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The 1964 Great Alaska Earthquake, Good Friday, March 27th

  1. Mar 27, 2016 #1


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    The matter of large tsunamis come up in another thread on thermonuclear explosions. Very large earthquakes Mag 8 and greater, produce energies well above the largest manmade thermonuclear detonation. They can also create large tsunamis. The 1964 earthquake produced large tsunamis in Alaska and the northern Pacific. I thought we had discussed this in a thread here, but I can't find such a discussion.

    1 Mag 9 earthquake ~ 900 Mag 7 earthquakes

    USGS has a number of articles on the 1964 Great Alaska earthquake.
    http://earthquake.usgs.gov/earthquakes/states/events/1964_03_28.php [Broken]

    There was a large tsunami in Lituya Bay, Alaska in 1958. A large landslide at the head of the bay caused an immense tsunami in the bay.

    Such large earthquakes could happen in the future since the subduction of the Pacific Plate at the Aleutian Trench is ongoing.
    Last edited by a moderator: May 7, 2017
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  3. Mar 27, 2016 #2


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    Is there any way to predict earthquakes ahead of time aside from just arbitrarily knowing that it "could happen in the future" that scientists have successfully tried? By the time seismographs have detected vibrations, it's already to late to do almost anything.
  4. Mar 27, 2016 #3


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    That is an ultimate goal in geophysics and seismology. There are numerous monitoring sites, which provide data, and these data can be fed into models. However, we would still nears decades or centuries of measuring and simulation to better determine the probability of an event. It's difficult to predict where exactly an event will happen since we cannot map precisely the structure below the surface. We would need to know the elastic and creep/flow properties of the various rock and mineral formations. The deeper (kms) the earth the less knowledge we have.
  5. Mar 27, 2016 #4


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    The answer to that is very involved ... simple answer regarding short term predictions " large event will happen within a say 12 month time between 2 set dates" is currently impossible. And it's still virtually impossible to do that within any set decade period.

    For any active major fault system eg around any of the worlds plate boundaries your "could happen in the future" is a definitive "WILL happen in the future"
    The activity has been happening there for 100's of millions of years and will continue to happen. Taking a particular stretch of plate boundary, we can look at historical large events along that section of fault line. Doing this seismologists came up with the seismic gap idea. This looks at where events have/have not occurred and predicts where the next event is most likely to occur

    Here's a good paper on a region I am reasonably familiar with
    This is looking at historical events and seismic gaps along the Sumatera - Java trench prior to the 2004 major M9+ event


    Things considered when doing this research on gaps...
    1) Time of the last event
    2) Size of the event
    3) Offset on the fault caused by the event -- how much it slipped -- in metres
    4) The amount of the fault that ruptured, 10's to 100's of kilometres
    5) The relative plate motion for that fault line -- measured in mm / year some of the largest motions are up around 75mm ( 7.5cm) / year
    6) The strain rates across the region ( calculated from info from #5) -- this tells the researchers how quickly the strain and stresses are building up across the faulted region.

    It gets more complicated when larger events happen away from the main boundaries and on specific faults that may not have been identified and studied to work out strain/compression rates or avg return periods of large events

    Eg. the very damaging events that occurred in and around the Christchurch region from Sept 2010 to present.

    The first of the events, a M 7.1, occurred on an unmapped fault under farmland and also 10's of metres of alluvium deposits. It produced maximum offset of up to 4 metres and a surface rupture length of around 21 km. There hadn't been a large event in that region in recorded history and no small events in the previous few years.

    Last edited: Mar 28, 2016
  6. Mar 28, 2016 #5


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    Since I'm in California (notorious for quakes) I've lived with people always saying, "Seriously, we're due for a big one" and nothing ever comes. I've thought about this before and wondered whether there would patterns in the rock and underground to allow us to predict earth quakes easily. Now I realize that there are so many contributing factors that makes it difficult.
    I can't even comprehend on how seismologists and geophysicists can go about even trying to predict a quake away from boundaries and faults.

