by: COURTESY USGS - This graphic from the U.S. Geological Survey, as of November 8th, shows the 110 earthquakes in the unusual swarm that began with a 7.8 shaker north of Vancouver Island on October 27 and continued every day after that - and it also shows the two earthquakes, one of which measured 6.3, that took place to the south on the same fault system, just west of Vancouver Island itself, on November 8.On Thursday, November 8, the news agency Reuters reported, “A strong earthquake off the coast of Guatemala killed at least 48 people and trapped others under rubble [on November 7th], crushing homes and cars, destroying roads, and forcing evacuations as far away as Mexico City.

“Most of the dead were buried under debris in San Marcos state, a mountainous region near the Mexican border. Landslides triggered by the 7.4-magnitude quake blocked highways, and complicated rescue efforts.

“It was the strongest earthquake to hit the Central American nation since 1976, when a 7.5-magnitude quake killed more than 20,000 people.”

This earthquake occurred on the subduction zone fault which encircles the Pacific Basin, resulting in the nickname “Ring of Fire”. To the north of Guatemala, this fault is known as the San Andreas fault in California; it goes offshore near Eureka, and parallels the Oregon coast some distance from the coast, as the Cascadia Fault Zone.

It is there, in the strip from the northern California coast to the tip of Vancouver Island in British Columbia, that every 300 to 900 years – as far back as geologists can determine – a giant subduction zone earthquake around magnitude 9 has caused enormous damage, augmented by a major tsunami washing ashore, along six hundred miles or more of the Northwest coastline.

The last one dropped the coast by some three feet, submerging coastal stands of forest and making big changes in the landscape. This megathrust earthquake happened at about 9 pm on January 26, 1700, and is estimated to have registered between magnitude 8.7 and 9.2.

Geologists famously determined the time and date by finding, in Japanese records, an “orphan tsunami” that struck that country without an accompanying earthquake. Turns out, it came from here.

Since that was 312 years ago, we are now officially in the danger period, although the variable intervals between past quakes of that type here have ranged up to 900 years. It could happen again tomorrow – or a centuries from now. But it will happen. You’ve heard all that before.

What you might not have heard about is that on October 27, at the far northern end of the affected zone of the last such quake, there was, at 6:04 pm, an earthquake significantly stronger than that Guatemalan earthquake described by Reuters.

It was a magnitude 7.8.

That doesn’t sound much more powerful, but on the scale in which each whole number equals a magnitude change of 30, that means this earthquake – centered on the offshore island just north of Vancouver Island – was 12 times stronger than the Guatemalan event. It didn’t make much news, because the area affected is lightly inhabited and apparently there were no deaths. But a 7.8 is a pretty major earthquake, and it was right on the Cascadia fault line in the area, not too far north of us.

That makes it newsworthy for those of us living in Oregon. It was big, and it happened on our fault. Does it foretell a bigger one soon? Or did it relieve some pressure, and thus delay the “big one”?

There was one other aspect of this quake in particular that was particularly unusual, and THE BEE sought a geologist to speculate about it, but we did not find one willing to do so by the time this issue of the paper went to press. And that’s not surprising, because geologists are scientists, and there is no scientific way at present to know the answer to these questions.

But what really caught our attention was what happened after that earthquake.

First, within a few minutes, there were two smaller but sizeble earthquakes in Asia, and a small one in Montana. An odd coincidence. And then the aftershocks began.

We have checked in at the U.S. Geological Survey worldwide website for earthquakes nearly every day for years. We have only seen one earthquake in all that time that made its fault “ring like a bell” with a massive number of aftershocks, over a significant period of time. That was the huge Japanese subduction quake in March of last year.

This October 27 earthquake was much smaller than that one, but it too produced a lengthy swarm of aftershocks in exactly the same way…some of them quite strong. Two of them measured 6.2 and 6.3.

By November 8th there had been 109 aftershocks right around the site of the original shaker, and two more had occurred on November 8 itself just west of Vancouver Island – one of those measured 6.3. (See the accompanying USGS illustration of that area.)

Again, nobody knows whether or not this unique activity suggests that the “big one” is now imminent, or now has been delayed – or, if its eventual timing has changed at all. But this swarm of aftershocks is certainly unusual.

And, it is a reminder that we are in danger of an absolutely huge earthquake which will keep shaking for perhaps as much as five straight minutes. Brick buildings and poorly built structures may collapse, and strong buildings in unstable areas may find the ground liquefying under them.

Most if not all of the Willamette River bridges may collapse or be unsafe to use, and at least one and maybe both of the Columbia River Interstate Five bridges may fall into the river. (That’s a major reason for the ongoing effort to replace them with something built to handle this sort of earthquake.)

Experts expect Interstate Five to be unusable for months because of overpasses collapsed onto the roadway, as well as broken pavement in the highway itself. The tsunami will devastate the coast, wipe out Highway 1, and kill anyone not able to get to high ground there in the minutes before the huge wave rolls ashore.

Immediate relief and supplies to Portland may rely entirely on air transport for a time – but only when the airport runways are repaired enough to accept aircraft. Helicopters may be the first to land there.

If you have followed the inconvenience, damage, and death that accompanied Hurricane Sandy in the New York and New Jersey area early in November, you have only a mild taste of what we will face. Are you ready?

Chances are you are not. Lay in batteries for flashlights and radios. Be aware that when the power is out, so are telephones connected to television cable service. Cellular phones are not much more useful; cell systems are not designed to accommodate the massive demand that they will be confronted with; and anyway, the cell sites also require electrical power to work, as do Internet services.

There are batteries at cell towers to provide temporary service in outages, but these outages could last weeks…or longer. Without a way of receiving gasoline, the gas stations will run out of gas…if they have the electricity to pump it.

Give some thought to preparations you can make that can make things easier for you and your family should this massive earthquake strike within your lifetime. You should have water stored where you can get at it if needed, since what water supply that remains after this pipe-bursting disruption may be contaminated, and we can’t live long without water. Tents and warm clothing would be useful if it occurs again in the winter, and if your home is too damaged to live in.

In the meantime, consider joining the NET Team in your neighborhood. These Neighborhood Emergency Teams are composed of volunteers who receive training from the Fire Bureau in rescue and assistance, and they organize to make a difference. Ham radio operators are often among these volunteers, since their radio equipment is often the only way word can get into and out of the stricken area on who survived and who didn’t, and what damage there is, and what needs there are.

If you are interested, check with your neighborhood association about joining your NET team.

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