Mag 7.8 Earthquake, 101km ESE of Suva, Fiji

[Astronuc]

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
https://earthquake.usgs.gov/earthquakes/eventpage/us2000h9e2#executive

Very deep. A number of deep aftershocks have occurred.

Potential for Mag 8 earthquakes in that region - along the Tonga trench.
https://earthquake.usgs.gov/earthquakes/tectonic/images/emaustralia_tsum.pdf

The Australia-Pacific plate intersection takes a jog in that region. I'm expecting a Mag 9+ somewhere around that region one day.

 

[davenn]

my seismogram and location map

http://www.sydneystormcity.com/seismograms.htm

Dave

 

[davenn]

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

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

It was also very deep

 

[Astronuc]

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

It was also very deep

https://www.physicsforums.com/threads/m-8-2-major-quake-fiji-now.953609/
Also around 600 km below the surface, about 360 east of Suva.

 

[davenn]

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

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)

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
Dave

 

[Stavros Kiri]

Dave

How was it felt in Australia?

 

[davenn]

How was it felt in Australia?

not felt, too far away

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

 

[Astronuc]

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)

Some information on the region. Where the earthquakes occurred is the Lau Basin.
https://www.researchgate.net/profile/Bernard_Pelletier2/publication/32972654_The_North_Fiji_Basin_Geology_Structure_and_Geodynamic_Evolution/links/573005d908ae744151905014/The-North-Fiji-Basin-Geology-Structure-and-Geodynamic-Evolution.pdf

To the east is the Tonga Trench and to the north is the Vitiaz Trench.
https://en.wikipedia.org/wiki/Lau_Basin

The Geology of the Lau Basin
https://www.researchgate.net/publication/289951173_The_Geology_of_the_Lau_Basin

A three-plate kinematic model for Lau Basin opening
http://www.soest.hawaii.edu/GG/FACULTY/TAYLOR/kz_lau_g3.pdf

 

[ensign_nemo]

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.

 

[davenn]

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.

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

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

I cannot answer that till you answer my previous question

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.

you still need to define your distance definition

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

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

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...

Dave

 

[ensign_nemo]

Wikipedia is not a good source for formal academics, but it's usually OK for straightforward reporting of facts, such as earthquake magnitudes.

https://en.wikipedia.org/wiki/List_of_earthquakes_in_2018

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?

 

[ensign_nemo]

For a graphical version of 6.0 or larger worldwide since August 19, try this link:

https://earthquake.usgs.gov/earthqu...0Z","minmagnitude":6,"orderby":"magnitude"}}}

 

[OmCheeto]

...
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?

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...

You also need to understand that those events you are referring to were likely to have happened anyway in the near future.

That's kind of my take, even though I know nearly nothing of the subject.

 

[davenn]

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.

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

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.

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

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?

That isn't impossible, and I did cover that in my previous post. Did you read it ? and another M6.5 this morning in the Solomons on my recorder

cheers
Dave

 

[ensign_nemo]

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

 

[ensign_nemo]

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.

 

[OmCheeto]

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.

IMHO, that is the least of our problems.

I tried to break up the "wall of numbers" into three more bite-sized sets and it appears that it STILL confused people.

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. [https://earthquake.usgs.gov/research/induced/myths.php]
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.

 

[davenn]

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

yeah, ok 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.

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.

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

 

[BWV]

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?

 

[davenn]

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?

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 generatorsDave

 

[BWV]

Thanks - looking on Wikipedia it says epicenters of both the 2004 Indian Ocean and 2011 Japanese Tsunamis were at a depth of around 30km?

 

[Astronuc]

Thanks - looking on Wikipedia it says epicenters of both the 2004 Indian Ocean and 2011 Japanese Tsunamis were at a depth of around 30km?

Whether or not there is a tsunami event depends on magnitude, location (e.g., coast or undersea) and depth. The larger the magnitude, the deeper the epicenter can be and still generate a tsunami.