    Thanks for sharing that paper; I'll read it in depth after school and see if I can make any sense out of it past the abstract :rolleyes:
  7. Mar 28, 2016 #6


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    They can still study strain rates across a region. Looking at those figures annually shows them how much the region is compressing
    This has become much easier and more accurate over the last 20 yrs since the advent of GPS location measuring
    a few ( less than 5mm) mm of accuracy is easily accomplished with the equip being produced over the last 10 years.
    Yup, it really is that accurate ! ... I service Trimble GPS equip for machine control and surveying
    here's a screenshot off a bit of gear I worked on .....


    NOTE the 7mm vertical and 3 mm horizontal accuracy. Horizontal acc. is always around 2 times better than vertical
    Also note that the GPS accuracy of your avg public in car or tramping handheld GPS unit is around 10 - 10 metres in comparison !!

    So various organisations eg USGS (US Geological Survey) have a wide network of GPS receivers that they collect data from continuously
    Tiny variations in equip inaccuracy can be averaged out over time and the true motion of the land can easily be observed.
    They can measure how much the land is compressing up ( or maybe stretching out) over a period of time and from that work out strain rates across a region.

  8. Mar 28, 2016 #7
    Lituya Bay is a loch, narrow with high walls. The wave reached over 500 feet.

    Several boats were anchored in the bay. A man and his son battened down the hatches and survived.
  9. Mar 28, 2016 #8


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    I had posted the following about the Lituya wave in the thread on nuclear explosions being able to cause tsunamis:
    A landslide in Lituya Bay, Alaska did cause a runup in the bay of ~1720 feet directly across from the landslide. As the wave made its way down the bay, the height diminished rapidly.
  10. Mar 31, 2016 #9
    I'll never forget that day in 1964. I lived in Port Alberni, BC, a small town app. 50 miles inland at the end of a fjord on Vancouver Island. When the wave hit us, it was still about 3 metres or so in height. It did a great deal of damage. A graveyard shift was at work already at the sawmill and plywood mill and pulp mill. Workers on the ground floor stated they saw a floor to ceiling wall of water come at them. They all floated momentarily and were then deposited back on the floor. NO ONE was killed and only a few were hurt. Several homes along the inlet at sea level were floated away. Again, no one killed or injured badly. What was really weird was that the inlet on which Port Alberni is situated is at a 90 degree angle to the direction the wave would have had to traverse. In other words, when the wave reached the west coast of Vancouver Island it took a hard left turn to travel app. 50 miles in a north-easterly direction in order to reach Port Alberni. Yet it came in from the north! Check it out on a map. Perhaps you can explain that to me. I believe that was also the year the Pacific Northwest got hit with a rare cyclone but I digress. It was definitely a year to be in awe of Mother Nature.
  11. Apr 1, 2016 #10


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    Hi ebos

    Looking at your location on google earth, I can see that Port Alberni is at the head of a very long narrow inlet ( fiord?)


    The waves don't have to turn like that. keep in mind that the waves only form relatively close to shore.
    How close depends on the topography of the seafloor in the region. Prior to the waves forming in the shallower waters,
    the tsunami is just a big swell in the level of the sea level. The swell will flow into any bay/inlet area and the overall depth
    of the water will dramatically rise. At some point as the swell approaches the shore, waves will likely form.
    In your area, I could imagine the would increase the sea level broadly over that very wide bay and that there would be
    a significant funnelling effect up into all those small narrow areas. The direction of the swell and any waves would to totally controlled by local topography, above and below sealevel.

    This effect was observed in the Japanese quake/tsunami of 2011 in many of the videos. as in this video


    It is just like a fast incoming tide that just kept rising ... no huge 10m (give or take) wave. This swell just follows
    the estuary and river channel as it moves inland. Take note particularly from around the 5 minute mark ...
    there's no huge wave that overruns that concrete barrier wall, just the overall rising swell.

  12. Apr 2, 2016 #11
  13. Apr 2, 2016 #12
    Whoops, I got so excited I clicked the wrong button. Anyways, thanks for elucidating my fading memories. And, it IS called an 'inlet' as in Somass Inlet but 'fjord' sounds sexier.
    So I guess that's why they used to call them "tidal waves". They're just like a tide except 10 times as high and 100 times as fast. Amazing video to watch again. I still can't get over the people that were still just driving or strolling by without a clue. I guess that's probably what the video-taking crew were yelling about - trying to warn the pedestrians what was coming.
    Thanks again Dave. Now I can understand how that 'wave' got up the Somass.
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