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Sep 25 2013

Prelude to a Catastrophe: “The Unusual Character of the Seismic Activity Became Clear”

This is the trouble with beginnings: the beginning is often subtle, and unrecognizable at the time. It’s only in retrospect that we can go back, look at sequences of events until we find a place to stab a finger down and say, “Here. Here is where it began. This is the time, the place, the event.” Even then, it’s usually only a beginning. There are many places to put the finger, many events to choose.

It began with earthquakes.

Mount St. Helens from Spirit Lake. Skamania County, Washington. August, 1975. Image courtesy Donal Mullineaux, USGS.

Mount St. Helens had always been more seismically noisy than her siblings. Studies in the early 1970s found two types of earthquake: typically “volcanic” quakes high on the mountain, and classically tectonic quakes a few kilometers beneath to the northeast and southwest. Further studies revealed the earthquakes with “volcanic” signatures to be glaciers grating down the mountain. None of this was unusual for an ice-covered volcano in a tectonically feisty area.

The moving finger moves on through the 1970s, reaches March of 1980, hovers and stops. Here. March 16th.

It was just a small uptick in earthquake counts around the volcano. No one pointed to it that day, when the count went up to 13; or on the 17th, 18th or 19th, when the counts had reached 34 per day. Although that daily total nearly equaled the number of earthquakes located near Mount St. Helens for the entire previous five years, it was too early to cry eruption. Loggers were busy building roads: the recorded seismicity could merely be a result of their blasts. No cause for concern. No beginning, until in the midst of the action, prior records are scrutinized in light of certain knowledge, and the moving finger can point with confidence. Here.

There is another beginning.

Seismogram of moderate earthquake recorded on the afternoon of March 20 at station SHW. Image Courtesy USGS.

3:47pm Pacific Standard Time, March 20th, 1980. This earthquake is strong, shallow, unlike any of the others. It shakes the ground around the volcano hard enough for people to notice. In Seattle, Washington, a joint group from the University of Washington and the United States Geological Survey, studying seismicity around the mountain in search of geothermal resources, see their instruments record a magnitude 4.2. They don’t know what’s beginning, but they suspect. By the next day, they’re installing new seismometers on and around the mountain. Mount St. Helens is too dangerous to take chances. They need to know.

US Forest Service officials and the UW/USGS seismic research group geophysicists contact Dan Miller, Donal Mullineaux and Dwight Crandell at the USGS Volcanic Hazards Project to report the increase in earthquake activity and seek advice. The three of them had studied the volcano extensively and published a paper in 1978 that outlined her hazards and predicted she would erupt again, possibly as early as the end of the century. This large earthquake and the ongoing aftershocks could be an indication of eruptions to come. Experts in her behavior would be essential.

Seismogram, station SHW, March 21 to March 23, 1980. Frequent earthquakes are being recorded, and the counts are increasing. Image courtesy USGS.

On March 22nd, another magnitude 4 earthquake rattled the mountain, and by the 23rd, it was apparent that the seismometers weren’t recording an aftershock sequence. Earthquakes averaged 15 per hour, the rate was increasing, and though most of the magnitudes remained low – less than 2% reached magnitude 3 or higher – the sheer vigor of the activity made it clear to geologists that they were in the midst of an earthquake swarm unlike anything ever seen in the Cascade Range. And that swarm could be volcanic in origin.

Officials and scientists reacted fast once they identified the sequence as a swarm. As four earthquakes of magnitude 4 or higher, one reaching 4.7, rocked the mountain on March 24th, Pacific Power and Light workers lowered Swift Reservoir to accommodate possible mudflows predicted by Miller, Mullineaux and Crandell’s 1978 hazards paper. The seismology lab at the University of Washington moved to a 24-hour schedule, while seismologists and USFS officials advised the public that staying away from Mount St. Helens would be very wise indeed. Miller, Mullineaux and Crandell obtained data from the National Weather Service and NOAA’s Air Resource Laboratories that would allow them to plot out areas that could be affected by ash fall in the case of an eruption: these predictions would be updated daily. Hazard maps were generated and distributed, allowing officials to plan operations around the mountain during the next six days.

And a plane took to the skies to inspect the mountain for changes.

Oblique aerial photographs of the Mount St. Helens summit area showing historic thermal areas. Distance from False Summit to summit about 600 m. Viewed from southwest, thermal area B appears as a cluster of snow-free patches of ground. Photo by D. Frank. Skamania County, Washington. March 24, 1980. Portion of Figure 149-B, U.S. Geological Survey Professional paper 1250. Image courtesy USGS.

Nothing. No major changes other than a few avalanches, no deformation or fracturing. The historic thermal areas hadn’t heated up. Everything appeared quite normal, despite the rumblings below. The old girl was definitely up to something, but you’d never know it from her serene exterior.

The moving finger, flitting restlessly between events of increasing interest, stabs down on March 25th with a decisive thump. Here. Here’s where it begins getting dramatic.

Within a single hour, five earthquakes larger than magnitude four rock the volcano, and before the day is done, the total of magnitude 4+ quakes reaches 25. The poor seismic station on the western flank can’t keep up. It’s saturated with quakes by early afternoon. Little ones are drowned out by larger; the graph, usually full of discrete wobbles, is nearly solid black. As the FAA restricts airspace over the mountain, scientists and photographers flying over the summit during peak seismic activity see new fractures cracking glaciers; rockfalls and avalanches cascade down the slopes. Photographer Bud Kimball tells the USGS he’s spotted a considerable crack in the snow at the summit. Things are obviously getting intense.

