When prediction is useless
We have seen before on StatsChat that, worldwide, there’s no relationship between the position of the moon and the risk of earthquakes. Suppose, for the sake of argument, that there was some relationship in New Zealand. Imagine that in Wellington, 100% of big earthquakes happened in the 24-hour period centered on the moon’s closest approach to the earth. The real figure is more like 0%, since Sunday’s earthquake missed the window by a few hours (perigee was 8:28am Monday) and the 1855 Wairarapa quake and the 1848 Marlborough quake missed by days, but we’re running a thought experiment here. Would this level of prediction be useful?
At one or two big quakes per century, even if they all happened on a predictable day of the lunar month, that’s a risk of between 0.075% and 0.15% per month. At one extreme, you couldn’t evacuate Wellington every month to get around the risk (and even if you did, it would probably cause more injuries each month than happened in Sunday’s quake). At the other extreme, you could make sure you had a few days supply of water and food, and a plan for communicating with friends and relatives, but that’s a good idea even in the real world where earthquakes are unpredictable. The only thing I could think of is that you wouldn’t schedule major single-day tourist events (World Cup games, royal visits) or the most delicate pieces of construction work for that day.
[If you want to look up lunar distances, there’s a convenient online calculator. Note that the times are in UTC, so the NZ standard time is 12 hours later than given]
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 »
It would be interesting to know when it was high tide over the quake zone. It might not directly be the moon but the volume of water pressing down on the fault, kinda like reservoir-induced seismicity with dams. IIRC The tides are different at either end of Cook Strait and that can trap a large volume of water in the Strait. There are also underground rivers in Cook Strait flowing out from below Lower Hutt (although I am not sure if they make it as far south as the quake zone) and with all the heavy rain we’ve had recently that could have added to the volume of water pressing down on the fault.
Even if the quake didn’t happen on the day, but rather the day before, it could still be the moon’s/sun’s fault. If there is some graviational pull that triggers a quake then that trigger amount could occur before the amount is at it’s maximum hence the earthquake happens before the maximum.
11 years ago
It could be true, but if you look at the earlier post there really is not the slightest suggestion in world earthquake data that it is true.
11 years ago
There is no evidence, so far, based on the confounders you’ve looked at.
The thing about the Chch earthquakes is that it happened when there wasn’t a lot of water pressing on the plates (Feb – summer dry) and when there was a lot of water pressing on plates (Sept – IIRC – cumulative winter wet). So I think the whole moon/gravity/water systems must have some effect but probably small and most likely useless for prediction.
It would be intersting to look at the data (moon distance versus earthquake date) by season or rainfall (since monsoons happen in the summer).
11 years ago
You have not defined an earthquake. Without a working definition you cannot count them. In fact an earthquake happens on every sea tide, because an earthquake is an exaggerated land tide, such that high land tide is the low sea tide time. The best way of thinking about earthquakes is to consider moonquakes, which occur every time the earth passes closer, as was discovered by NASA from the seismological equipment left on the lunar surface. That scenario is mutual.
11 years ago
I have not come up with a personal definition of an earthquake, no. I have used the term as defined by the organisations who define and collect the information, such as the US Geological Survey and NZ Geonet. I don’t see a problem with this — or any real disagreement with any practical definition.
It seems strange to me that the best way of thinking about earthquakes, which primarily occur on tectonic plate boundaries and which have been studied in great detail for a long time would be to think about moonquakes, which happen on a body that has no tectonic plates and where there is enormously less data. But even if that is the best way to understand what is really going on, it doesn’t affect anything I’ve written here.
11 years ago
Just because an earthquake is seen to occur along tectonic boundaries does not confine it to that, because geology is the study of what makes things occur, not merely the descriptive study of what has been seen already to have occurred. It is rather nonsensical to assume that tectonic plates that do not descend more than 100kms, and over most of the ocean floor about 10kms, can cause an earthquake 400kms down. It is much more likely that the plates are disturbed by internal stress release passing upwards and parting the plates, this ruling out plate movement as something causal. The trouble with analysing numbers of quakes is that the gathered number does not include all the thousands of daily underwater fissures, eruptions and volcanoes that do not register or register away from recording apparatus, but nevertheless are indicative of disturbances to earth’s electromagnetic field that cause the internal release that may finally get to be called one particular earthquake at one particular location.
