Stephan Mueller died at the age of 66, only one and a half years after his retirement in 1995 from the Chair of Geophysics and Directorship of the Swiss Seismological Service at the Swiss Federal Institute of Technology (ETH) and from the Chair of Geophysics at the University of Zürich.
Born in Marktredwitz, Bavaria (Germany), he went to school in Passau and Stuttgart. He studied physics at the University of Stuttgart (Diploma in 1957) and electrical engineering at Columbia University in New York (M.Sc. in 1959). During his studies he obtained scholarships at the Technical University in Berlin and at Columbia University. Also at the University of Stuttgart, Stephan came into contact with Wilhelm Hiller, a classical seismologist and head of the State Seismological Service, who aroused Stephan's interest in geophysics. This quickly evolved into his major pursuit of study. While at Columbia, he sought out the Lamont Geological Observatory, already then one of the worldwide leading geophysics institutes. Maurice Ewing and his group at Lamont strongly supported Stephan's enthusiasm for geophysics. Returning to Stuttgart in 1962, he completed a PhD thesis on "Synthesis of Normally Dispersed Wave Trains by Means of Linear System Theory" under the supervision of Hiller in close cooperation with Ewing. His interests focused on the structure of the Earth's lithosphere-asthenosphere system and especially on its seismic properties. This theme was to dominate his lifetime research. Using hand-digitized records, Stephan and Leon Knopoff in 1960 were the first to determine upper mantle structures from surface waves by spectral analysis.
After 1962, Stephan spent two more years in Stuttgart as scientific assistant and lecturer with intermittent extended research visits to the Southwest Center for Advanced Studies in Texas. Together with Mark Landisman, he demonstrated the wide-spread existence of zones with reduced seismic velocities in the continental crust.
In 1964, he became full professor and head of the newly-created Geophysical Institute at the University of Karlsruhe, a position that he held for seven years. Based in Karlsruhe at the center of the Rhinegraben, Stephan worked with Karl Fuchs and Henning Illies on the structure and evolution of rifts, a topic that became another favourite in his broad spectrum of lithospheric research. Expanding on his earlier studies with Knopoff on the propagation of surface waves, he looked to the wider Alpine-Mediterranean region where he participated in the ongoing European cooperative deep seismic sounding experiments as part of the International Upper Mantle Project. The extremely fruitful cooperation with other geoscientific groups at Karlsruhe led him, together with the staff at the Geophysical Institute of the University of Stuttgart, to found the Black Forest Observatory (BFO) for the study of long period surface waves, free Earth oscillations, and Earth tides. He was also responsible for the operation of the new Seismological Central-Observatory Gräfenberg, Erlangen (SZGRF) in its critical initial years.
The next challenge for Stephan came with the call from the Swiss Federal Institute of Technology (ETH) to succeed Fritz Gassmann as head of the then small Institute of Geophysics. Up to that time geophysical research in Zürich focused primarily on applied geophysics and shallow targets. The Faculty of Natural Sciences intended to broaden the scope of geophysics in Zürich. In accepting this challenge in 1971, Stephan became Professor of Geophysics and Director of the Swiss Seismological Service at the ETH. With his characteristically encompassing and clear vision, he eagerly incorporated almost all branches of geophysics in his studies of the entire Earth from crust to core with strong emphasis on including geology and geodesy. Only three years later, following his initiative, a professorship of rock- and palaeomagnetism was created and filled in Zürich. While remaining responsible for the Swiss Seismological Service and Experimental Seismology, Stephan added research groups in geothermics and gravity. In 1977, he was co-appointed full Professor of Geophysics at the University of Zürich. Thanks to his insistence, and supported by a general awareness of the importance of environmental sciences, a third full professorship in applied geophysics was installed in 1992 at the ETH.
He continuously worked towards building a modern seismic station network for the Swiss Seismological Service. His early education in electrical engineering enabled Stephan to promote and apply modern technical strategies to create a leading research and monitoring tool in earthquake sciences. It was under his directorship that Erhard Wielandt developed the broad-band seismometer that is now in use throughout the world. During this time, he enthusiastically taught undergraduate students and supervised more than sixty doctoral theses.
When appreciating the work of Stephan Mueller, his academic activities can hardly be separated from his scientific interests and achievements. Based on a large number of crustal deep seismic soundings, he developed in 1977 a basic model of the continental crust. Probing the Earth from the surface to the upper mantle transition zone by refraction seismics and analysis of seismic surface waves in many different areas showed that lateral variability was obviously related to tectonic evolution in time and space. After years of analyzing phase velocities of surface waves, Giuliano Panza, Stephan, and Gildo Calcagnile produced in 1980 the first map of lithospheric thickness and lithospheric and asthenospheric shear-wave velocities of Europe. They also documented the presence of a deep-reaching high-velocity body beneath the Alps.
