for his exceptional and wide-ranging contributions to Earth Sciences, for his role in initiating new research areas in geophysics and his encouragement of the younger generation of geophysicists

Nicolaas Jacob Vlaar received his PhD in 1963. His thesis was entitled "The transient electromagnetic field of a vertical dipole antenna". During 1963-1973 he carried out research at several places in the United States and was appointed full professor in theoretical geophysics at the University of Utrecht in 1993.

During his career Nicolaas Vlaar has continuously explored new directions of research in geophysics. In particular, he made important contributions to the following fields. 

In 1968 Vlaar developed the ray theory for the propagation of elastic waves through anisotropic media. This work still forms the basis of recent advances in exploration geophysics where seismic anisotropy is becoming increasingly important.

Together with his graduate student Durk Doornbos, Vlaar showed in 1973 that the core-mantle boundary exhibits significant lateral variations which give rise to significant scattering of body waves. It took an additional ten years before the seismological community started publishing a large number of contributions on the lateral variations of the core-mantle boundary. Vlaar contributed to the development of the concept of the moment tensor as a description of the source of earthquakes and he actively explored the effects of gravity on the Earth’s normal modes.

Since the mid-seventies, Vlaar has been exploring ways of increasing the impact of geophysical methods in geology. This refocused his research on tectonophysical problems, which has led to a number of studies in which numerical modelling techniques were used to study the details of subduction processes. These modelling studies made Vlaar realize that subduction does not always take the form of steeply dipping lithospheric plates. He proposed that under certain conditions lithospheric plates can slide virtually horizontally under other plates, and he called this process "lithospheric doubling".

In the mid-eighties, the research interest of Vlaar shifted to the pattern of plate tectonics in other era. This has let to his theory that plate tectonics is only possible under restricted thermal and petrological conditions and that plate tectonics is a relatively recent phenomenon. Finally, his application of seismic tomography for studying the structure of subduction zones deserves special mention.

Another important side of the research style of Nicolaas Vlaar was that he has continuously helped young researchers to establish themselves in geophysics by allowing them to bring the newly developed research fields to a more mature stage. In this way, Vlaar has been at the basis of the growth and development of an impressive number of successful geophysicists.

R.K. Snieder, M.J.R. Wortel and S.A.P.L. Cloetingh

Newsletter 59, 34, 1996