Massive Earthquake Shakes Aleutian Islands

By Tom Yulsman | June 23, 2014 9:24 pm

Days earlier, the area around a remote Aleutian volcano was rattled by dozens of smaller quakes, prompting fears of a possible eruption

Aleutian Islands

The magnitude 7.9 earthquake that struck Alaska today occurred 15 miles southeast of Little Sitkin Island in the Aleutian chain. (Source: Google Earth)

 

Sixty-seven miles beneath the western Aleutian Islands, a portion of the massive Pacific tectonic plate was straining mightily today to plunge deeper into the Earth’s interior — along the arcing seafloor trench seen in the Google Earth image above.

And then, at a little before 1 p.m. local time, it suddenly wrenched free, triggering a magnitude 7.9 earthquake that shook the tiny island town of Adak about 200 miles to the East.

The temblor caused several minutes of “steady shake and roll,” said eyewitness Vince Tutiakoff Jr., quoted in the Alaska Daily News. It also prompted a tsunami warning, which has now been downgraded to a watch by the National Tsunami Warning Center. (But not before the National Weather Service in Anchorage reported that water was seen leaving Adak harbor — often a sign of an impending tsunami.)

Aleutian Islands

Little Sitkin Island and its volcano. (Source: Alaska Volcano Observatory)

The epicenter of the quake was located about 15 miles southeast of Little Sitkin Island, according to the U.S. Geological Survey. Click on the thumbnail image at right to see a photograph of the island and its snow-draped volcano taken by an astronaut aboard the International Space Station in October of 2002.

Little Sitkin and the other Aleutian Islands have been built by magma rising up from the mantle and spewing out to form a chain of volcanoes. This happens as the Pacific Plate moves northward and subducts beneath the North America plate.

As the Pacific Plate plunges into the Earth, temperature and pressure increase dramatically. This causes water that has been dragged down within the rocks and minerals of the descending plate to be released upward into the material above it. The water, in turn, reduces the melting point of the material, causing it to melt. This magma burns its way upward and spews out to create the Aleutian island chain. (For more detail, see this excellent explanation of subduction zone volcanism.)

Aleutian Islands

Source: Wikimedia Commons

All of this takes place in slow motion, of course. The Pacific Plate is “plunging” beneath the North American Plate at a rate of approximately 59 millimeters per year. That’s a little over two inches.

But it doesn’t happen smoothly. It only stands to reason that one massive slab of the Earth’s crust cannot glide easily beneath another. So the movement happens in fits and starts all along the island chain.

Today, a portion of the subducting slab broke free along a fault, triggering the earthquake.

Aleutian Islands

The Aleutian Islands arc for 1,800 miles across the northern Pacific Ocean. This is where the Pacific Plate subducts into the mantle beneath the North America Plate. Today’s earthquake (not indicated on this map) occurred not far from the 1965 magnitude 8.7 earthquake in the western part of the island chain. (Source: U.S. Geological Survey Earthquake Hazards Program.)

Because of the plate motions, this region is very seismically active, as the map above indicates. Since 1900, 26 events of magnitude 7 or greater have occurred within 250 kilometers of today’s quake. According to the USGS:

Notable events include an M 8.4 in 1906, M 8.7 in 1965, and an M 7.9 in 1996 . Unlike the June 23 event, many of these events occurred at shallower depths, likely along the plate boundary interface.

Interestingly, in a story headlined “Five Active Volcanoes Keeping Alaska Scientists Busy,” the Anchorage Daily News reported on June 13 that dozens of small earthquakes had been recorded by monitors in and around the Semisopochnoi volcano in the Western Aleutians. The earthquake swarms were a sign that magma was rising up.

But this does not mean that an eruption is imminent. In fact, according to the Anchorage Daily News story, earthquake swarms occurred in 2012 around Little Sitkin Island, and also at Mount Iliamna, but neither resulted in volcanic activity.

Even so, earthquake swarms often do presage eruptions — which is why the Alaska Volcano Observatory currently lists Semisopochnoi as being under a volcano alert advisory. This means it “is exhibiting signs of elevated unrest above known background level.”

Lastly, what’s the connection between the earthquake swarms and what happened near Little Sitkin Island today? The answer is simple: subduction. It’s happening across the sweeping arc of the Aleutians. That’s precisely why they’re there.

CATEGORIZED UNDER: Earth Science, Geology, select, Top Posts
  • disqus_atlq8Zmtsd

    I always feel like Tolkien’s tree ents and tectonic plates probably share a great kinship.