The 2004 Indian Ocean earthquake occurred at 00:58:53 UTC on 26 December, with an epicentre off the west coast of northern Sumatra and a magnitude of 9.1–9.3 Mw, . . . Depth 30 km (19 mi) (Ref: https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami ), and

The 2011 earthquake off the Pacific coast of Tōhoku (東北地方太平洋沖地震, Tōhoku-chihō Taiheiyō Oki Jishin) was a magnitude 9.0–9.1 (Mw) undersea megathrust earthquake off the coast of Japan, . . . (Ref: https://en.wikipedia.org/wiki/2011_Tōhoku_earthquake_and_tsunami ) .

Both earthquakes were two of the largest recorded, and both were undersea megathrust earthquakes.

 

[BWV]

So if either of these quakes had occurred at a depth of 10M it would have been worse, conversely if it happened at the bottom of the Marianas Trench, it would have been a non-event?

 

[OmCheeto]

So if either of these quakes had occurred at a depth of 10M it would have been worse, conversely if it happened at the bottom of the Marianas Trench, it would have been a non-event?

Even as a layman in such things, your questions don't make much sense to me.
Though, I do put some of the blame on earthquakeologists, as "magnitude", "depth", and "epicenter", don't really capture the full nature of earthquakes.
IMHO, they should include duration, fault length, and depth variations. (Which they do, it you look closer.)

I spent about an hour this morning reading about the 2004 earthquake at wiki:

it had the longest duration of faulting ever observed, between 8.3 and 10 minutes.
The northern section of the Sunda megathrust ruptured over a length of 1,300 km (810 mi).​

I interpret this, along with much other information, that the earthquake was 4 dimensional, and varied in depth from zero to 30+ km.

[ref]

synopsis: Generating a conclusion from two data points, when there are a bazillion data points, is pointless.

 

[Astronuc]

So if either of these quakes had occurred at a depth of 10M it would have been worse, conversely if it happened at the bottom of the Marianas Trench, it would have been a non-event?

More or less yes. It depends on the volume of water displaced and how much is pushed up, as in vertical thrust. In terms of depth, the deepest part of the Marianas trench is about 36,000 feet (~11,000 m), so a 9+ mag earthquake pushing vertically might produce a tsunami. One would have to simulate such an event.

Some insight from the Great Tohoku earthquake
https://authors.library.caltech.edu/28770/1/Lay2011p16763Phys_Today.pdf

Shear sliding on the fault where the Pacific Plate thrusts below Japan lasted for 150 anxiety-filled seconds, shifted the coast of Japan up to 5 m eastward, and lifted the sea floor by as much as 5 m over 15 000 km², an area comparable to the state of Connecticut. Displacements as large as 60 to 80 m — the largest ever measured for an earthquake — occurred near the subduction trench, and a total strain energy equivalent to a 100-megaton explosion was released during the sliding.

 

[davenn]

Thanks - looking on Wikipedia it says epicenters of both the 2004 Indian Ocean and 2011 Japanese Tsunamis were at a depth of around 30km?

Yes, but that's only part of the story

So if either of these quakes had occurred at a depth of 10M it would have been worse,

no it wouldn't have because, and this is the important bit ……

and varied in depth from zero to 30+ km.

The important bit being that the rupture of the fault line came to the surface ( of the sea floor)
IF the seafloor doesn't rupture/break/change in levels, then tsunami generation becomes very difficult to impossible.
As with the 2 recent major quakes in the Fiji region, the 8.2 and the 7.8 where no tsunami was generated.

So yes, the focus of those events was around 30 km but the ruptures propagated upwards along the fault interface to the surface
as well as along the length of the fault. 30km is nothing for a major quake, considering it ruptured some 1300 km horizontally along the fault.

OK I hope this little bit of art and text helps with the understanding ….
Base graphics from
http://rsta.royalsocietypublishing.org/content/roypta/364/1845/1947/F2.large.jpg

I have added a few extra arrows for more clarity and a text description

The same is for the Japanese area. Just reverse the orientation of the diagram. And for any other mega thrust region around the world,
eg, Fiji and south to New Zealand, The Aleutian Islands, The Cascadia zone offshore of Oregon and Washington states, USA and
down, along the west coast of South America, to name the major zones

Dave

 

[BWV]

Thanks - yes have seen some documentaries on how potentially scary the Cascadia Zone could be