Moon and earthquakes
So, there’s a new analysis in Nature Geoscience, with a story on Radio NZ, about how some earthquakes do appear to be linked to tides.
There’s a couple of things to note, though. First, the correlation is only found for the very largest quakes (in their analysis, they found it for magnitudes 8.2 or higher, and possibly for magnitudes 7.5 and higher). Second, they needed to look not just at the position of the moon, but at the orientation of the tidal stress relative to the fault line that slipped. Thirdly, the correlation isn’t anywhere near big enough to use for predictions.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
My thought with finding the correlation for the highly select group was the extent to which other groups were also checked, creating a multiple testing artefact.
But (and I freely admit I am going to vent here) also, in pointing to the full moon and new moon as times of correlation, during the full moon the sun is on the opposite side of the earth to the moon, making the nett gravitational affect a minima compared to the new moon maxima.
Doing the calculations for nett gravitational effect, the moons is much less than the sun, but fluctuates much more due to the relative closeness. The sun is acting a a long term cycle basis, the moon a medium term one, and the earth rotation causing daily variation for items on the surface with the movement relative to the other two. The difference the position of the moon relative to the sun makes about a half a percent difference in surface gravitational attraction compared to the total pull of the sun (based on hurried unchecked workings).
Also, the gravitational strength of the moon varies depending on how close it is, a cycle that has nothing to do with the angle to the sun (and the distance to the sun varies through the year as well). If you want greatest tidal stress, the question should be how many earthquakes are in early July during new moon lunar perigees (moon has strongest effect) compared to a lunar apogee full moon in early January (moon has weakest effect compared to sun). I haven’t bothered to test that.
8 years ago
The Nature Geoscience paper did look at tidal shear stress along the fault, not at moon phase.
8 years ago
I wrote this as a follow up “The sun causes earthquakes”
https://thoughtfulbloke.wordpress.com/2016/09/14/the-sun-causes-earthquakes/
Because the results were completely against everything I have ever read on the matter, it would be fair to say they surprised me.
8 years ago
IIRC there was some work done a 3 or 4 years ago where they* buried sensors along a line of longitude in the lower North Island and then detonated stuff to see what the sensors picked up. The extra day time earthquakes could be man made.
* “they”, I think, was a whole group of agencies that included VuW.
8 years ago
Yes, but the extra earthquakes are at night not the day- The initial graph is in UTC, so the conversion to New Zealand time has around 25% more earthquakes (45% to 55%) at night.
8 years ago
I don’t actually know what time they did the detonations but my point was that the “excess” earthquakes could be man made.
They are certainly not independent events either.
8 years ago
An article in the Guardian about it is here
https://www.theguardian.com/science/2015/feb/05/geologists-solve-tectonic-plate-mystery-with-half-a-tonne-of-explosives
And I found out the detonations were done between 2am and 3am in the morning.
I thought they had been done the detonations out at sea but it sounds like on land.
8 years ago
I just tested under the ocean vs under land earthquakes- on the theory that if it is generated by man-made activities there should be a difference between earthquakes under where people are and earthquakes in areas people cannot be. There was no big difference, under the ocean 57% of 35322 earthquakes occur at night, under land 56% of 66605 earthquakes occur at night.
This was done by a quick and dirty is the point within a polygon that forms a map of New Zealand
8 years ago
How correlated do earthquakes have to be before it starts explaining the excess?
If you look at the animation of the Chch earthquakes, it’s pretty plain that one initial earthquake event triggers thousands of aftershocks dampening out over time
http://www.christchurchquakemap.co.nz/all
A small number of extra trigger earthquakes at night, that are just natural random variation, could explain the excess (essentially after-shock) earthquakes at night.
~
Of course, that means we need to know decide what a trigger earthquake is. Are the St Arnaud, Seddon, Porangahau, Eketahuna, and Te Araroa earthquakes all trigger earthquakes? Or have they just being triggered by the Christchurch earthquake as the resulting pressure changes on the fault line work their way north?
8 years ago
I’m hoping to get some advice from a geologist in the next few days (assuming I don’t come off as a crank). Then I can filter out quakes near in space and time for whatever standard definitions of “near” are used.
But I think I can rule out the few big events hypothesis, as the pattern is consistent between months. If it was a few big events then those events would cause a spike in the months they were in.
8 years ago
If you go and look at geonet – there are tremors in Christchurn, Seddon, Porangahau, Eketahuna, and Te Araroa still happening right now
https://www.geonet.org.nz/quakes/all
So the time horizon for related events is a really, really long time.
8 years ago
Yes, but when the related events are going on for long enough to be express in both day and night, then they should be equally likely to occur in the day and at night. Thus are not the explanation for the excess. The only way I can see for related events to be contributing is the intense concentration of aftershocks in the few hours after a major quake (longer than that and they start contributing to the opposite diurnal category and it all balances out). The short term mass of aftershocks from a large event as explanation for the excess is ruled out by the lack of variation between months in the day/night proportion.
8 years ago
If the count of after shocks per hour happens according to exponential decay then there will always be more aftershocks in the next 12 hours than in the following 12 hours – there is no balancing out across 24 hours. So if a trigger event happens late in the day or early in the night than night time counts of earthquakes are going to outdo daytime counts for however long the trigger event generates aftershocks.
[If a trigger event happens away from the twelve hour boundaries between day and night than counts between day and night will be closer depending on the paremeters of the decay model and how far into a twelve hour period the trigger event is.]
~
Wouldn’t you expect variation in the rate of earthquakes per month in your model? The distance increase between NZ facing the sun and NZ not facing the sun is about the diameter of the earth 12,000 km. The difference in distance between the sun and the earth in Jan and June is 6 million km.
So if the sun is having an effect it should be greater in Jan versus June than in day versus night.
8 years ago
I don’t know I would say I have a model at this stage, I am 48 hours into having an observation.
But I would observe, in a force vector kind of way, that the seasonal variation on the sun’s gravity is tiny in comparison to the daily variation between the sun being below the horizon and the sun’s gravity adding to the earth’s gravity in pushing bits of crust against each other compared to the sun being above the horizon so mitigating the earth’s gravity pushing bits of crust into other crust.
At least that is as close to a model asI am at the moment- ultimately for things on the earth’s crust the earth is the winner gravity-wise. The sun is in second place, the moon a distant third.
8 years ago
“There’s a couple of things to note, though. First, the correlation is only found for the very largest quakes (in their analysis, they found it for magnitudes 8.2 or higher,….
That is not the truth what you were saying.
You can read in our paper
Gravitational Moon-Earth Forces Triggering
Earthquakes in Subduction Zones
7 years ago
I am reporting their analysis and that is what their analysis found.
7 years ago
Thomas,
Happy new year. My evidence over 40 years suggests that during OCT NOV the Sun creates approx 20% more quakes than average. ie 6.5Mag and above. Odd that this
is 6 weeks ahead of nearest Sun time
I have curves and data if you wish
Syd BAA member
7 years ago