Deep-Focus Earthquakes

Deep-focus earthquakes provide conclusive evidence that there is active mantle convection to depths of 660 km. Since the lower mantle is expected to contain significant concentrations of radioactive isotopes, we expect that mantle convection will occur in the lower mantle in order to transport the resulting heat.

Three alternative models for mantle convection have been proposed: a Whole mantle convection. If significant amounts of subducted lithosphere can enter the lower mantle beneath 660 km, then there must be a complementary mantle upwelling. In this case the geotherm for the entire mantle is likely to be adiabatic. The expected geotherm is illustrated in Figure 4–57 as curve a. The primary arguments against whole mantle convection come from chemical geodynamic studies.

b Layered mantle convection. Two separate convection systems are operating in the upper and lower mantle. This would be the case if the density discontinuity at a depth of 660 km completely blocks convection. An upper convective system associated with plate tectonics would be restricted to the upper 660 km of the mantle; a lower, separate system would operate between a depth of 660 km and the core mantle boundary. In this case a thermal boundary layer would be expected to develop at a depth of 660 km similar to the lithosphere. However, it is very difficult to estimate the change in temperature ssociated with this boundary layer.

An expected geotherm for layered mantle convection is given as curve b in Figure 4–57. Although deep-focus earthquakes do not occur at depths greater than 660 km, studies using mantle tomography indicate that at least some subducted slabs penetrate through this boundary. This is taken as convincing evidence that there is significant material transport between the upper and lower mantle.

c Hybrid models. Hybrid models have been proposed that involve a strong time dependence and/or a barrier to convection within the lower mantle. If the 660-km seismic discontinuity acts as a partial barrier to mantle convection, then mantle “avalanches” may be triggered that would lead to a strongly time-dependent mantle convection. Dense subducted litho sphere could “pile up” on the 660-km deep seismic discontinuity until a finite-amplitude instability resulted in a mantle “overturn” or avalanche.

Episodic mantle overturns have been proposed as an explanation for apparent episodicities in the geological record.