Shijie Zhong (Postdoc '94-'97), Planetary Geophysicist
In planetary science and geophysics, Earth can be studied as "just" another planetary body, albeit one that offers far richer datasets. Shijie Zhong has been engaged in studying the physical processes that shape planetary evolution of Earth and other planets for over thirty years, and advances in seismological techniques, space exploration and optical resolution have yielded important insights on the thermal evolution of terrestrial planets and on the structure and composition of planetary interiors.
After completing his graduate research at the University of Michigan, Zhong arrived at Caltech's Seismo Lab as a postdoctoral scholar in 1994. The connecting point was Mike Gurnis, who was Zhong's thesis advisor before he moved to Caltech. In the discussion below, Zhong recalls the Seismo Lab's vibrant research culture and the free exchange of ideas and research between and among faculty and students. He emphasizes the import of parallel computing, which both revolutionized seismology and further integrated this subdiscipline within the broader field of geophysics.
As he looks to the future, Zhong wants to focus on plate tectonics. While this is an established theory which has matured over the past fifty years, Zhong's interest is indicative of the fact fundamental questions remain, particularly because of outstanding incongruities in laboratory experiments and field observations.
DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Friday, October 7th, 2022. I'm very happy to be here with Professor Shijie Zhong. Shijie, it's so nice to be with you. Thank you so much for joining me today.
SHIJIE ZHONG: Thank you.
ZIERLER: Shijie, to start, would you please tell me your title and institutional affiliation?
ZHONG: I'm currently Professor of Physics in the physics department at the University of Colorado.
ZIERLER: Now, does the University of Colorado have a broader seismology and geophysics program, or is that all rolled into physics?
ZHONG: We actually have geophysics across a few departments here on campus. We have a geological science department that's got some geophysicists there. We also have what we call the APS department, that's astrophysical and planetary sciences, and they have some geophysicists there. We also have a department of atmospheric and oceanic sciences that has some geophysicists. It's like everywhere. [laugh]
ZIERLER: Tell me about some of the major research projects you've been involved in.
ZHONG: My work has been mostly, of course, on solid earth geodynamics. But I also work extensively on planetary sciences on terrestrial planets, primarily, and the moon, Venus, Mars, and things like that.
Current Focus on Mantle Convection
ZIERLER: What are you working on currently?
ZHONG: Well, at the moment, most of my projects are about the Earth. Of course, mantle convection, mantle dynamics is one of the topics that I work on, as always. But, in the last few years, I started to move more to problems like sea-level change from the isostatic glacier adjustment. This is to look at, since the Last Glacial Maximum how the deglaciation contributed to the seal level. Of course, it's about vertical motion of the Earth's surface, and the implication is on mantle rheology, viscosity, and things of that nature. I've been spending quite some of my time working on those sorts of problems recently.
ZIERLER: Shijie, when is your research looking at the commonalities between Earth and other planets, and when is it specific to Earth itself, without thinking of comparisons?
ZHONG: When I look at problems of very large-scale, so the features, say, on Earth, we're talking about maybe the supercontinent cycles, that is, how Pangaea and Rodinia, those are supercontinents, get formed and then break apart into different pieces. Those are really large-scale features on Earth. When I try to understand their dynamics then, of course, I would naturally try to compare with the other planets because if you look at the moon, Mars, in particular, they have very broad, very long wavelength features. We call them long wavelength, large-scale features, even looking at its surface. Naturally, we like to understand some common underlying physical processes that may be responsible for not only the Earth's large-scale structure formation but also Mars and the moon.
ZIERLER: Shijie, what are some of the technologies that are most useful in your research?
ZHONG: Primarily, we do mantle convection simulations and numerical modeling. We develop numerical models. In our case, mostly this code that I started working when I was at Caltech, and this is Citcom. We call it CitcomS now, really, a Citcom series of codes, from Cartesian geometry to the spherical geometry. For mantle convection problems or glacial isostatic glacial adjustment problems, we use the codes that are derived and modified from Citcom. That's the code that I started to work with when I was at Caltech as a postdoc.
ZIERLER: Well, before you got to Caltech, where were you, and how did you hear about the Seismo Lab?
ZHONG: [laugh] I was a graduate student at the University of Michigan in Ann Arbor. I finished my PhD there in the summer of 1994. Then, of course, my thesis advisor was Michael Gurnis. He moved to Caltech I think around '93. I was at Michigan all by myself for about a year when Michael was at Caltech. Of course, I heard about Caltech. [laugh] Then I moved to Caltech to continue to work with Michael as a postdoc.
From Michigan to Caltech
ZIERLER: Tell me about your thesis research at Michigan. What did you focus on?
