A major international study has shed new light on the mechanisms through which earthquakes are triggered up to 40km beneath the earth’s surface.
While such earthquakes are unusual, because rocks at those depth are expected to creep slowly and aseismically, they account for around 30 per cent of intracontinental seismic activity. Recent examples include a significant proportion of seismicity in the Himalaya as well as aftershocks associated with the 2001 Bhuj earthquake in India.
However, very little is presently known about what causes them, in large part due to the fact that any effects are normally hidden deep underground.
The current study, published in Nature Communications and funded by the Natural Environment Research Council, sought to understand how such deep earthquakes may be generated.
They showed that earthquake ruptures may be encouraged by the interaction of different shear zones that are creeping slowly and aseismically. This interaction loads the adjacent blocks of stiff rocks in the deep crust, until they cannot sustain the rising stress anymore, and snap - generating earthquakes.
Emphasising observations of quite complex networks created by earthquake-generated faults, they suggest that this context is characterised by repeating cycles of deformation, with long-term slow creep on the shear zones punctuated by episodic earthquakes.
Although only a transient component of such deformation cycles, the earthquakes release a significant proportion of the accumulated stress across the region.
The research was led by the University of Plymouth (UK) and University of Oslo (Norway), with scientists conducting geological observations of seismic structures in exhumed lower crustal rocks on the Lofoten Islands.
The region is home to one of the few well-exposed large sections of exhumed continental lower crust in the world, exposed during the opening of the North Atlantic Ocean.
Scientists spent several months in the region, conducting a detailed analysis of the exposed rock and in particular pristine pseudotachylytes (solidified melt produced during seismic slip regarded as ‘fossil earthquakes’) which decorate fault sets linking adjacent or intersecting shear zones.
They also collected samples from the region which were then analysed using cutting edge technology in the University’s Plymouth Electron Microscopy Centre.