In the 1990s, McKenzie had worked with Derek Fairhead on continental topography and gravity anomalies, concluding that the elastic behaviour of the continental lithosphere is controlled by the parts of the crustal layer strong enough to give rise to earthquakes, which includes the lower crust. McKenzie went on to examine, with Alessia MaggiAlessia Maggi (1975-present) is an Italian seismologist currently teaching and researching at the Université de Strasbourg, Observatoire des Sciences de la Terre (EOST). She gained her PhD in seismology at Bullard Laboratories, University of Cambridge in 2002., James JacksonJames Jackson (1954-present) of the University of Cambridge works on active deformation of the continents, focusing on earthquake source seismology alongside geomorphology, space geodesy and remote sensing. He is interested in quantifying not only the deformation from individual fault ruptures but also in how individual earthquakes combine to produce the landscape of active tectonic regions. and Keith PriestleyKeith Priestley (1946-present) of the University of Cambridge is a seismologist with an interest in the structure and mechanical behaviour of the continents. He combines comprehensive seismological information with geochemistry, petrology and mineral physics to delve into the fundamental processes that shape the Earth. , the depths at which earthquakes occur across the continents.
Their paper marks a return to the distribution of earthquakes, a data source that proved so helpful in developing the ideas of McKenzie and ParkerBob Parker (1942-present)joined Madingley Rise as a PhD student in 1963, alongside McKenzie; they shared an office and had the same supervisor, Teddy Bullard. Parker was working on the propagation of the electromagnetic field from the ionosphere into the Earth, which demanded an understanding of the oceans. Parker went to IGPP Scripps for post-doctoral research and there he and McKenzie worked out the paving stone model of rigid plates rotating on a sphere about axes. Parker had produced a flexible computer programme to draw maps and he helped McKenzie incorporate earthquake first motions around the edges of the plates as instantaneous velocity vectors, a crucial step in the arguments of McKenzie and Parker 1967. (1967), but this time on the continents; again, improved earthquake location provided the key. This paper’s careful survey of gravity anomalies and earthquake locations in the continents overturned accepted ideas of crust and mantle strength in the continents. They established the position of the crust-mantle boundary and found no earthquakes originating in the mantle; they were all crustal. This overturns the idea that the upper mantle beneath the continents was strong enough to generate earthquakes, as it is in the oceans. The strength of the continental lithosphere, they conclude, is dominated by the strength of the crust; the upper mantle is weak.