would continue to be the case.

Mother Nature, of course, always has the last word.

Jim Drummond

Qualicum Beach

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