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Geodynamics

Geodynamics

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

A belated happy new year!

A belated happy new year!

It was that time of the year again: holidays! Time to take a break from work, relax, and see all your friends and family again. The blog team is no different: we took a break from blogging for a little while as well, so you had to survive the holidays without us! Did you survive Christmas day without one of our blogposts? It must’ve been dreadful, I know, but that’s life! Luckily, we have some good news: we are back with some belated happy new year wishes and wintersport recommendations. We also tried to write limericks. Also also, we discuss chocolate and peppermint. Because we can. Cheers to a good blog year in 2019! 

Iris van Zelst

I once tried to ski down a slope
as friends thought there might be hope
I was covered in snow
from my head to my toe
If they invite me again it’s a ‘nope’

So, as many of you might have guessed, winter sports (or any sports, really) are not entirely my thing. Particularly skiing did not go down well for me. However, as a true Dutch girl, I do really enjoy ice skating and can recommend it thoroughly! However, this year no winter sports at all for me: I flew towards the sun in an effort to actually destress from work (feeble attempt as I brought my laptop, but still, kudos for trying, right?). I hope everyone had a very nice holiday and relaxing break. May all your (academic) wishes come true in 2019!

I also tried cross-country skiing once. That was infinitely better than alpine skiing. It was actually fun!

Grace Shephard

In hemispheric defiance of the “wintersport” edition, I am currently back Down Under where I have replaced the (seemingly eternal) television coverage of cross-country skiing with cricket, swapped a toboggan for me ‘togs’, and exchanged a pull-over for some ‘pluggers.’ I wish all of our blog readers a very happy and safe end to the year that was, and a fabulous start to the next!

What Aussies call swimming-related attire from bit.ly/AusWords

Anne Glerum

This year I spend winter in Berlin,
Where no snow has fallen and the ice is too thin.
So I drink myself heavy,
With hot chocolate and Pfeffi,
And wait for the fresh air of spring!

In the weeks before Christmas, Christmas markets dominate the streets of Berlin. Besides delicious food, they offer mulled wine and, as I discovered this year, hot chocolate with peppermintliqueur. A green version of the liqueur is made by Pfeffi, while a colorless Berlin-made peppermintliqueur is called Berliner Luft. It’s as clear and fresh as Berlin’s air according to the manufacturer. Although the freshness of Berlin’s air is debatable, the combination of chocolate and peppermint is delicious. I wish everybody a fresh start of the New Year with loads of hapiness!

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

Presentation skills – 2. Speech

Presentation skills – 2. Speech

Presenting: some people love it, some people hate it. I firmly place myself in the first category and apparently, this presentation joy translates itself into being a good – and confident – speaker. Over the years, quite a few people have asked me for my secrets to presenting (which – immediate full disclosure – I do not have) and this is the result: a running series on the EGU GD Blog that covers my own personal tips and experience in the hope that it will help someone (you?) become a better and – more importantly – more confident speaker. Last time, we discussed your presentation voice. In this second instalment, I discuss everything related to how you speak.

1. Get rid of ‘uh’

Counting the number of times a speaker says ‘uh’ during a presentation is a fun game, but ideally you would like your audience to focus on the non-uh segments of your talk. Therefore, getting rid of ‘uh’ (or any other filler word for that matter) is important. I have two main tips to get rid of ‘uh’:

Write down your speech and practice (but don’t hold on to it religiously)

Practice. Practice. And practice it again. Maybe a few more times. Almost… no: practice it again.
I am being serious here. If you know exactly what you want to say, you won’t hesitate and fill that moment of hesitation with a prolonged uuuuuhhh. The added benefit of writing down your presentation and practising it religiously is that it will help you with timing your presentation as well. I also find it helpful to read through it (instead of practising it out loud) when I am in a situation that doesn’t allow me to go into full presentation mode (on the plane to AGU for example). However, make sure to practise your presentation out loud even though you wrote it all down: thinking speed (or reading in your head) and talking speed are not the same!