ZHONG: At the time, we were trying to model and understand the plate boundary dynamics, for example, the subduction zone and the thrust faults in subduction zone environments, how they would impact the dynamics of subduction zones, including the surface features like trench topography, the plate motion, and deformation of slabs. I also worked with Michael on supercontinents.
ZIERLER: What were your impressions of the Seismo Lab when you got there?
ZHONG: It was great. Of course, Seismo lab has a lot of history in geophysics, and in Earth science in general. I remember when I first arrived at Caltech as a postdoc and go to the office, I went to the Benioff Room where people would get together to talk about science, and saw pictures of Richter, Gutenberg, and people like that. Then, of course, I got to meet people like Don Anderson, Hiroo Kanamori, Donald Helmberger, … and you got a chance to talk to them every day. It's great. [laugh]
ZIERLER: Shijie, what were some of the big topics of interest or debates in the Seismo Lab when you were there as a postdoc?
ZHONG: That's interesting. At the time, there were these coffee hours: one in the morning; one in the afternoon. Each of the coffee hours could potentially last for an hour, really, like an hour. You almost got an hour in the morning, and an hour in the afternoon to talk about science. Students, postdocs, like myself, students, everybody would just, around 9:30-ish in the morning, and 2:30 in the afternoon, started to drip into the Benioff Room. People would just bring a piece of paper or a figure that you created in the last day or two, or in the last week, if you find it interesting. You bring that piece of material or results to see if anyone would actually jump on it. Don Anderson would be always there, maybe the first one to get there, Professor Don Anderson, and then Hiroo Kanamori [laugh], and all these people. They will bring coffee there and sit, and you'd try to show your results to them. Then they would start to comment on them; start to discuss about them. It's kind of hard to say what would be the most debated question at the time. The topics varied from day to day. Say, if a major earthquake happened somewhere, and then, certainly, you would see a lot of discussions about the earthquake, earthquake itself, and the tectonics around it, and the geology and everything relevant to that topic. You would hear discussions about that, maybe for days. It was great.
ZIERLER: Shijie, at Caltech, was that when you first started thinking about planets other than Earth?
ZHONG: I would say Professor David Stevenson had an impact on me. He didn't really go to [laugh] coffee hours that much. But, of course, I got chances to talk with him over the years, starting at Caltech. The planetary sciences, to people like myself who are interested in geodynamics, it's always an interesting topic. It's hard to say when I started to get interested in planets. Certainly, by the time I got to MIT to work with Professor Maria Zuber, that was full-time planetary science for me, and that was great. I really learned a lot about the planetary sciences there as well.
ZIERLER: Shijie, what were your main areas of research during your postdoc at the Seismo Lab? What did you work on?
ZHONG: There's a technique aspect I worked. At the time, Louis Moresi was also a postdoc in Mike Gurnis' group, and we overlapped quite a bit, I think maybe for two years. He was the original developer of Citcom code. But, at the time, Citcom was mostly for two-dimensional work, and it only worked for a single CPU. I got a copy of the code from Louis for Cartesian single CPU code. Then I started to mess around with it. Really, one of the major technical projects I worked on was to put that code onto a parallel machine. At the time, Caltech had one of the best parallel computers in the academic world. I think it was called the Intel Paragon. I got access to that parallel computer with about 512 CPUs at the time. Each CPU was not very powerful, compared to today's computers. But I was able to parallelize that code on that Intel Paragon machine using intel's own communication protocol, and then also turn the code into a spherical shell geometry, so it can work on real planets. That was one major part of my effort. Of course, I also used the code to explore various kinds of science problems, working to incorporate the tectonic faults into dynamic models to see how plate tectonics works with mantle convection. It was an interesting time. Even now, those codes are still very useful to people in our field.
The Revolution of Parallel Computing
ZIERLER: Shijie, I wonder if you can explain what was so useful or even revolutionary about parallel computing for geophysics.
ZHONG: It enables us to explore problems of a much larger scale and with adequate resolutions, which of course would be impossible if you just deal with single-CPU computers. Gradually, with time, it's just so clear that for 3D models, you really need big parallel computers to work with. The code that we parallelized at the time was really a good investment in that sense. You could see really just around the middle of the 1990s, when parallel computing started to be so popular. Then you start to see the PC clusters started to be accessible to individual research groups. You don't need a big budget to build a PC cluster of a reasonable size that actually can run all the parallel jobs.
ZIERLER: Shijie, is it more about doing the computation more efficiently with parallel computing, or are you able to do new kinds of research, pose new kinds of questions?
ZHONG: Oh, yes, absolutely, new and different kinds of physics can be explored, and you can really start to be able to compare models with real observations of the Earth. The Earth's observations are always from three-dimensional. These are more powerful computer models from the parallel computers, and then they really opened the door for us to compare with the real data and observations.
