Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mag 7.8 Earthquake, 101km ESE of Suva, Fiji

  1. Sep 6, 2018 #1


    User Avatar
    Staff Emeritus
    Science Advisor

    M 7.8 - 101km ESE of Suva, Fiji
    2018-09-06 15:49:14 UTC
    Location 18.494°S 179.332°E, 608.6 km depth

    Very deep. A number of deep aftershocks have occurred.

    Potential for Mag 8 earthquakes in that region - along the Tonga trench.

    The Australia-Pacific plate intersection takes a jog in that region. I'm expecting a Mag 9+ somewhere around that region one day.
  2. jcsd
  3. Sep 6, 2018 #2


    User Avatar
    Science Advisor
    Gold Member

  4. Sep 6, 2018 #3


    User Avatar
    Science Advisor
    Gold Member

    there was the M 8.2 E of there last month ( August)

    It was also very deep
  5. Sep 7, 2018 #4


    User Avatar
    Staff Emeritus
    Science Advisor

  6. Sep 7, 2018 #5


    User Avatar
    Science Advisor
    Gold Member

    now upgraded to a Mw 7.9 and substantially deeper

    some interesting things for this and the previous event

    The M 7.9 is almost due west of the M 8.2. This puts it almost directly down dip from the 8.2. ( "down dip" = down slope)

    180919 and 0907  UT M 8.2 - blue, 7.9 - yellow E of Fiji1a.JPG

    180919 and 0907  UT M 8.2 - blue, 7.9 - yellow E of Fiji2a.JPG

    Cross section


    remember this cross-section is only based on the 2 listed events -- so the shape of the slope above the 8.2 location is just inferred

  7. Sep 7, 2018 #6
    How was it felt in Australia?
  8. Sep 7, 2018 #7


    User Avatar
    Science Advisor
    Gold Member

    not felt, too far away :smile:

    over 3000km from me in Sydney
    over 2000km from Brisbane city

  9. Sep 7, 2018 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    Some information on the region. Where the earthquakes occurred is the Lau Basin.

    To the east is the Tonga Trench and to the north is the Vitiaz Trench.

    The Geology of the Lau Basin

    A three-plate kinematic model for Lau Basin opening
  10. Sep 8, 2018 #9
    If I recall correctly, there were many smaller earthquakes that were triggered by the 8.2 magnitude one last month, and many of them were quite some distance from the original 8.2 magnitude quake.

    Is there any evidence that there are similar aftershocks or triggered quakes at a large distance from the new earthquake?

    Here's a general reference to this process: https://www.sciencedaily.com/releases/2018/08/180802102352.htm

    I'm not referring to small ones right on top of the big one, but to medium to large earthquakes at a large distance at a frequency that is much higher than random.
  11. Sep 8, 2018 #10


    User Avatar
    Science Advisor
    Gold Member

    Have you got a reference for that ?
    what is your definition of "quite some distance" ?
    The aftershocks area can be seen from my map above first map in post #5

    I cannot answer that till you answer my previous question

    you still need to define your distance definition

    There are none that I am aware of that fit that definition

    That isn't new info …. The 1992 Landers quake of M7.3 ( southern California) was noted to have stirred up activity in northern California and further north.

    You also need to understand that those events you are referring to were likely to have happened anyway in the near future.
    It's just that the large passing seismic waves through the region has changed the stresses enough to allow these addition events to
    occur a little earlier than they would have naturally done so.
    Events of M8 and greater have the Earth ringing like a bell ( at very low frequencies) often for several days after the initial event occurred.
    It's quite amazing to watch this on a seismograph that is capable to record periods of 20 - 50 seconds / cycle.
    With my own seismo system, I high-pass filter from about 22 seconds so I don't see those really long periods. The system isn't stable
    enough below around 22 - 25 sec. Mainly because of it's domestic/suburbia location here in Sydney, Australia.

    The most sensitive seismometers are usually located well away from people (man-made noise) and ocean. They are in underground
    vaults where air pressure and temperatures are very stable.

    Here is the Charters Towers unit with is part of the WWSSN ( World Wide Standard Seismic Network) now known as the GSN (Global Seismic Network) and it's in the state of Queensland, Australia about 1400km to the north of me.

    outside view ….

    inside view....


    Last edited: Sep 8, 2018
  12. Sep 9, 2018 #11
    Wikipedia is not a good source for formal academics, but it's usually OK for straightforward reporting of facts, such as earthquake magnitudes.


    Here's my attempt to re-format the large earthquakes reported for the first nine months of 2018.

    Number by magnitude

    ............ Jan Feb Mar
    8.0−8.9 0 0 0
    7.0−7.9 3 2 0
    6.0−6.9 8 9 8
    5.0−5.9 118 145 133
    4.0−4.9 1076 922 953

    ............ Apr May Jun
    8.0−8.9 0 0 0
    7.0−7.9 0 0 0
    6.0−6.9 7 6 1
    5.0−5.9 90 139 123
    4.0−4.9 917 930 905

    ............ Jul Aug
    8.0−8.9 0 1
    7.0−7.9 0 3
    6.0−6.9 9 19
    5.0−5.9 125 164
    4.0−4.9 863 727

    Note that if we count the number of earthquakes of magnitude 6.0 or higher the monthly numbers are:

    11, 11, 8, 7, 6, 1, 9, and 23.

    August was busy, with quite a few big ones after the August 19 8.2.

