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Tomography and plate tectonics

Tomography and plate tectonics

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to Professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our first ‘Geodynamics 101’ post for 2019, Assistant Prof. Jonny Wu from the University of Houston explains how to delve into the subduction record via seismic tomography and presents some fascinating 3D workflow images with which to test an identified oceanic slab. 

Jonny Wu, U. Houston

Tomography… wait, isn’t that what happens in your CAT scan? Although the general public might associate tomography with medical imaging, Earth scientists are well aware that ‘seismic tomography’ has enabled us to peer deeper, and with more clarity, into the Earth’s interior (Fig. 1). What are some of the ways we can download and display tomography to inform our scientific discoveries? Why has seismic tomography been a valuable tool for plate reconstructions? And what are some new approaches for incorporating seismic tomography within plate tectonic models?

Figure 1: Tomographic transect across the East Asian mantle under the Eurasian-South China Sea margin, the Philippine Sea and the western Pacific from Wu and Suppe (2018). The displayed tomography is the MITP08 global P-wave model (Li et al., 2008).

Downloading and displaying seismic tomography

Seismic tomography is a technique for imaging the Earth’s interior in 3-D using seismic waves. For complete beginners, IRIS (Incorporated Research Institutions for Seismology) has an excellent introduction that compares seismic tomography to medical CT scans.

A dizzying number of new, high quality seismic tomographic models are being published every year. For example, the IRIS EMC-EarthModels catalogue  currently contains 64 diverse tomographic models that cover most of the Earth, from global to regional scales. From my personal count, at least seven of these models have been added in the past half year – about one new model a month. Aside from the IRIS catalog, a plethora of other tomographic models are also publicly-available from journal data suppositories, personal webpages, or by an e-mail request to the author.

Downloading a tomographic model is just the first step. If one does not have access to custom workflows and scripts to display tomography, consider visiting an online tomography viewer. I have listed a few of these websites at the end of this blog post. Of these websites, a personal favourite of mine is the Hades Underworld Explorer built by Douwe van Hinsbergen and colleagues at Utrecht University, which uses a familiar Google Maps user interface. By simply dragging a left and right pin on the map, a user can display a global tomographic section in real time. The displayed tomographic section can be displayed in either a polar or Cartesian view and exported to a .svg file. Another tool I have found useful are tomographic ‘vote maps’, which provide indications of lower mantle slab imaging robustness by comparison of multiple tomographic models (Shephard et al., 2017). Vote maps can be downloaded from the original paper above or from the SubMachine website (Hosseini et al. (2018); see more in the website list below).

Using tomography for plate tectonic reconstructions

Tomography has played an increasing role in plate tectonic studies over the past decades. A major reason is because classical plate tectonic inputs (e.g. seafloor magnetic anomalies, palaeomagnetism, magmatism, geology) are independent from the seismological inputs for tomographic images. This means that tomography can be used to augment or test classic plate reconstructions in a relatively independent fashion. For example, classical plate tectonic models can be tested by searching tomography for slab-like anomalies below or near predicted subduction zone locations. These ‘conventional’ plate modelling workflows have challenges at convergent margins, however, when the geological record has been significantly destroyed from subduction. In these cases, the plate modeller is forced to describe details of past plate kinematics using an overly sparse geological record.

Figure 2: Tomographic plate modeling workflow proposed by Wu et al. (2016). The final plate model in c) is fully-kinematic and makes testable geological predictions for magmatic histories, terrane paleolatitudes and other geology (e.g. collisions) that can be compared against the remnant geology in d), which are relatively independent.

A ‘tomographic plate modelling’ workflow (Fig. 2) was proposed by Wu et al. (2016) that essentially reversed the conventional plate modelling workflow. In this method, slabs are mapped from tomography and unfolded (i.e. retro-deformed) (Fig. 2a). The unfolded slabs are then populated into a seafloor spreading-based global plate model. Plate motions are assigned in a hierarchical fashion depending on available kinematic constraints (Fig. 2b). The plate modelling will result in either a single unique plate reconstruction, or several families of possible plate models (Fig. 2c). The final plate models (Fig. 2c) are fully-kinematic and make testable geological predictions for magmatic histories, palaeolatitudes and other geological events (e.g. collisions). These predictions can then be systematically compared against remnant geology (Fig. 2d), which are independent from the tomographic inputs (Fig. 2a).

