Thorne Lay has made contributions to our knowledge and understanding of deep Earth structure that have had an exceptional multidisciplinary impact, as have his contributions to the study of earthquakes.
As a graduate student with Don Helmberger in the early 1980s, his discovery of the discontinuity at the top of the D˝ region in the lowermost mantle, now sometimes called the “Lay” discontinuity, attracted the attention of the community to a region of the mantle that previously had been somewhat neglected. At the time, this was quite an achievement, using for the most part analog, noisy data from a not-so-large global seismic network. These observations have since been confirmed by many independent studies. Perhaps most importantly, they have inspired the mineral physics community, culminating in the discovery, 10 years ago, of the postperovskite (pPv) transition in magnesium at pressures and temperatures corresponding to those in the vicinity of Earth’s core-mantle boundary.
Since then, Thorne has been at the forefront of intellectual efforts to characterize the consequences of these findings for the interpretation of complexities in the seismic structure at the base of the mantle, such as the possible double occurrence of the pPv, folded slabs, and partial melting at the base of the mantle.
Even before the pPv transition “explosion,” Thorne and his students, postdocs, and collaborators continued over the years to document lateral variations in the D˝ discontinuity as well as other complexities, such as seismic anisotropy and velocity gradients. He largely contributed to the development of the broad picture that we now have of the fine-scale structure of the D˝ region.
So far, I’ve only described one aspect of Thorne’s research, the one most relevant to the “theme” of the Inge Lehmann Medal. I also wish to mention his important contributions to the question of how megaearthquake ruptures develop on the rupture plane. Starting as a graduate student with the development of the “asperity” model with Hiroo Kanamori, he has picked up this question very actively in recent years, taking advantage of the newly available high-quality seismic broadband array data, which have coincided with the occurrence of several megaearthquakes in the last decade, and he has often been among the first to document their unusual rupture behaviors.
Finally, it is important to stress Thorne Lay’s community leadership, as manifested by his role in the Incorporated Research Institutions for Seismology and his endless dedication as a spokesman for the solid Earth community, where we often rely on his clear and articulate thinking.
—Barbara Romanowicz, University of California, Berkeley, and Collège de France, Paris, France
Thank you, Barbara, for the generous citation and for working with other colleagues to advance my nomination for the Lehmann Medal. I am very aware that recognition like this stems from the efforts of many folks, and it is humbling and deeply satisfying to be selected as a contributor to our understanding of the deep Earth.
I first became an AGU member in 1978, and it is remarkable to recall the limited state of understanding of the vast region of the lower mantle at that time. Seismic velocity models and associated geodynamical and mineralogical interpretations were not dramatically different from those available in the days of Inge Lehmann’s seminal work on the inner core. The accumulation of analog recordings by the worldwide standardized seismological network and advances in numerical methods for computing seismic waves for one-dimensional Earth models had set the stage for moving forward, but few seismologists were working on deep-mantle problems. Indeed, my own work with Don Helmberger was initially focused on quantifying upper mantle lateral variations, and when we first advanced interpretations of deep-mantle discontinuity structure, the general response by the few who cared was rather dismissive skepticism.
Fast-forwarding to today, progress has been dazzling, with a large and vigorous international interdisciplinary research community advancing the frontiers of our knowledge. This is reflected in a proliferation of unpronounceable acronyms like LLSVP (large low-shear velocity provinces), ULVZ (ultralow velocity zones), and pPv (postperovskite) and the integrated efforts by organizations like SEDI (Study of the Earth’s Deep Interior) to understand the detailed chemistry, transport properties, and evolution of the deep mantle and core. Topics such as deep-mantle anisotropy, barely suggested in work preceding 1978, now engage joint seismological, geodynamical, and mineralogical modeling efforts that build upon state-of-the-art capabilities of different disciplinary efforts.
Our understanding of the deep mantle and core is now sophisticated, but great uncertainties and challenges remain; I am sure that the next generation of results will revise some of our current paradigms, and hopefully, it will provide new acronyms that are easier to say.
I’ve been very fortunate to work on seismology of the deep mantle and core with wonderful mentors, colleagues, and graduate students, along with receiving institutional support from great programs at the California Institute of Technology (Caltech), the University of Michigan, and the University of California, Santa Cruz. This recognition is shared among us all, and I deeply appreciate the many collaborations.
—Thorne Lay, University of California, Santa Cruz, Calif.
Citation: AGU (2015), Thorne Lay receives 2014 Inge Lehmann Medal, Eos, 96, doi:10.1029/2015EO022039.