Rainwater may play an important role in the process that triggers earthquakes, according to new research.
Researchers identified the sources and fluxes of the geothermal fluids and mineral veins from the Southern Alps of New Zealand where the Pacific and Australian Plates collide along the Alpine Fault.
From careful chemical analyses, they discovered that fluids originating from the mantle, the layer below Earth's crust, and fluids derived from rainwater, are channelled up the Alpine Fault.
By calculating how much fluid is flowing through the fault zone at depth, the researchers showed for the first time that enough rainwater is present to promote earthquake rupture on this major plate boundary fault.
"Large, continental-scale faults can cause catastrophic earthquakes, but the trigger mechanisms for major seismic events are not well known," said lead researcher Catriona Menzies, from the University of Southampton in UK.
"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," Menzies said.
"Fluids are important in controlling the evolution of faults between earthquake ruptures," she said.
"Chemical reactions may alter the strength and permeability of rocks, and if enough fluid is present at sufficiently high pressures they may aid earthquake rupture by 'pumping up' the fault zone," Menzies said.
The Alpine Fault is a major strike-slip fault that fails in very large (more than magnitude 8) earthquakes around every 300 years.
It last ruptured in 1717 AD and consequently it is under intense scientific scrutiny because it is a rare example of a major fault that is late in the strain-build up before rupture.
"We show that the Alpine Fault acts as a barrier to lateral fluid flow from the high mountains of the Southern Alps towards the Tasman Sea in the west," said 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," she said.
"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," Menzies said.
"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," she added.
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