It’s the rain’s fault.
Scientists have been trying to figure out how to predict earthquakes for years, and some seismologists have suggested that electromagnetic anomalies could be used as a precursor for earthquakes, an idea that is currently being researched.
But in a first, a group of researchers from the University of Southampton, GNS Science (New Zealand), the University of Otago, and GFZ Potsdam (Germany), has found evidence showing that rainwater may play an important role in the process that triggers earthquakes.
By studying the sources and movements of the geothermal fluids and mineral veins (which form when minerals are carried by water within the rock and are deposited) from the Southern Alps of New Zealand where the Pacific and Australian plates collide along the Alpine Fault, careful chemical analyses have revealed that the fluids originating from the mantle — the layer below Earth’s crust — and fluids from rainwater, are being channeled up the Alpine Fault.
The researchers then calculated how much fluid is flowing through the fault zone, and showed that if enough rainwater is present it can promote an earthquake on this major plate boundary.
The Alpine Fault is a major “strike-slip fault” — a vertical (or nearly vertical) fracture where the movement of the plates is mainly in a horizontal direction (seen in the animation below). It results in very large earthquakes (magnitude 8 or more) roughly once every 300 years, and since it last ruptured in 1717, it is almost due.
A strike-slip fault occurs where rock is displaced mainly in a horizontal direction, parallel to the fault line.
Lead researcher Catriona Menzies, from the Ocean and Earth Science at the University of Southampton, said in a press release, "Large, continental-scale faults can cause catastrophic earthquakes, but the trigger mechanisms for major seismic events are not well known. Geologists have long suspected that deep groundwaters may be important for the initiation of earthquakes as these fluids can weaken the fault zones by increasing pressures or through chemical reactions."
The chemical reactions could alter the strength and permeability of rocks, and if there is enough fluid present at sufficiently high pressures deep within the Earth, they may encourage earthquakes by "pumping up" the fault zone.
"We show that the Alpine Fault acts as a barrier to lateral [horizontal] fluid flow from the high mountains of the Southern Alps towards the Tasman Sea in the west,” explained Dr. Menzies. "However, the presence of mantle-derived fluids indicates that the fault also acts as a channel for fluids, from more than 35 km depth, to ascend to the surface."
And it turns out that only a little bit of surface rainfall is needed to increase the pressure within the fault zone.
"As well as mantle derived fluids, our calculations indicate that 0.02-0.05 per cent of surface rainfall reaches around six kilometres depth but this is enough to overwhelm contributions from the mantle and fluids generated during mountain-building by metamorphic reactions in hot rocks," said Menzies. "This rainwater is then focused onto the fault, forced by the hydraulic head of the high mountains above and the sub-vertical fluid flow barrier of the Alpine Fault."
With further research, geologists may be able to use the results as a means of predicting when this fault line will rupture next.
The study was published in the journal Earth and Planetary Science Letters.