The proposed 3D slab mapping workflow of Wu et al. (2016) assumed that the most robust feature of tomographic slabs is likely the slab center. The slab mapping workflow involved manual picking of a mid-slab ‘curve’ along hundreds (and sometimes thousands!) of variably oriented 2D cross-sections using software GOCAD (Figs. 3a, b). A 3-D triangulated mid-slab surface is then constructed from the mid-slab curves (Fig. 3c). Inspired by 3D seismic interpretation techniques from petroleum geoscience, the tomographic velocities can be extracted along the mid-slab surface for further tectonic analysis (Fig. 3d).


Figure 3: Slab unfolding workflow proposed by Wu et al. (2016) shown for the subducted Ryukyu slab along the northern Philippine Sea plate. The displayed tomography in a), d) and e) is from the MITP08 global P-wave model (Li et al., 2008).

For relatively undeformed upper mantle slabs, a pre-subduction slab size and shape can be estimated by unfolding the mid-slab surface to a spherical Earth model, minimizing distortions and changes to surface area (Fig. 3e). Interestingly, the slab unfolding algorithm can also be applied to shoe design, where there is a need to flatten shoe materials to build cut patterns (Bennis et al., 1991).  The three-dimensional slab mapping within GOCAD allows a self-consistent 3-D Earth model of the mapped slabs to be developed and maintained. This had advantages for East Asia (Wu et al., 2016), where many slabs have apparently subducted in close proximity to each other (Fig. 1).

Web resources for displaying tomography

Hades Underworld Explorer : http://www.atlas-of-the-underworld.org/hades-underworld-explorer/

Seismic Tomography Globe : http://dagik.org/misc/gst/user-guide/index.html

SubMachine : https://www.earth.ox.ac.uk/~smachine/cgi/index.php

 

References

Bennis, C., Vezien, J.-M., Iglesias, G., 1991. Piecewise surface flattening for non-distorted texture mapping. Proceedings of the 18th annual conference on Computer graphics and interactive techniques 25, 237-246.

Hosseini, K. , Matthews, K. J., Sigloch, K. , Shephard, G. E., Domeier, M. and Tsekhmistrenko, M., 2018. SubMachine: Web-Based tools for exploring seismic tomography and other models of Earth's deep interior. Geochemistry, Geophysics, Geosystems, 19. 

Li, C., van der Hilst, R.D., Engdahl, E.R., Burdick, S., 2008. A new global model for P wave speed variations in Earth's mantle. Geochemistry, Geophysics, Geosystems 9, Q05018.

Shephard, G.E., Matthews, K.J., Hosseini, K., Domeier, M., 2017. On the consistency of seismically imaged lower mantle slabs. Scientific Reports 7, 10976.

Wu, J., Suppe, J., 2018. Proto-South China Sea Plate Tectonics Using Subducted Slab Constraints from Tomography. Journal of Earth Science 29, 1304-1318.

Wu, J., Suppe, J., Lu, R., Kanda, R., 2016. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods. Journal of Geophysical Research: Solid Earth 121, 4670-4741

Get conference ready!

Get conference ready!

It’s almost time for the AGU fall meeting 2018! Are you ready? Have you prepared your schedule and set up all your important business meetings? Here are some final tips to nail your presentation and/or poster!

Nailing your presentation
The art of the 15-minute talk: how to design the best 15-minute talk
Presentation skills – 1. Voice: how to get the most out of your presentation voice
Presentation skills – 2. Speech: how to stop staying ‘uh’

Making the best poster
Poster presentation tips: how to design the best poster layout
The rainbow colour map (repeatedly) considered harmful: how to make the best scientific figures

The art of the 15-minute talk

The art of the 15-minute talk

We’ve all attended conferences with those dreaded 15-minute talks and we have no problem picking out which talks were amazing and which talks were abysmal. However, when it comes to our own talks, it’s hard to judge them, find out how they can be improved or break away from long-established habits (such as our layout or talking pace). This week, Matthew Herman, postdoc at the Tectonophysics research group at Utrecht University in the Netherlands, guides you towards your best 15-minute talk yet!

At some point in your career as an Earth Scientist, you will hopefully have a chance to give a 15-minute talk at a meeting, a colloquium series, or simply in your lab group. This provides a great opportunity to advertise your hard work to your colleagues in an amount of time that is well within a human attention span. Ultimately, your goal in this talk is to effectively communicate your discovery to your audience. In the process, you get to explain the importance of your field, pose a crucial research question in that field, demonstrate cutting-edge analyses and applications, and, finally, provide an answer to that initial research question, sometimes for the very first time.