If you write down your presentation, and you know exactly what you want to say, you have to take care to evade another (new) pitfall for saying ‘uh’: now that you know exactly what you want to say and how to say it most efficiently, you start saying ‘uh’ when you can’t remember the exact wording. Let it go. Writing down your speech helps you to clarify the vocabulary needed for your speech, but if you don’t say the exact sentences, just go with something else. You will have a well thought out speech anyway. Just go with the flow and try not to say ‘uh’.

The second main tip for getting rid of ‘uh’ is to

Realise that it is okay to stay silent for a while

If you forget the word you wanted to say and you need some time to think, you can take a break. You can stay silent. You don’t need to fill up the silence with ‘uh’. In fact, a break often seems more natural. Realise that you forgot something, don’t panic, take a breath, take a break (don’t eat a KitKat at this point in your presentation), and then continue when you know what to say again. Even if you don’t forget the exact words or phrasings, taking a breath and pausing in your narrative can be helpful for your audience to take a breath as well. It will seem as if your presentation is relaxed: you are not rushing through 50 slides in 12 minutes. You are prepared, you are in control, you can even take a break to take a breath.

2. Speed

A lot of (conference) presentations will have a fixed time. At the big conferences, like EGU and AGU, you get 12 minutes and not a second more or less. Well, of course you can talk longer than 12 minutes, but this will result in less (if any) time for questions.

I don’t think the conveners will kill you, but don’t pin me down on it

And on top of that, everyone (well, me at the very least) will be annoyed at you for not sticking to the time.

So: sticking to your time limit is important!

But how can you actually do this? Well, there are a few important factors:
1. Preparation: know exactly what you want to say (we will cover this more in a later instalment of this series)
2. The speed at which you speak.

We will be discussing the latter point in this blog entry. For me (and many other people), I know I can stick to the rule of “one slide per minute”, but I always have a little buffer in that I count the title slide as a slide as well. So, my 12-minute long presentation would have 12 slides in total (including the title slides). This actually spreads my 12 minutes over 11 scientific slides, so I can talk a little bit longer about each slide. It also gives me piece of mind to know that I have a bit of extra time. However, the speed at which you talk might be completely different. Therefore, the most important rule about timing your presentations is:

Knowing how fast you (will) speak

I always practice my short presentations a lot. If they are 30 minutes or longer, I like to freewheel with the one slide per minute rule. But for shorter presentations, I require a lot of practice. I always time every presentation attempt and make a point of finishing each attempt (even if the first part goes badly). Otherwise you run the risk of rehearsing the first part of your presentation very well, and kind of forgetting about the second part. When I time my presentation during practice, I always speak too long. For a 12 minute presentation, I usually end up at the 13.5 minute mark. However, I know that when I speak in front of an audience, I (subconsciously?) speed up my speech, so when I time 13.5 minutes, I know that my actual presentation will be a perfect 12 minutes.

The only way to figure out how you change or start to behave in front of an audience is by simply giving a lot of presentations. Try to do that and figure out whether you increase or decrease the speed of your speech during your talk. Take note and remember it for the next time you time your presentation. In the end, presenting with skill and confidence is all about knowing yourself.

3. Articulation and accent

There are as many accents to be heard at a conference as there are scientists talking. Everyone has there own accent, articulation, (presentation) voice, etc. This means that

You should not feel self-conscious about your accent

Some accents are stronger than others and may be more difficult for others to follow. Native speakers are by no means necessarily better speakers and depending on whom you ask, their accent might also not be better than anyone else’s.
Of course your accent might become an issue if people can’t understand you. You can try and consider the following things to make yourself understandable for a big audience:
1. Articulate well.
2. Adapt the speed at which you talk

Some languages are apparently faster than others. French is quite fast for example, whereas (British) English is a slower language. You have to take this into account when switching languages. If you match the pace of the language you are speaking, your accent will be less noticeable, because you avoid any ingrained rythm patterns that are language specific. Then you might still have your accent shine through in your pronunciation of the words, but it will not shine through in the rhythm of your speech.
In addition, you can consider asking a native speaker for help if you are unsure of how to pronounce certain words. Listening or watching many English/American/Australian tv series/films/youtube will also help with your pronunciation.

And that, ladies and gentlemen, is about everything I have to say on the matter of speech. You should now have full control over your presentation voice and all the actual words you are going to say. Next time, we go one step further and discuss your posture during the presentation and your movements.