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.
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.
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