• Matthew Blackett

The summer solstice & no earthquakes?

It's summer solstice today (21 June) and for many, this conjures up images of druids, and those of a more spiritual disposition, gathering at Stonehenge to watch the rising sun and to celebrate the start of the astronomical summer. In Neolithic times, this must have felt like a new lease of life, with productive and long days of growth and harvest ahead.

Sunrise over Stonehenge, Wiltshire (source).

From an astrological point of view, the summer solstice is when the Earth's North Pole has the greatest tilt towards the sun. The consequence of this is that the sun is overhead at the Tropic of Cancer and there is no darkness in the Arctic Circle (see below). And for anyone who knows a bit about the tides, they will understand that it is such positions of the sun (and moon), relative to the Earth, that drives tides. How then, might the solstice affect the occurrence of earthquakes? Well, the moving mass of enormous bodies of water over the Earth's crust is often enough to destablise the underlying tectonic plates and has been associated with several earthquake events. This old study from 1983 for example, appeared to show periodicity in Californian tremors that coincided with ocean tides in the south of the state. Ocean tides are not solely responsible however.

Midnight sun in northern Russia on summer solstice (source).

It is not just oceans that have tides; the solid Earth has them too, with deformation on the scale of up to several metres occurring due to these extra-terrestrial gravitational pulls. Several studies have attempted to show how Earth tides might be related to volcanic and seismic earthquake activity. One paper announced "Earth’s rotation variability triggers explosive eruptions in subduction zones" and another one found "clear evidence for small but significant increase in earthquake rates near low tide" but also "no evidence for an increase in seismicity during intervals of large tidal range". So the situation is clearly confusing, inconsistent and far from conclusive as to possible earthquake predictions; the influence also seems only to be related to smaller events.

So, what are we to expect on the summer solstice? It seems the probability of significant earthquakes is actually lower on 21 June than on any other day of the year. My only guess as to why this might be would be related to there being less water in the Northern Hemisphere so be displaced (as opposed to the vastly more oceanic Southern Hemisphere, see below). So perhaps we can sigh with a breath of relief today, that a large earthquake is unlikely. This said, statistics can be used to show whatever one wants and I can't guarantee that the summer solstice will consistently reduce tectonic activity!

Global oceans. Note that there is much less coverage in the Northern Hemisphere and potentially, less infience of moving oceans on the summer solstaice than at other times (source).