Despite all the latent potential for a 15-minute talk to captivate and teach the audience, many of these presentations end up being uninformative. I do not intend this as a judgment regarding the significance or quality of the science. I have seen incomprehensible talks from people whose research is crucial to our understanding of the Earth system. Alternatively, I have seen talks presenting incremental scientific advancements that were truly enlightening. But from all the diverse presentations I have seen, there are common elements that either dramatically improved or reduced my understanding of the subject matter. My aim here is to provide what I think are some of these key characteristics that make up a really excellent talk, so that next time you have the opportunity to present, you will inspire your audience.

I think there are two general things to keep in mind for your 15-minute talk: (a) you have limited time with your audience, and (b) the expertise of your audience can vary a lot. This means that you should design a presentation that fits your extensive understanding into a brief window and tailor the details for the particular audience that will be attending. If this makes it seem like it will take time and effort to construct an effective talk, that is because it is true! Even if you have a well-received publication, simply transferring figures, analyses, and interpretations from the paper into your talk is almost guaranteed to lead to an ineffective presentation – it will probably be too long, too technical, and too difficult to see from the back of the room. If you really want your audience to concentrate on your work for the full 15 minutes, take the time (potentially up to a few weeks) to craft a great talk. And one more thing: you really should practice your talk ahead of time. Actually, I cannot emphasize this point enough: PRACTICE.

Note: If you are short on time right now, I have included a checklist at the end to summarize the main points.

How long?

Imagine: you are in the audience and the end of the talk is not in sight. You shift in your seat uncomfortably as you glance at your watch. The speaker does not appear to notice the amount of time since they started, but you definitely do: 14:30… 15:00… 15:30. Finally, two full minutes after the end of the scheduled time slot, the speaker asks if there are any questions, but of course there is no time for that. Many otherwise good talks have been ruined for me by the presenters going into overtime. All I can now remember about them is by how much they exceeded the final bell. As a speaker, you have 15 minutes – choose a topic and present it in the allotted time frame. In fact, target your talk for 12-13 minutes so your audience can ask questions at the end.

This, and that, and these…

The detailed structure of the talk is flexible, but should probably contain the following items: background/motivation (Why should we, the audience, care?); a research question or hypothesis (What is being tested?); observations, models, and analysis (How is the research question being answered?); and interpretations and conclusions (GIVE US THE ANSWERS!).

i. Background
Try to avoid dwelling on the background for too long. I know many of us (myself included) enjoy pedantically explaining the rich history of our field leading up to the present day. But you do not have the time in a 15-minute talk. As you are constructing your presentation, you should budget no more than 2-3 minutes at the start to establish the context for your research problem. At that point, your audience should be oriented and ready to be amazed by your results.

Example of an introduction/background slide

ii. Research Question
Do not assume that your research question or hypothesis is obvious to everyone. People come to talks for a lot of different reasons; sometimes they are experts in the field, but other times they saw a keyword in your title or abstract, or maybe there were no other interesting sessions. In any case, it is likely that a good percentage of your audience does not know what specifically you are testing if you do not tell them. After setting up the background, verbally or on the screen state your research question or hypothesis.

iii. Observations, Models, and Analysis
This will be the bulk of your presentation. Tailoring your 15-minute talk for your specific audience means you will want to use just the right amount of technical terminology. You should assume some foundational level of knowledge because there is no way to define every term and present the complete theory for your research. But for the most part, I think you should try to minimize technical jargon (particularly uncommon acronyms) in talks. If and when you need to use a term repeatedly, then take 15-30 of your precious seconds to concisely explain the concept, ideally without patronizing or condescending. [Did I mention this was a difficult balance?] Incidentally, explaining a concept has the added benefit of forcing you to understand the concept sufficiently that you can distill its definition into a compact form for your listeners.

The precise minimum level of knowledge you assume for your audience depends on the setting. In the large lecture hall of an international meeting like the EGU General Assembly, the audience may be weighted towards less experience in your field, whereas a special meeting focused on your subject area will likely have a higher percentage of experts.