    In fact, the next 14 earthquakes reported from August 19-29 were *all* 6.0 or larger.

    That seems to be an anomaly. I don't know if the reporting is accurate, or if they just omit the small ones if a big one hits on the same day, but it does seem strange to have 14 big ones in ten days, even with a few listed as official aftershocks of the 8.2.

    Some are probably too far away (Alaska) to be triggered by a South Pacific quake but others are in the same general area, and a quake 600 km down might be more effective at transmitting energy long distances than a shallow quake. Note that the New Madrid earthquakes in the middle of the US were felt at much greater distances than California quakes, so local geology also matters.

    I think that this data is more suggestive than conclusive, but combined with the paper referenced before, it does at least beg the question: were some of these quakes, other than the official aftershocks, actually remotely triggered by the big and deep one?
  13. Sep 9, 2018 #12
  14. Sep 9, 2018 #13


    User Avatar
    Gold Member

    Interesting paper. Looking at the data for the last year for quakes 6.0 and larger, the September 8, 2017, 8.2 earthquake in Mexico seems to go against their hypothesis; "...provided discernible evidence that in the three days following one large quake, other earthquakes were more likely to occur."


    There wasn't a 6.0+ earthquake on the planet for another 11½ days.

    Just sayin......

    That's kind of my take, even though I know nearly nothing of the subject.
  15. Sep 9, 2018 #14


    User Avatar
    Science Advisor
    Gold Member

    that list doesn't agree with your previous list in the same post ????

    8, 9, 8, 7, 6, 1, 9, 19

    where did your 2 lots of 11 come from for Jan and Feb ?

    The long term world wide averages are ....

    M6.0 - 6.9 --- 2.5 / week ( avg 140 / yr)
    M7.0 - 7.9 --- 1 roughly every 21 days (avg 18 / yr)
    M8.0 ++ --- avg 1 / year ( sometimes there are none in a year, sometimes there are two. I have vague memories of one year having 4 x M8+ events.
    Now that was a bit above long term averages)

    the first 2 months, Jan and Feb were good for M6's and M7's then it died right off
    Then finally in August, a rise in activity saw the averages getting restored

    So, Assuming your sources are correct ? ( you really should go directly to the USGS), for M6.0 - 6.9 we so far ( excluding this mornings'
    event had 64 in that range, with 4 months of the year left to get another around 75 in that range. That's not going to be easy to so
    without a large escalation in events between now and the end of December. and that is just to hit the average, not break it :wink:

    not really an anomaly ... as I stated above, just restoring the averages :smile:

    That isn't impossible, and I did cover that in my previous post. Did you read it ? :smile:

    and another M6.5 this morning in the Solomons on my recorder


    Last edited: Sep 10, 2018
  16. Sep 10, 2018 #15
    I wrote "6.0 or higher", which includes anything greater than a 6.9.

    There were three mag seven-somethings in January, two in February, and three in August plus the 8.2.

    Add 3, 2, and 4 in those months to your numbers and you get my numbers:

    .. 8, 9, 8, 7, 6, 1, 9, 19
    + 3, 2, 0, 0, 0, 0, 0, 4
    11, 11, 8, 7, 6, 1, 9, 23
  17. Sep 10, 2018 #16
    Part of the problem with presenting this sort of data is that it's hard to copy tables without a common format across web platforms, so it's harder to read.

    I tried to break up the "wall of numbers" into three more bite-sized sets and it appears that it STILL confused people.
  18. Sep 10, 2018 #17


    User Avatar
    Gold Member

    IMHO, that is the least of our problems.
    I'm confused as to what your point is?
    As Dave mentioned earlier, these quakes were going to happen anyways, so "something" is going to trigger them.

    The positions of the sun and moon have been shown to trigger earthquakes. [ref]
    Sometimes human activity causes earthquakes. [ref]
    And your reference says that earthquakes cause earthquakes. I doubt that anyone is doubting that.

    I think we are arguing, for the sake of arguing.
  19. Sep 10, 2018 #18


    User Avatar
    Science Advisor
    Gold Member

    yeah, ok :smile: but it is better and normal to keep the magnitude ranges separate as I have shown... it gives better details as to what is happening.

    no, what was confusing was, as I stated, that you clumped magnitude ranges together ... I could read your tables with no problem

    so, again, this is the important bit ......

    The long term world wide averages are ....

    M6.0 - 6.9 --- 2.5 / week ( avg 140 / yr)
    M7.0 - 7.9 --- 1 roughly every 21 days (avg 18 / yr)
    M8.0 ++ --- avg 1 / year ( sometimes there are none in a year, sometimes there are two. I have vague memories of one year having 4 x M8+ events.

    the last 24 hrs has been really busy .... the numbers are slowly climbing towards the yearly averages

    6.5 Solomons
    6.9 Kermadecs
    6.3 Loyalty Isl

    180909-10  UT M 6.5 Solomons, 6.9 Kermadecs, 6.3 Loyalty Isl zhi.gif
  20. Sep 27, 2018 #19


    User Avatar

    Why does it seem that so few of these earthquakes trigger tsunamis? how good are we at determining whether a particular offshore earthquake will generate one?
  21. Sep 28, 2018 #20


    User Avatar
    Science Advisor
    Gold Member

    we know because it was VERY deep 600 - 700 km ….. the sea floor wouldn't have even ruptured and that is what is needed for a tsunami o be generated.

    Shallow events 0 to around 10km maximum depth would be tsunami generators

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?