    • Buddy199

      What?

      • disqus_atlq8Zmtsd

        “All of this takes place in slow motion, of course. The Pacific Plate is “plunging” beneath the North American Plate at a rate of approximately 59 millimeters per year. That’s a little over two inches.”

        I pictured Treebeard sagely saying, “Anything worth saying………. Takes a long time to say.”

  • http://www.facebook.com/jane.carter.98434997 Jane Carter

    As a wine enthusiast, my travels have taken me to a wide variety of vineyards and wine areas of France and Italy, and of course to the wonderful gardens of the wineries and surrounding places. Such spectacular fountains, planters and statuary! Citrus and olive trees in beautiful planters, stone statuary in the midst of bubbling fountains, elaborate terra cotta creations…. Imagine my pleasure, then, when I walked into Authentic Provence in West Palm Beach, Florida. In a beautiful environment of running water and good smells, the owners have sourced one of the finest collections of European garden antiques that I have seen in the USA: statues, fountains, planters (note especially the classic Caisse de Versailles, and Anduze pottery), terra cotta shields, stone animals, copper pots, garden spouts, etc. They also have beautiful stone fireplaces, re-purposed tiles, and many other specialty items. They are available online at http://authenticprovence.com, and can arrange shipping anywhere in the US. Well worth a visit!

  • http://phusankhoa.com Bác sĩ Như Hoàn

    Looking good – and looking forward to be able to contribute more in a batch way. http://phusankhoa.com/

    • Buddy199

      Great, Thai porn. Why is this blog a fly strip for insane people and morons?

      • Tom Yulsman

        This will be deleted momentarily. AAARRGGGHHHH! I don’t want to do it, but I may just institute a policy of reviewing all comments before they go up.

        • disqus_atlq8Zmtsd

          I think that might only encourage me to seek out the most bizarrely relevant analogies possible just to mess with you.

          Granted comparing tree ents and tectonic plates without any context might be hard to top.

      • Tom Yulsman

        What is your theory Buddy?

        • Scott McCain

          I manage a group of over 6k people so I feel your pain. Depending on your traffic you may not want to have to review all comments. I know it would be a full time job for me. The best you can do is make a couple of checks a day to make sure you clean up. Most people will tolerate small amounts of SPAM for brief periods. It’s only when the SPAM becomes the majority or it sticks around when I get nasty grams from my users.

          • Tom Yulsman

            Thanks Scott. I think you are right. I didn’t start out by reviewing all traffic because I knew how time consuming it would be — and this blog is not my main job! So I don’t think I’ll start now. But I’m finding I have to delete more things than ever. (I’m also growing less and less tolerant of bullshit.)

        • Buddy199

          I’m at a loss, frankly. This sound like a job for Neuroskeptic!

  • Stephen Green

    Thank goodness I live here in New England, a very low chance of any siesmic activity..

    • Jerry Stuckle

      That’s what we thought here in the Washington, DC area until 2011.

  • http://blogs.discovermagazine.com John Kuipers

    Other than what you can see, there is no proof that they are correct, it’s a theory of what man thinks is happening, but the theory can’t be proven in real life, so people feel safe to make the assumption that the theory is correct.

    • mbkeefer

      When a hypothesis reaches the level of a theory, that means it is well supported by the available evidence and is the best explaination that fits the currently known facts.
      Your idea of a scientific theory, is probably a hypothesis, which is somebody’s best guest based on the knowledge they have on hand. The preceding sentence is my hypothesis based on your entry. Additional data may support of disprove my hypothesis.

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ImaGeo

ImaGeo is a visual blog focusing on the intersection of imagery, imagination and Earth. It focuses on spectacular visuals related to the science of our planet, with an emphasis (although not an exclusive one) on the unfolding Anthropocene Epoch.

About Tom Yulsman

Tom Yulsman is Director of the Center for Environmental Journalism and a Professor of Journalism at the University of Colorado, Boulder. He also continues to work as a science and environmental journalist with more than 30 years of experience producing content for major publications. His work has appeared in the New York Times, Washington Post, Audubon, Climate Central, Columbia Journalism Review, Discover, Nieman Reports, and many other publications. He has held a variety of editorial positions over the years, including a stint as editor-in-chief of Earth magazine. Yulsman has written one book: Origins: the Quest for Our Cosmic Roots, published by the Institute of Physics in 2003.

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