A related point is that you should avoid all but the most straightforward equations. The reality is that any audience member who does not already understand the equation is not going to understand it from your talk. There is not enough time, and the medium is not amenable to higher level math. Simple equations with a couple variables are okay, but anything with multiple terms, powers, derivatives, etc. are a waste of time.

iv. Interpretations and Conclusions
Honestly, most people are pretty good at this part. This is the most fun and exciting aspect of the talk, plus it means the end is near. A couple minor pieces of advice: (a) make sure you have drawn a clear path from the background through the analyses and into the interpretations, with the common thread being answering your research question; and (b) I think it is best to limit the number of conclusions to 3-4 (consider this in the preliminary design stage of the talk as well!).

Example of a results slide

Good looks matter

I try to follow the advice of the great Jim Henson when it comes to designing the look for my talks: “Simple is good.” I will not harp on making figures, because many other people have discussed how to design good ones. In a nutshell: make them big, use good color schemes and large fonts, and keep them uncluttered. Resist the urge to copy figures straight from papers to your talk. You will probably need to simplify a figure from the published version in order to make it optimal for your talk. Sometimes you just need to design and produce a totally new figure. In fact, making figures is where I spend at least 65% of my time when I am preparing a talk.

In terms of slide layout, use the whole slide. Borders, icons, and backgrounds can be pretty flourishes, but they take up valuable real estate. Every centimeter you use for a border is a centimeter you can no longer use for a making a figure nice and big. And remember there will be people, some with poor eyesight, in the back of the room. As on figures, limit the amount of text. When you do need labels or bullet points, use a classic, simple font (I will scream if I see Comic Sans one more time…) in a large size – I typically use no smaller than 24-point font Helvetica.

Closing remarks

Many of my suggestions are more like guidelines than hard rules. I enjoy seeing creative and innovative presentations. As long as you give yourself enough time to craft an excellent presentation, then take time to practice it in front of friends, it will turn out well. Hopefully we will all see a large collection of great talks in the next few meetings. See you there!


Checklist
Remember: the goal of the talk is for your audience to understand your science!

Preparation
• Take time (up to several weeks) to construct your presentation
• Practice before the date of your talk, if possible in front of a test audience

Structure
• Target talk length for 12-13 minutes (do not go over 14!)
• Limit background or introductory information to 2-3 minutes
• Explicitly state research question
• Link background, analysis, and interpretations to research question
• Limit conclusions to ≤ 4

Scientific Content
• Choose technical jargon at level appropriate for audience
• Define critical terminology in 30 seconds or less
• Limit acronyms
• Avoid complex equations
• Avoid tables

Visual Content
• Fill space on slide, especially with figures
• Make thin frames to not waste precious room
• Choose large font sizes (≥ 24 pt) in a standard font
• Adjust figures from published version
• Check figure color contrasts (avoid blue/black, yellow/white)
• Use perceptually linear color palettes (no rainbow!)
• Avoid cartoons, animations, and sounds

General Life Advice
• Use common sense (e.g., do not include pictures from the bar in your talk)

The quest of a numerical modelling hero

The quest of a numerical modelling hero

Numerical modelling is not always a walk in the park. In fact, it resembles a heroic quest more often than not. In this month’s Wit & Wisdom post, Cedric Thieulot, assistant professor at the Mantle dynamics & theoretical geophysics group at Utrecht University in The Netherlands, tells the story of his heroic quest to save the princess from the dragon clear a code from bugs and shows that failed models can be the best models.

Heroes are also artists. I am a hero, therefore I am an artist. Sometimes against my will. In other words, sometimes the code works; most of the time it doesn’t.

A true hero embarks on his noble steed upon a long and perilous quest as the fire-breathing dragon who keeps the princess hostage awaits him in its lair.
“I am a hero too!” says my programmer ego although I spend most of my time sitting on ikea chairs looking for bugs. Yes, bugs. Bugs I have put there myself. Yup.

On my quest, I sometimes get lost in impossible mazes!

I have to cross mysterious mountain ranges

… but I am rewarded by a beautiful sunset on another planet.

I can’t believe what I C(++)

My quest can be dangerous.
Sometimes the enemy is tiny but viruses can be deadly too!

I sometimes feel like I am drowning in a petri dish.


Sometimes I encounter weird beings on my quest…



I even have to fight improbable gnu-snakes!

And the spirits of old viking warriors creep up in my models…

I need some candy to keep my spirits up.

And then… I find the bug and defeat the mighty bug! Fireworks!


Time to switch on the disco balls!

It’s party time!