When
you look at the sky at a starry night, you might be curious about the stars in
the space. But when you look at the ground, have you ever been wondering how
the unseen underground world would look like? Seismology is a powerful tool to
help us discover the invisible underground world because seismic waves can
travel through Earth interior at different depth and tell us valuable
information about Earth’s structure. However, earthquakes usually happen at
active tectonic regions like the plate boundaries in Japan. What if we want to
study the underground structure of a intra-plate region that is far away from
plate boundaries? We don’t want to waste our time waiting a couple of years for
an earthquake to happen. Is there a way to image underground structure without
earthquakes? Yes, it is ambient seismic noise!
Figure 1. Ubiquitous ambient noise recorded by two stations (R. Weaver, 2005) .
Ambient
seismic noise is ubiquitous ground motions excited by human activities, ocean
bottom seismicities or wind. Ambient seismic noise is usually thought to be
useless in traditional seismology studies. However, recent studies prove that
seismic waves like surface wave and body wave can be extracted from ambient
noise, which means we can also use noise to replace an earthquake to look into Earth’s
interior. When the ambient noise travels through two different stations, the
seismograms of these two stations will have certain amount of coherence, which
can be retrieved by calculating their cross-correlations. The waveform of the
cross-correlations are similar to that of an earthquake and it can represent
the seismic waves traveling from one station to the other. We can take an
“X-ray photo” of Earth by digging into the noise.
Ambient
seismic noise is not dependent on earthquakes so we can image underground
structures anytime and anywhere we want. Moreover, the seismic waves extracted
from ambient noise contain enough high frequency information to make a high
resolution image of underground structure. Surface wave is sensitive to shallow
structures so we can use it to study velocity structure of the crust. Body wave
is more sensitive to deep structure so we can use it study the mantle and core
structure. Ambient noise is also popular in oil industry because it doesn’t
require an active source like explosion or strike by truck. What you only need
to do is to deploy an array of seismometers and keep listening to noise. It can
save lots of money for oil industry companies. Ambient noise is a huge
supplement to traditional seismology study because it breaks the time and space
limits of earthquakes and offers a more flexible way to image Earth interior
structure.
Figure 3. Surface wave tomography from ambient noise (Shapiro et al., 2005).
This is a good start on your blog and generally accessible. I would work more on integrating the images with the text.
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ReplyDeleteOn the one hand, you seem to have improved your speaking ability from your graduate talk last May. Good job. You could still work on your transitions though, since you seem to jump to much between disparate topics. That is also a comment on your text as well, as the comment relevant for both.
ReplyDeleteI agree with Jay, especially for figure 3. It is a great image, which captures the attention of the reader but you don't discuss it.
ReplyDeleteI liked some of the analogies you used to relate seismic methods to everyday things (like going to the dentist). However, I still think there is some jargon you could simplify.
ReplyDeleteGreat improvement! There's still some technical terms, like cross-correlation and coherence that most readers may not understand, but your figures are helpful.
ReplyDeleteYou improved some of the language to make it more accessible. The way you have it now would be a good introduction to say a beginning seismology or geology class.
ReplyDeleteAfter your personal introduction, you do a nice job of introducing seismology and what it can be used for. Again, I don't think you explicitly link seismic waves to earthquakes; however, overall I think this improved greatly.
ReplyDeleteFigure 1: How can those arrows represent the noise? Are they vectors for the direction and magnitude of the movement?
Figure 3: How does velocity relate to structure?
The first figure is just a cartoon so the arrows just represent how random the noise is on the ground. The direction and magnitude are not in the real scale.
DeleteDifferent rocks have different seismic velocities. In the third figure, sedimentary rocks have a lower velocity and volcanic rocks have a higher velocity so that's how we distinguish them on the map.
I really like the introduction to the topic relating to xray imagery. Cross-correlation may need a quick definition, but I think the graphical representation in your figures helps a lot.
ReplyDeleteI like how you starting with asking people if we want to know the structure of underground, It makes the audience feels engaged. Maybe I had missed it during your presentation, but I feel you need to expalin the term like the cross-correlation more. Over all, great presentation.
ReplyDeleteThis is so much easier for me to understand as someone who know's nothing about seismology. Much, much improved!
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