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Thread: M 5.3 earthquake near Point MacKenzie, Alaska

  1. #1
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    M 5.3 earthquake near Point MacKenzie, Alaska

    M 5.3 - 3 km SSW of Point MacKenzie, Alaska
    2021-02-27, 18:59:25 (UTC)
    61.329N 149.999W, 42.1 km depth

    That places the epicenter about 20 miles away from our house. It was quite the rumbler. It started modestly like many do and I gave it the 'wait and see' pause like any true Alaskan. It didn't let up, however and seemed to build, so I did the old duck-and-cover thing for what seemed like a long time. I haven't seen an official duration but in total, I'd say it was 30-40 seconds. No damage here at the house, though. Nothing fell off of shelves or walls. Just a perturbed cat who is now wandering the house, meowing at the top of his voice.
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    Not damage, exactly but I did find the back door ajar and it wouldn't latch without persuasion from a small pry bar. I'll need to re-shim the door frame.
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    At 42 km that seems fairly deep for such a long duration. But at the same time the Alaska Earthquake Center said it was "an aftershock of the November 2018 M7.1 Anchorage earthquake."

    Here are the waveforms from a number of stations:
    http://earthquake.alaska.edu/event/0212o88mof/waveforms

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    Glad you got thru it a-ok, hope no other problems found. My older brother was stationed in Anchorage during the 64 quake, he had quite a ride too. Our cat went bonkers in the house during a hail storm 3 summers ago (aren't they supposed to warn us about these things? ;-)

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    Quote Originally Posted by schlaugh View Post
    At 42 km that seems fairly deep for such a long duration. But at the same time the Alaska Earthquake Center said it was "an aftershock of the November 2018 M7.1 Anchorage earthquake."

    Here are the waveforms from a number of stations:
    http://earthquake.alaska.edu/event/0212o88mof/waveforms
    I figured my perception of time was pretty skewed but I was looking at those very waveforms and a few others earlier. My guess now is that it was around 20-30 seconds.
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    Quote Originally Posted by PetersCreek View Post
    M 5.3 - 3 km SSW of Point MacKenzie, Alaska
    2021-02-27, 18:59:25 (UTC)
    61.329N 149.999W, 42.1 km depth

    That places the epicenter about 20 miles away from our house. It was quite the rumbler. It started modestly like many do and I gave it the 'wait and see' pause like any true Alaskan. It didn't let up, however and seemed to build...
    Yeah, I totally understand that. When the 3/11 earthquake hit here I was in downtown Tokyo, and it started building up just like other earthquakes, and I was thinking that it would soon stop. But then it didn't. It just kept getting bigger, and bigger, and bigger. And then all the stuff started flying off my desk.
    As above, so below

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    "an aftershock of the November 2018 M7.1 Anchorage earthquake."
    How long after a 7.1 quake do you get aftershocks?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    "an aftershock of the November 2018 M7.1 Anchorage earthquake."
    How long after a 7.1 quake do you get aftershocks?
    Apparently the recent quake in Japan was called an aftershock to the big event in 2011.

    https://www.cnn.com/2021/02/13/asia/...ntl/index.html

    By Junko Ogura, CNN
    Updated 11:34 AM ET, Sun February 14, 2021

    Tokyo, Japan (CNN) - A powerful earthquake that hit Japan on Saturday was an aftershock of the devastating 9.0 magnitude quake that struck the same area almost 10 years ago, according to the national Meteorological Agency.

    The 7.1 magnitude earthquake struck the country's east coast at 11:07 p.m. Saturday. At least 48 injuries were reported in Fukushima and Miyagi prefectures, according to state broadcaster NHK, but there were no major casualties.

    The epicenter hit about 46 miles (74 kilometers) northeast of Namie, a coastal town 60 miles from Fukushima, according to the United States Geological Survey. The earthquake measured about 36 miles in depth. No tsunami warning was issued.

    Saturday's quake took place in the same area as the March 11, 2011, earthquake that caused the country's worst nuclear disaster on record, when three reactors at the Fukushima Daiichi nuclear plant melted down, releasing radioactive materials into the air.

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    Quote Originally Posted by Tom Mazanec View Post
    How long after a 7.1 quake do you get aftershocks?
    From the USGS glossary:

    Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.
    And this, from the Southern California Earthquake Center (emphasis added):

    Aftershocks are earthquakes that usually occur near the mainshock. The stress on the mainshock's fault changes during the mainshock and most of the aftershocks occur on the same fault. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well.

    An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly. The day after the mainshock has about half the aftershocks of the first day. Ten days after the mainshock there are only a tenth the number of aftershocks. An earthquake will be called an aftershock as long as the rate of earthquakes is higher than it was before the mainshock. For big earthquakes this might go on for decades.
    Here's a USGS search of earthquakes (M1.0+) during the year following and in the vicinity of the 2018 M7.1 quake. (image included if the link doesn't work for you.) Not all of them are aftershocks, of course, but 10405 in that small area is considerable, given that we had about 55,000 quakes that year, statewide.

    Capture.JPG


    And from the 2020 Seismicity Year in Review:

    Quote Originally Posted by Alaska Earthquake Center
    Aftershocks from the November 30, 2018 M7.1 Anchorage Earthquake continued into their second year at an average pace of about 26 earthquakes per week. Several days of renewed aftershock activity spurred by a M5.1 aftershock on November 7 were felt across Anchorage and the Mat-Su valley. Approximately 1,200 aftershocks were reported for 2020 (figure 4), bringing the total count to more than 12,000.
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    This is the USGS website on aftershock forecasts; has some nice graphs of aftershocks over time from various size quakes.

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