Does the Moon cause earthquakes – or is that a bit looney?

The magnitude 7.8 earthquake that hit New Zealand on 14 November – and the sequence of events that followed – has been complex in every sense.

US Navy photo of a total lunar eclipse in 2004, by Photographer's Mate 2nd Class Scott Taylor. Public domain, via Wikipedia.
US Navy photo of a total lunar eclipse in 2004, by Photographer’s Mate 2nd Class Scott Taylor. Public domain, via Wikipedia.

The main shock itself was a highly complex rupture of multiple faults that extended northwards and delivered a hefty punch to Wellington, well distant from the putative epicentre. That was followed by a ‘slow slip’ quake on the east coast – a microscopic but relentless movement at mid-depth on the plate interface, detectable only by GPS data, that was triggered by the earlier shock.

Other quakes blipped and rattled around New Zealand, some of them consequent on the slow-slip event, others aftershocks to the magnitude 7.8 Kaikoura event.

One of the things that surprised me was the fact that the lunatic fringe were soon declaring it was a US oil-drilling plot, but nobody popped up to finger the Moon. After all, the quake struck on the night of the supermoon. There’s a particularly lunatic member of the lunatic fringe, here in New Zealand, who believes he can ‘predict’ earthquakes – specifically – from lunar phases.

So what’s the science behind it? The Moon does influence the Earth, of course, through tides. And they occur in rock, just like they do in the ocean. Because rock doesn’t flex the way water does, the total energy (heat) generated by these tides is only about 4 percent of the total tidal energy, and movement is in the order of a few centimetres.

Into this picture come the supermoons. They occur because the Moon’s orbit around Earth isn’t close to circular. The distance of the Moon actually varies by up to 39,000km. At the perigee (closest) point the moon is about 14 percent visibly larger. Because of the inverse-cube law, the tidal force is about 19 percent greater – but that translates as less than 1 percent difference for the rock. There have been six supermoons this year – the next is due on 15 December. For reasons deriving from the fact that the Moon’s orbit is inclined, and the general gravitational/orbital relationship between Moon, Earth and Sun, the perigee distance varies depending on the direction the major axis (a straight line drawn between the furthest points of an ellipse) happens to be pointing. Consequently, not all supermoons are equal. It happened that the 14 November supermoon was the closest in decades. But the difference is marginal.

Buildings in Featherston street, Wellington, damaged by the Mw 7.8 quake on 14 November 2016.
Buildings in Featherston street, Wellington, damaged by the Mw 7.8 quake on 14 November 2016.

Does any of  this cause earthquakes? Curiously, the seismometers left on the Moon by the Apollo astronauts reveal that the Moon suffers more quakes at perigee than at other times – caused, very probably, by tidal forces induced by Earth. However, Earth is 81 times more massive than the Moon – the effect doesn’t mirror back here much. The issue isn’t the gravitational force, but the gravitational gradient. And the fact remains that the primary driver behind terrestrial quakes is not lunar at all: it’s tectonic movement, driven by monstrous energies generated in the Earth’s lithosphere. These are colossal by comparison with tidal forces.

It’s possible that the Moon might be a triggering factor, sometimes – that a fault line stressed close to rupture point might be pushed over the edge by tidal flexing. But that has yet to be proven, and because tectonics are a highly complex system where triggers derive from a range of different factors, it obviously won’t be a factor every time a quake occurs. Indeed, efforts to establish a statistical relationship, to date, haven’t revealed any obvious patterns. What has been discovered, though, is that ocean tides might have an effect on shallow underwater fault lines – in part because the weight of the water flexes the rock – which indirectly brings the Moon into play.

Bottom line is that the Moon doesn’t cause earthquakes on Earth, and its phases or position in its orbit is not a useful way of ‘predicting’ earthquakes. In fact, quakes can’t be predicted – there are too many unknowns in the system, and it is too complex, for specific predictions to work. Forecasting (like the weather) is another matter of course – and seismologists forecast earthquakes all the time, as ranges of probability. The science is well worked out: but of that, more anon.

Copyright © Matthew Wright 2016

10 thoughts on “Does the Moon cause earthquakes – or is that a bit looney?

  1. In plate tectonics we learn that the Earth’s crust is always in motion creating slips and faults, etc. It’s true that the moon affects our tides, but extremely little, if anything at all, to do with the movement of the plates. The standard joke about California separating from the rest of the US is not a joke, it is actually the fastest moving plate of all I was once taught, and although we will not live to see it, Nevada will one day be able to sell ocean side property! Oh well,

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    1. Yes – given time this plate movement causes all kinds of changes to the geography. No question about California slip-sliding north at speed. Here in NZ the Southern Alps are product of the fastest uplift in the world – were it not for erosion they’d apparently be 20 km high.

