All shook up

The mainland, looking east from Quadra Island

The mainland, looking east from Quadra Island

Living on the west coast is marvelous. The weather is polite, the scenery spectacular, the people friendly.

What we forget is that we live in an active fault zone which has generated the most powerful quake recorded in modern times. Alaska experienced a 9.3+ quake in 1964. The geological record shows similar massive ruptures in the past as well. In the early 17th century, Japan was hit by an enormous “orphan” tsunami (no local earthquake before hand). The source has been found to be off northern Vancouver Island in what is known as the Cascadia subduction zone.

The Cascadia subduction zone is both large and complex, covering an area from the northern US coastal states to Alaska and from the coastal mountains in the east; well out into the Pacific Ocean. Mount St. Helens and Mount Baker rise in this zone and Vancouver Island comprises a large piece of it.

Earthquake theory predicts periods of gradual strain building along a fault until the pressure is released in an earthquake. What has been found during recent observations in the Cascadia subduction zone (hereafter called the CSZ) has been quite different. One of the most active segments of the CSZ is about 80 kilometers north-west of the tip of Vancouver Island. Here, small quake “swarms” occur every eighteen months or so, with values on the Richter scale rarely exceeding 5.0. It might be expected that these swarms would release strain along the fault and while this seems partially true, observations show strain drops only marginally and then begins to build again, rising fairly rapidly to levels above those found before the swarm. Core samples and coastal sediments indicate the last major quake in the central CSZ was over four hundred years ago, so we are well past the “normal” release period of a couple of hundred years between major quakes.

By comparison, the San Andreas fault, further south, ruptures fairly regularly with quakes occasionally over 8.0 releasing strain where those quakes occur. What seems (in my opinion) to be different in the CSZ is that while the San Andreas is fairly straight and the boundary between the Pacific plate and the continental plate fairly well defined, the CSZ has a large chunk of continental plate which is not North American interfering with subduction of the Pacific plate. That chunk drifted across the Pacific before piling into North America millions of years ago and is now known as Vancouver Island.

So what does this mean for those of us who live in the CSZ (and those around the Pacific Ocean who would be affected by a tsunami generated from a major quake)? Seattle, Tacoma, Portland, Vancouver and Victoria are major cities which have never experienced a major quake and if a quake of the expected possible magnitude were to occur, it could eclipse the largest in California by a factor of ten times or more. It remains to be seen what might be left standing if, no WHEN, this happens.

All we can really do is continue (albeit with more urgency) to retrofit old buildings to meet current earthquake standards, build to more stringent standards now and in the future, become intimately familiar with the tsunami warning system and personally prepare for significant disruptions in supplies and transportation at some time between this moment and the not too distant future.

We do live in interesting times


One Response to “All shook up”

  1. ceo Says:

    this is cheating. Your blog site automatically moved your last entry ( sept 21st) forward and, and I say, did not carry forward the “comments” I know, ’cause I left one last month.

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