An Emergency Coordination center is set up in Vancouver, WA, and the USFS, fearing avalanches could reach as far as Spirit Lake, close the information center there. They restrict access to the volcano above the treeline, and close several forest roads to keep the public out of harm’s way. Donal Mullineaux flies to Vancouver as intense earthquake activity continues through the night. Events have progressed so far that one of the three men who are most intimately familiar with St. Helens’s propensity for violence needs to be on-site to assess the situation.

Seismogram from stations SHW and CPW, for March 24 to 25, 1980. Earthquake activity has increased dramatically, saturating the records. Image courtesy USGS.

Mullineaux outlined her potential dangers in a March 26th meeting for government representatives and private industries affected by her antics. He explained the types of eruptions they might expect, and outlined what their result could be. His warnings prompted the USFS and county officials to close off areas of St. Helens beyond her flanks. As they worked to assess hazards and protect the public, earthquakes continued, including seven larger than magnitude 4. Those earthquakes were worryingly shallow: less than 5 kilometers (3 miles) beneath the volcano. Seismologist Stephen Malone and his team looked at the totals: 10 earthquakes of magnitude 2.6 or higher between March 20th and 11:00am PST, 174 between the 25th and the 26th. They would later state in a paper on their work, “We did not see how this activity could continue without something dramatic happening.”

Neither could any other geologist. With that exponential increase in earthquake activity, they take swift action. On the morning of March 27th, the USGS issues a Hazards Watch. The Army National Guard flies a reconnaissance mission over the volcano: at 11:20am PST, they spot a hole with a gray streak in the ice cap near the summit.

At 12:36pm, a loud boom echoes over the ridges and valleys near the volcano.

The moving finger stops. Here. Here is where Mount St. Helens proved all the scientists suspecting an eruption 100% correct.

Here is where her explosive phase begins.

Mount St. Helens’ eruption, first crater, from the north. USGS Photograph taken on March 27, 1980, by David Frank.

Previous: Prelude to a Catastrophe: “One of the Most Active and Most Explosive Volcanoes in the Cascade Range.”

Next: Prelude to a Catastrophe: “Something Dramatic.”

References:

Klimasauskas, E. and Topinka, L. (2000-2010): Mount St. Helens, Washington, Precursors to the May 18, 1980 Eruption. Cascades Volcano Observatory website, USGS (last accessed June 13th, 2012).

Korsec, M.A., Rigby, J.G., and Stoffel, K.L. (1980): The 1980 Eruption of Mount St. Helens, Washington. Department of Natural Resources Information Circular 71. (PDF)

Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.

 

Originally published at Scientific American/Rosetta Stones.

12 comments

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  1. 1
    rq

    Someone’s kicking at the seismograph again!
    More seriously, you build tension wonderfully. I’m re-reading this, and it still has me on edge and anticipatory. What was that about a book?

    1. 1.1
      petemoulton

      Yes, a book! I love your writing, Dana, and it’d be a great book. Pliny the Who?

    2. 1.2
      Lithified Detritus

      Look’s more like tap-dancing on it to me. ;-)

      Agreed – want book!

    3. 1.3
      Lithified Detritus

      Looks more like tap-dancing on it to me. ;-)

      Agreed – want book!

      1. Lithified Detritus

        Double post – my bad.

      2. rq

        Very small, rapid kicks… Definitely not soft-shoe. ;)

  2. 2
    embertine

    These are fantastic, Dana. The tension! I cannot bear it!

  3. 3
    Trebuchet

    I’m really enjoying reading these again. And I second the book idea.

    I suspect this is about the time I first heard the words “harmonic tremor”.

  4. 4
    M31

    Great writing, Dana! I love this.

    I have a (probably dumb) question, which is about how big Mt. St. Helens is. I tried to work it out from some maps and whatnot but I’m still not clear, and photos, since you can’t tell from how far away they’re taken aren’t exactly helpful. (and total height above sea level doesn’t help if I don’t know the level it starts at)

    So, in the first photo above, how far across is the visible portion of the mountain you can see?

    I have this feeling that it’s huger than I think, and I think it’s pretty huge.

    1. 4.1
      Al Dente

      The wikipedia article on Mt. St. Helens says:

      Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington. It stood out prominently from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone, earning it the nickname “Fuji-san of America”.[6] The peak rose more than 5,000 feet (1,500 m) above its base, where the lower flanks merge with adjacent ridges. The mountain is 6 miles (9.7 km) across at its base, which is at an altitude of 4,400 feet (1,300 m) on the northeastern side and 4,000 feet (1,200 m) elsewhere. At the pre-eruption tree line, the width of the cone was 4 miles (6.4 km).

      Incidentally, the mountain is named after Alleyne Fitzherbert, Lord St Helens, a friend of explorer George Vancouver.

  5. 5
    jane

    That picture of the first eruption crater on March 27 is awesome.

    Even though we all know the outcome, your writing makes it so exciting!

  6. 6
    hexidecima

    I remember this so clearly. all of the “nah, it won’t blow” and then it did.

    Dana, thought you might be interested in my visit to Watkins Glen state park in NY. much geological goodness: http://clubschadenfreude.wordpress.com/2013/09/27/what-the-boss-likes-finger-lakes-vacation-part-1-geology-food-and-drink/

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