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  2. To me, one of the most terrifying things about the Big One we are due for here on the west coast of North America is that there is no warning of an imminent earthquake, unlike a hurricane, wildfire, or even a volcanic eruption. It could happen right now, or in a hundred years. The only way to deal with this is to be as prepared as you can with emergency supplies and a plan (I’m not, actually) and not think about it.

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    1. It’s that warning that is the problem – the systems that drive earthquakes have far too many unknowns to make ‘prediction’ possible. Japan has a system that picks up the P-wave and gives a bit of warning of the more destructive and slower S-wave, which is handy. There is also work, as I understand it, to try and figure out how animals seem to pick up that something’s about to happen – the stories of birds falling silent and pets going crazy in the hours ahead of a major quake are too common to be ignored. It’s possible that they are picking up the pre-quake subsonics, which we (usually) miss. Though sometimes we pick them up – there are stories I’ve heard of people feeling suddenly uneasy, needing to get out of where they are – and then a quake hits. There is some solid physics behind why that happens, of course – the ground ‘groans’ ahead of the main tremor, but the problem is turning that into some sort of method for picking quakes up even minutes ahead of the moment.

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      1. A warning system has apparently been developed by a guy called Kent Johansen at the University of British Columbia, and installed in some Catholic schools, but has not been widely adopted. It can give a 30 to 90 second warning. Sensors have been installed along the Cascadia Subduction Zone. So eventually, I suppose predictability will improve.


  3. Well darn it! I had my hopes set on a top-secret gravity weapon test from the former nazi base (taken over by Russian Spetznaz commandos in ’68 before US SpaceSEALS took it ’89) on the moon. Now you’ve spoiled my theory! 😉

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    1. There is always the other secret Nazi base at the South Pole… ☺ (It’s likely that the US station there was built directly on top of the main entrance, unknowingly but fortuitously trapping the Nazis inside and thus preventing them trying to take over the world with Stormtroopers riding robo T-Rexes…)

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  4. Don’t dismiss the US oil drilling plot just yet. Even we don’t know of what we are capable, witness our current political scene. And you don’t want Donald Trump to read about it because he’ll believe it and no one can ever predict where that will lead. I know, enough already…. 😉

    As I mentioned to you in another post, you must be exhausted with all of the quakes and aftershocks. And, of course, no one can predict just when that will end, and I do not imagine a forecast right now is very comforting, either. Yet, it must be a comfort to know that you can still enjoy supermoons without worrying about earthquakes. Great post, Matthew, as always; thanks for the science.


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    1. I’m not too worried about the earthquakes personally – don’t have any fear of them, though that doesn’t mean being reckless. I don’t want anything to fall on my head, so I am fairly careful about where I go in town (I did get bashed on the head with a beam of 4 x 2″ timber, just after the main quakes, but that wasn’t due to the earthquake – it fell out of the runner-bean frame in my back yard while I was trying to wrestle another beam into position).

      However, the likely dislocation of a major shock and longer term issues do concern me. Our seismologists have a year-ahead forecast for the likely aftershock sequence of the current series, based on earlier experience – and it will tail off – but there are disturbing signs that the whole lot is symptomatic of a much larger and interconnected period of general movement that can be traced to a major quake in Fijordland, in our deep south, in 2009. It’s been marching north up the country – Christchurch, then the Seddon/Kaikoura region (which is where the 2013 quakes that hit Wellington emanated from, as did the latest). The next in line is Wellington itself, and for all the shaking we’ve had, it’s not been the feared ‘big one’ that the local faults will cause. Again, I have no concerns if I’m at home, which is well set up; but to be caught down town wouldn’t be fun.

      There’s precedent for this ‘march of quakes’ too – it’s hard to pick up traces of old quakes, but our geologists have identified a devastating series that ripped across NZ around 1460, all 8+ magnitude, causing massive destruction and tsunami. Maori called the Wellington district one ‘Haowhenua’ – it caused uplift that created the land on which Peter Jackson’s studio complex sits today. The name means ‘land eater’, which was puzzling, but I published my explanation a couple of years ago – for Maori what was important was their food resource, and the tidal waves destroyed their gardens, ‘eating’ the land.

      We don’t want a set like that today but the seismic reality is that this is exactly what will happen, sooner or later. Hopefully later…

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