All earthquake and volcanic activity is ultimately driven by deep-seated motions within the Earth's mantle. Such motions drive tectonic plates and deform the crust and mantle. Earthquakes and volcanic eruptions are direct short-term responses to this deformation. Deformation at the Earth's surface is monitored by modern geodetic techniques. From the Trench to the Core-Mantle Boundary .
Subduction zones have special societal relevance.
As pointed out by Kincaid, all of the great earthquakes in recorded history have occurred at sites of subduction. Depth-Age Revisited . Seismic tomography provides a view of the internal structure and dynamics of the mantle and Adam Dziewonski summarizes the latest improvements to this picture in his paper Global Seismic Tomography of the Mantle . The seismic tomographic image of the Earth's interior has sharpened over the last four years mainly through the incorporation of new data types into the inversion. The deep, global-scale motions of the Earth's mantle and core are determined in large part by the composition, structure, and thermodynamic properties of the mantle and core.