ZIERLER: Besides Mike Gurnis, who else did you work with at the Seismo Lab?
ZHONG: I learned from a lot of people. I would say Don Anderson and Hiroo Kanamori, I learned a great deal from them, of course. They're really great scientists. Then, of course, Don Helmberger and David Stevenson, it's just great to have all these people around.
ZIERLER: What was so useful about coffee hour, and hearing about what other people were working on?
ZHONG: I think, after Caltech, I visited so many different places. Some places, they also have coffee hours, but they're mostly for social. I think the Seismo Lab coffee hour is just different. That's really for science. You didn't have to bring coffee. You don't have to drink anything. [laugh] You just go there and sit and listen, and participate in the discussions about the science. Often, sometimes, you feel like, oh, one hour has gone. [laugh] But it's not an hour of waste. Really, that one hour is an hour of science. That's just so unique. I think I really miss that.
ZIERLER: Did you take advantage of JPL at all while you were at Caltech?
ZHONG: No, I guess not. No, not for me. [laugh] Even up to today, I don't think I've ever visited JPL. I think, at the time, I was still a Chinese citizen. I think there's one time I remember someone was going to JPL to give a talk, and then I would like to go. I think then they said, "OK, you have to apply for permission to get onto the campus." I said, "Oh, OK, forget it," [laugh] and so I didn't go. I never really visited JPL.
ZIERLER: Shijie, for the last part of our talk, just a few questions about how your experience at Caltech has stayed with you in your career. On that note, what are some of the things that you learned at Caltech in terms of the collaboration, in terms of the research, that really have guided the things that you've done since?
ZHONG: One thing that I feel like I benefited a lot from being at Caltech for a few years as a postdoc was those seminars; sometimes two, three seminars a day. Of course, I was a postdoc and had a lot of freedom. Those seminars were on all different kinds of topics: planetary, geology, everything. I just learned so much from them. That really got with me all these years. Sometimes, you say, "Oh, I feel familiar with this topic because of a seminar I went to [laugh] years ago at Caltech. I heard something about this." That's certainly something that I feel like was so valuable to me. It really helped me a lot. I think another is, just in general, at Caltech, the Seismo Lab people are friendly; a very friendly environment; easygoing. The senior professors were so easy to talk with. You just feel like, hey, this is really a great culture. It just feels so comfortable. [laugh] I always recommend the Seismo Lab to any junior people that I know if they ask me for opinions of where to study. I actually sent quite a few of my own PhD students to Caltech to the Seismo Lab as postdocs. I think three of my former students ended up at Caltech as postdocs, because I've just got so many good things to say about my own experience there.
ZIERLER: Shijie, I wonder if, by sending your postdocs to Caltech, that's been a way for you to stay connected to the Seismo Lab over the years?
ZHONG: Yes, sure. That's certainly true. Also, of course, I feel like the Seismo Lab would be a good experience for them as well.
ZIERLER: Finally, Shijie, for my last question, just to get a sense of what's been accomplished since the mid-1990s when you were at Caltech, what would you say were some of the big open questions in geophysics at that time that had been resolved or there's been major progress, and what questions might feel like there as open today as they were 25 or 30 years ago?
ZHONG: Well, that's really a good question. That depends on the whole geophysics field as a whole, or just really from my own perspective of geodynamics. Maybe I can say a little bit just from the dynamics of the Earth and the planets' kind of point of view. I would say, at the time, when I was there, the community was really interested in the overall question of the generation of plate tectonics, that is, why the Earth's got plate tectonics, and the other planets—at least at the moment—don't show plate tectonics. I think around the early 1990s when I was at Caltech, that people started to really ask that. I think now we have made quite some progress over the last, say, 20-some years. But I would say still we're not yet there, and this question is still there. But, certainly, we have made a lot of progress. I would say that's still an interesting and, yet, unresolved question.
Rheology and Plate Tectonics
ZIERLER: For you personally, what are the big projects that you want to focus on that you haven't yet?
ZHONG: I still wish I would understand better about the origin of the generation of plate tectonics. I'm still working on that problem with a student at this moment. I think it's unresolved, and from my own point of view, is that how to incorporate the realistic rheology, not the rheology of just mathematical equations. The realistic rheology can be constrained by the laboratory studies, and then also can be constrained by the field observations. I also think in these days about how the solid Earth interacts with the surface processes on problems like sea level change, environment, and the climate, I think there is still a lot of room, a lot of things to be learned there as well. To make geophysics and geodynamics even more societally relevant, I think, is the kind of questions that interest as well to me.
ZIERLER: Shijie, this has been a great talk. I'd like to thank you so much for spending this time with me.
ZHONG: Well, thank you too.