11 years ago
As much as an earthquake is only a tide, it is predictable. As for an increased magnitude shake in one location over specified time, a definite article, I have proven this is predictable to within 80%: https://twitter.com/kenringweather/status/23856729753
If we use the indefinite article “an” then it is generic and cannot be predicted until closer defined. It is therefore incumbent on the speaker to first remove linguistic ambiguity. Failure to do that incurs confirmation bias.
11 years ago
OK, I know I am like a dog worrying a bone but…
the other thing with the Seddon quakes was the sudden water and snow load on the Kaikouras and Tararuas in the weeks before the earthquakes. We’ve had a long, hot dry summer well into autumn (making the ground lighter than usual) and them followed by a lot of rain and a large dumping of snow (making the ground heavier). And that could put stress on the plates. (Snow isn’t tidal though).
With your analysis in the “The moon and earthquakes” posts did you take into account that your observations may not be independent? A moon/tidal/water triggered earthquake may be the first but then that sets off a whole lot of other earthquakes in response to the new stresses caused by the first. With the Chch earthquakes, IIRC they said there could be aftershocks for up to a year. How about just counting the first earthquake over 4 and then not counting all the other earthquakes within 2 degrees long and lat for the next year?
ps How did you calculate the distance between the moon and the earth between Apogee and Perigee – linear interpolation or an equation for travelling in an ellipse?
11 years ago
I do not regard the weight of water in the form of rain or snow as causal for earthquakes. If that was the case then every high tide would cause an earthquake at every beach twice per day. The land tide is several thousand kms thick, moving vertically a foot a day and back again in NZ. I fail to see how an extra few metres of water/snow is going to hold down the land tide or exert enough pressure to increase it.
As to “aftershocks” I don’t accept them. It’s like saying some showers are “afterfalls” left over from yesterday, or big waves are “afterwaves” from yesterday’s high tide, ones that arrived too late and missed the bus? Nah, sorry, to my mind it is a nonsense argument. The aftershock story is absurd. Every new shake is its own pressure release. And how about the 6.1mag on 21 July in South Africa, or the 6.6mag on that day in China? Were they afrershocks of our one or was ours an aftershock of theirs?
“Stress on the plates” can’t cause the formation of earthquakes several hundred kms beyond the bottom of the plates. What stress on them? The stress is beneath the plates. Does anyone seriously believe there is more stress on the plates from above than from beneath? If so then earthquakes would descend rather than ascend.
I think earth science needs to go right back to the drawing board. Tectonic theory doesn’t explain anything. The moon and sun do. All earthquakes are caused by internal stress. We must ask what gravitational forces can cause/come up with such huge stress as to be capable of moving the whole South island over by an inch towards Australia (the 2009 Te Anau 8.4mag full moon shake) a city like Christchurch over by a metre and a country like Japan over by 4 metres. We must ask why when the moon’s daily transit disturbs the earth’s electromagnetic field in the manner of a dynamo, that it would not have a solenoidal effect on a huge piece of subterranean iron ore and make it thrust forward enough to transmit that pressure upwards to burst through the earth’s crust where the subground material was least dense.
The earth/moon distance for any location at any time can be obtained from various programmes. I use Starry Night.
11 years ago
@Ken Ring
Then what about reservoir-induced seismicity? That’s caused by a large body of water pressing down just like snow is a large body of water pressing down (although more dispersed).
A high tide has being doing it’s thing for millions of years so plates get used to taking the pressure, it’s the sudden extreme difference that’s the trigger i.e. extra high tides from the super moon, sudden rain and snow fall in the weeks before, the aftermath of the winter storm with winds from the south pushing water into the strait (and not able to exit quickly from the bottle neck to the north).
Wellington really did have an onset of very extreme weather conditions.
11 years ago
An extra high tide from a supermoon may add another half metre of water to a land tide of several thousand kms. That is about an increase of pressure of 0.000025%. I do not see that as a significant sudden extra weight capable of causing enough stress to effect seismic pressure 400-500kms down through solid rock that was not already accruing. Instead, if anything, extra weight should suppress pressure release.
Also I do not understand this statement “plates get used to taking the pressure”. Doesn’t this apply to all of nature all the time? Nothing then is really any surprise because it has all happened millions of times before. But we are discussing causes, not whether nature surprises herself. Nothing happens without pressure. Something that erupts massively upwards (enough to cause a row of alps to appear) can only come from massive pressure from below. Without trying to be disrespectful, to me that is scientific common sense.
11 years ago