Posts Tagged ‘Northwest earthquakes’

After starting this blog with the intent of describing primarily the Nisqually earthquake, it’s come to have the broader goal of chronicling a wide variety of sizable quakes in the Northwest through the decades. With that in mind, I thought some statistical descriptions of those earthquakes were in order. So, here is a chart showing the frequency of 4+ magnitude earthquakes with an epicenter in Washington or Oregon, from 1970 through 2010:

This is an alternate presentation of the above data, in the form of scattered dots. It’s interesting to see that the year-to-year variation seems to diminish when you look at dots rather than bars of color to represent the number of quakes:

You’ve probably noticed that both of the above charts omit 1980, the year of the St. Helens eruption. The reason is that the number of 4.0 plus quakes connected to the eruption dwarfs the cumulative number of quakes in all other years of these four decades. Here’s a chart that includes 1980:

By the way, according to the PNSN, the total number of Northwest quakes since 1872 with at least 4 magnitude is 519. Of those, the number associated with the St. Helens eruption in 1980 was 315, from March 24 to May 21. Also, the PNSN notes that “this list is incomplete prior to 1970. Since 1970, the list should be complete, and include all WA and OR earthquakes magnitude 4.0 and greater.” As of mid-2011, the number since May 21, 1980 was 79. There were 24 on St. Helens on May 18, 1980. 13 of the ones since 1872 had at least a 6 magnitude, and 3 had at least a 7 magnitude.

By way of comparison, here is a chart showing the number of 7 plus magnitude earthquakes happening around the world in each year from 1973 through 2010:

And, again, the same data in the form of scattered dots:

And a final comment: although the 22 7 plus quakes in 2010 is the greatest in any year since 1973, it is not extraordinarily greater than the 15 to 18 quakes that happened in many years of the past four decades. In the U.S., we tend to think of years that include devastating quakes in our country as being especially bad: and yet 1989, the year of the Loma Prieta quake, had just 7 large quakes globally, and 1994, the year of the Northridge quake, had just 13, about an average year.

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The Pacific Northwest Seismic Northwest has a vast archive of material on the history of earthquakes in the region, including both data and written reports on the intensity and damage caused by individual quakes.

On Saturday, December 14, in 1872, at the estimated time of 9:40 p.m., the North Cascades earthquake happened. It’s been estimated by the PNSN to be a 6.8 magnitude quake. Here, from the PNSN’s page on the earthquake, is a description by the Weekly Mountaineer of The Dalles, Oregon:

On Saturday evening last, at about half past 9 o’clock, this section of country was visited by a shock of earthquake, which, as far as we are able to learn, did little or no damage. The vibrations lasted probably thirty seconds, and seemed to be from the east to west. The sensation we felt was a very peculiar one and had a tendency in a moment to destroy the illusion and faith we have always had in the stability of the surface of the earth. Animals, especially cows, dogs, and swine, seemed to experience the disturbance, if we judge from the commotion they made at that time. This we believe is the second one that has ever been felt at The Dalles, the former being some six years ago and was quite light. It has been supposed by many that a calm, an oppressive heat and misty horizon are always the fore runners of this phenomenon; but, we are happy to says that in this instance these signs all failed, for we did not observe any thing peculiar, either in the air, or other wise, about that time. The cause of earthquakes is supposed to come directly from volcanic force. For instance, when a volcano is in active operation, or as we might term it, “letting off steam,” there is no danger of an earthquake; but should it quiet down for a length of time and be followed by a large fall of rain, terrible explosions and quaking of the earth may be expected. The wave of an earthquake is said to travel at the rate of several miles in a second, until it expends its force.

In November 1994, the Wenatchee World revisited the quake. A fellow named John McBride, who was living on the Wenatchee River a few miles upstream from its confluence with the Columbia, said “the third shock, which occurred about 11 o’clock p.m., was preceded by an explosion – apparently on the mountain – sounding like the discharge of several pieces of artillery simultaneously.”

The explosion was a landslide on a hill north of Entiat that formed Ribbon Cliff: the debris fell into the Columbia, damming the river for about a day. And back in 1920, Yakima pioneer Moses Splawn told Rufus Woods, publisher of the Wenatchee Daily World, that Indians showed him two cracks in the earth “up the hogback east of the Columbia” that formed after the quake. Splawn said, “Deep down in the earth every five minutes there was an explosion like the shot of a cannon, and out of the cracks in the earth a dark fluid was oozing which hardened as it ran down the mountainside and cooled. I took some of that dark material and had it analyzed and the analysis showed that it was oil.”

The Weekly Pacific Tribune in Olympia reported: “In one of the saloons a party of men were intently engaged in gambling. The first shock startled them, but did not break up the game. When the second shock was felt, they concluded they had a call outside, and left the table in such haste that they forgot to carry off the stakes, which remained on the table until their alarm had subsided.

“It is said that in one of the Masonic Lodges, in session at the time, a gentleman was being initiated. He did not suspect the cause, thinking the rumbling and shaking a part of the initiation ceremonies; but the members were so alarmed that they sought safety in flight, leaving the candidate alone in the hall.

“A number of ladies and gentlemen were in St. John’s church, rehearsing for the Old Folks’ Concert. Among their selections for the occasion is the familiar anthem, `Joy to the world! the Lord has come,’ which words they had just sung, we are told, when the earthquake was felt. Several of the company thought surely the event proclaimed in the anthem was then transpiring.”

A 1902 Post-Intelligencer story on this 1872 quake reported: “Lake Union was like a sheet of glass just before the disturbance. Just as it was over large rollers approaching a tidal wave, came in a number of feet above high water mark. There was also a disturbance of a like character on the waters of the bay. The tall fir trees that stood thick around Lake Union at that time swayed back and forth as if a heavy wind were blowing. Indians living on the lake shore near the writer’s home were terribly alarmed and rushed from their houses screaming in excitement.”

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When that earthquake hit, I was in middle school, in our sex-ed class. It was weird. I noticed that the pens in the pen holder on the dry-erase board on the wall were rattling a bit. I thought that was very odd, and so I jumped under my desk. No one else thought much of it until a couple seconds later when the floor started shaking, then everyone else was under their desks too. We all went out to the track and waited for our parents to come get us. I remember being kind of terrified, and extremely excited at the same time. Since then, I have had this sort of desire to become a geologist. I study geology as much as I can. Plate tectonics have completely entranced me. I have been checking the earthquake listings on the USGS website frequently, and I have noticed that the intensity and frequency of major earthquakes is growing. I’m intrigued by this. I kind of feel like something major is about to happen with the planet. Everything is changing – it’s beautiful, but it is something we need to prepare ourselves for.

I remember in my geology class, we watched a video about the Pacific Northwest being on a subduction zone, which naturally means we’re earthquake-prone. In this video, they said that once every 500 years or so, a HUGE earthquake occurs off the coast of Washington/Oregon. Like… a 10.0 sort of deal, triggering tsunamis of massive scale, that hit every coastline in the Pacific Ocean. They also said that this earthquake hasn’t come for several hundreds of years, so we’re due for a BIG one. Kind of exciting, eh?

By Rebecca

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