To unravel the tectonic evolution of the Alps, much more detailed information was needed about the crustal structure of this continental collision zone. Based on experiences in other areas, Stephan in 1983, together with Peter Fricker, Ernst Niggli and Rudolf Trümpy initiated a proposal for a coordinated Swiss National Research Project (NRP20) "Deep Structure of the Swiss Alps". Near-vertical reflection seismic techniques were used for the first time to obtain continuous regional transects across an active continental collision zone. He supported and guided this national project actively throughout its twelve years running time. It was only one month after his untimely death that the results were published.
Even before the Swiss project was initiated, Stephan realized that to understand the lithosphere one had to encompass the more than 3.5 Ga span of continental evolution. In 1982, together with Trümpy and supported by the European Science Foundation, he initiated a project of integrated studies of the structure, physical properties, composition, and evolution of the continental lithosphere from northern Scandinavia to central Tunisia over a distance of 4600 km. This project, the European Geotraverse (EGT), was to become one of the most successful large-scale international Earth science projects of the last two decades. The EGT includes one of the broadest ranges of processes in which continental crust is built up, maintained, and destroyed along provinces occurring in succession geographically as well as in time. This transect provided one of the first opportunities to follow the tectonic development of continental lithosphere.
Stephan's broad interest appears again in his studies of crust-mantle evolution, structure, and dynamics of the Mediterranean ranging from Turkey to the Eastern Atlantic. In this capacity as member and, from 1981 to 1991, as Chief Project Scientist of the WEGENER-MEDLAS project, he focused on the relationships between lateral variations of lithospheric structure, stress distribution, and ongoing crustal movements. Rift structure and evolution represent another phase in the life of continental lithosphere that aroused Stephan's curiosity from his early days with the Rhinegraben to a dedicated cooperation in the worldwide compilation undertaken by the working group on Continental Rifts: Evolution, Structure, and Tectonics (CREST).
Stephan united several rare talents: a deep and broad scientific knowledge, the insight to define scientific questions and the intuition to recognize future research directions, the ability to find appropriate methods to tackle these problems and to synthesize the results with those from other Earth-science disciplines. He combined in an extraordinary manner steadiness in his research with the widest freedom possible to his coworkers. He was always open for new ideas without loosing sight of the original goals. Furthermore, he continuously promoted the geosciences in public and demonstrated their contributions and importance to society.
On a wider front, Stephan demonstrated his commitment to interdisciplinary research by initiating and promoting international cooperation and keeping it active over more than a decade, for example in projects such as the EGT. Stephan had the capacity to bring scientists to work together and to mediate between ambitious scientific plans and constraints imposed by funding and administration.
Among many national and international organizations he presided over the European Seismological Commission (1972-1976), the Governing Council of the International Seismological Centre (1975-1985), the European Geophysical Society (1978-1980), and the International Association of Seismology and Physics of the Earth's Interior (1987-1991), and was a member of the Executive Committee of the Academia Europaea (1988-1992).
He published more than 150 papers and served as editor and co-editor of numerous journals and books. His outstanding contributions to Earth sciences have been acknowledged by the geoscientific community with numerous awards and honourable appointments. To name only a few: he was Elected Fellow and Foreign Associate of the Royal Astronomical Society; Honorary Member and Badge Awardee of the European Geophysical Society; Elected Fellow of the American Geophysical Union; Elected Member of the Deutsche Akademie der Naturforscher LEOPOLDINA; Gustav Steinmann Medalist of the Geologische Vereinigung; Personal Member of the Swiss Academy of Sciences, Honorary Fellow of the Geological Society of London; Bearer of the Medaille de l'Ordre Grand-Ducal Luxembourgois de la Couronne de Chêne.
Stephan Mueller's work was characterized by his scientific curiosity and his quest for a better understanding of the Earth as a system. With this goal in mind he never forgot the human aspect and abilities of his students and co-workers. Many of us remember burning midnight oil together with Stephan. We lost a remarkable scientist, a mentor and teacher who aroused and stimulated the interest of those who were privileged to work with him, and a very good friend.
Stephan is survived by his wife Doris, two sons and their families.
Colleagues and former students at the Institute of Geophysics, Zürich