Why does the elastic rebound theory suggest that earthquakes cannot occur at great depths?

Answer 1

The elastic rebound theory states in simple terms that the rockmass around a fault zone deforms elastically. This means that it is storing energy (technically called elastic strain potential energy) in the same way that a squashed spring would. If the stresses on the rockmass exceed it's strength or that of the fault, it will rupture and all the stored energy will be released in the form of seismic waves producing an earthquake and the rock mass will spring back or rebound to its original shape (a lot like in our spring example where if the spring broke the pices would spring or rebound to their original size, releasing all their energy).

This can happen as the rocks forming the earth's lithosphere are under relatively low temperatures and pressures and so can deform elastically and fail in a brittle manner (where fractures form through the material and there is a consequent sudden release of energy)

However as the depth in the earth increases, the pressures and temperatures increase. At a depth of between 100 to 200 km (depending on exactly where on earth), due to this temperature increase the rocks stop behaving as a brittle solid and instead deform in a ductile manner (instead of squashing a spring or stretching an elastic band, this is like squashing a lump of clay - it stays squashed rather than springing back to it's original shape) . As they are deforming in a ductile manner rather than elastically they are not able to store strain energy and are not able to fail in a brittle manner (where fractures form through the material). This layer of the earth (located below the lithosphere) is known as the Asthenosphere.

As there can be no storage or sudden release of elastic strain as would normally occur in an earthquake (as is required by the elastic rebound theory described above summarised as an increase in elastic strain, followed by the stress exceeding the rockmass strength leading to fault slip and the rockmass rebounding back to it's original shape while the released energy creates the seismic waves) due to the fact that at greater depths the ductile rock deforms like a plastic, dissipating the energy rather than storing it then there can be no earthquake.

It is also worth noting that earthquakes can occur at greater depths than the above suggests where subduction is occurring and elastic strain can build in the subducting slab that is cooler than the surrounding material.

Answer 2

The elastic rebound theory suggests that earthquakes occur due to the accumulation and release of stress along faults. At great depths, the rocks are subjected to high pressure and temperature, causing them to deform in a ductile manner rather than accumulating stress and eventually rupturing in a brittle manner like at shallower depths. This means that earthquakes are unlikely to occur at great depths because there is no sudden release of stored energy.