Earthquakes generate “earthquake sounds” (terminologically called
seismic waves) that travel though rocks and fluids. Seismic waves have
different velocities in different materials. By listening to the sound from the
inside Earth, seismologists tell where the “sound” is from and what kind of
materials it might have traveled through. And that gives us the information
about the Earth interior. Figure 1 shows us how fast the seismic wave travels
at different depth of the Earth.
Figure 1:
Seismic velocities in the Earth
If we zoom in the velocity profile, we can see a low velocity
zone (LVZ) around depth 100 km. Seismic-wave velocities rely on the property of
the material in which the wave propagates. Generally, the harder and denser the
material is, the faster the wave travels in it. As depth increases, the rock
density gets higher. So the question now is why there is a velocity decrease at
the LVZ. Many studies were done on this topic, and currently the most
convincing interpretation is partial melt. As depth increases, the temperature
also increases. At LVZ, part of the rocks starts melting, which generates fluids
that cause the seismic velocity anomaly.
Figure 2: At about
depth 100 km underground, there is a low velocity zone.
However, the details of how the melt shape and amount affect
the velocity are not totally clear. Geoscientists want to find a quantitative
relationship between the melt and the seismic velocity change. First, they
measured the shape of the melt using X-ray scanning in the lab. Figure 3 shows
the melt shapes in partially molten rock cubes at different degrees of molten. The next step, which hasn’t
been done yet, is to simulate the seismic wave propagation on the computer and
then compare the calculated velocities with our observations. Since we are not
able to dig into any depth of our Earth, a combination of the seismic methods
and rock physics analysis would be particularly helpful with exploring the
Earth interior.
Figure 3: Melt
distribution in partially molten rock samples
Figure1 and figure 2 are from wikipedia, figure 3 is from Miller, Kevin J., et al. "Experimental quantification of permeability of partially molten mantle rock." Earth and Planetary Science Letters 388 (2014): 273-282.
I really like the logical flow of ideas here as a science article, but the information about melt between the rock grains should be highlighted right up front in the blog. Also there is a great reliance on the use of scientific terms that most of the audience will not understand. We will work on this to make the language more accessible.
ReplyDeleteHi Jay, thanks for the tips! I just finished the new version with a new organization and less tech terms.
DeleteThis is good, Jiangyi. Nice explanation of the complicated terms in the beginning. I would suggest that you do it more throughout (terms like 'partial melt,' 'quantitative,' 'seismic methods')
ReplyDeleteI would suggest removing figure 1, and leaving figure 2 and 3 only. As figure 2 is enough information for your audience. Maybe put a picture to the side to show what the rock might look like, and give your audience a better understanding of that.
In figure 3, tell people that the gray part is the melt, and the white is solid rock.
Hi Mark, you are absolutely right about using simplest words. After consideration, I think it's reasonable to remove figure 1. I reorganized the logic of the essay and added a new picture showing what melt looks like! Thanks for your help!
DeleteGood job Jiangyi, and I appreciate the straightforward graphs and explanation of what they mean (although a few terms are still out there--like what the D" layer/CMB is). Overall, well-written, but a heavy focus on technical terms.
ReplyDeleteHey Tom, you're right that terms like D" layer/CMB could be confusing. Because the picture is from google, I just can't change it (maybe I can if I use PhotoShop, but I just don't want to do that...). What I did is telling the readers in the caption that our interests focus on LVZ and we don't need to care about D" or CMB. Thanks for your advice!
DeleteDo you have a citation for "Many studies were done on this topic, and currently the most convincing interpretation is partial melt.", or is it the conclusion of research you are part of?
ReplyDeleteHey Anthony, actually, I was thinking about whether I should mention some papers and authors in the essay. But since it's a science blog for the public, I just want to avoid that much professional stuff and be simple. But still thanks for pointing out this.
DeleteI think you did a really good job simplifying some tricky concepts in the first two paragraphs but you got kind of technical in the third. Take some more time to clarify some of the concepts you introduce in the third paragraph and add a little more explanation about the figure 3. It's a really cool figure and I think most readers would think so too if they completely understood what it was. Overall good job!
ReplyDeleteHi Al, you're right that I didn't explain some terminology or figure 3 very clearly. I give a more detailed and easy-understanding caption in the new version. Thanks for your help!
Deletethe third figure is the toughest to understand. Be sure to dumb it down and explain it as well as you did everything else in the first paragraph
ReplyDeleteHey James, thanks for pointing this! Figure 3 should be explained more clearly, including what the colors mean, what the scale is and at what fraction the melt is present. Those stuff are added to the caption in my new version. Thanks!
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