Seismology
is a powerful tool to image Earth interior structure because seismic waves can
travel through Earth at different depth and carry valuable information about
Earth’s structure. However, earthquakes
usually happen at active tectonic regions like plate boundaries which is a big
limit to our study regions. If we want to study a seismically quiet region, we
have to use teleseismic waves which will lose much high frequency information
because they are easily attenuated. Is there a way to study Earth interior
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, oceanic
micro-seismicity or wind. Ambient seismic noise is usually thought to be
useless in traditional seismology studies. However, recent studies prove that
surface waves and body waves can both be extracted from ambient noise by calculating
their cross-correlations, which means we can also use noise to study
Earth’s structure. When the ambient
noise travels through two different stations, the seismograms of these two
stations will have certain amount of coherence. The coherence can be retrieved
by calculating the cross-correlations of these two seismograms. The waveform of
the cross-correlations are similar to that of earthquakes because it also
contains different body wave phases and surface waves. Ambient noise is
independent from earthquakes so we can use seismograms recorded at any station,
at any time. It can largely increase our data source.
Ambient
seismic noise is a new tool to study Earth’s structure at different depth. Surface
wave is the most obvious waves extracted from ambient noise and it can be used
for high resolution tomography for crust. Moreover, some core phases like PKIKP2
can also be extracted so we can use them to study the anisotropy of Earth’s
core. This study used to be limited by the location of earthquakes and stations
because it requires to sample Earth from different azimuth. It only requires to
have stations at different latitude with ambient noise method. Ambient noise is
also popular at oil industry because the companies can save huge amount of
money without making any explosions for oil exploration. 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).
You may want to define PKIKP2 or just say core phases, or seismic waves that pass through the core. Try to avoid phrases like "Surface waves are the most obvious...". it may not be obvious to non-seismologists.
ReplyDeletePerhaps a definition of teleseismic, or get rid of this term? The distinction between of coherence and cross-correlation might not be essential for people to grasp the concepts you are discussing. The second paragraph would probably be confusing for someone without a seismology background. I really like the figures though, and the usefulness of this technique really shines through.
ReplyDeleteReading through your text, it seems there is a bit too much a presumption of greater knowledge than the average reader may have. An example: micro-seismicity. How many people who are not seimologists will be able to read this and understand the distinction? Be careful, and always ask who will understand this.
ReplyDeleteI think you assume too much background knowledge, for instance that earthquakes cause seismic waves or that the reader even knows what a tectonically active region is. Since this is for a general audience, maybe focus in on the societal aspect. You talk about how human activity causes ambient seismic noise, it might be more connectable if you directly link how driving a car can help us understand the structure of the Earth. Also, make it clear why we care about the structure of the Earth in regions where there isn't a lot of tectonic activity.
ReplyDeletethere are a few words that a normal person may not understand. I think it was a nice overview and well written for geology grad students.
ReplyDeleteI agree with those above. Alot of the language won't be accessible to the general audience. Try using a few more metaphors that the general public understands.
ReplyDeleteSame general commentary as above. It's an interesting topic, but even words like "attenuated" will lose most people. For writing to the general public, I'd recommend using a thesaurus to find synonyms for words that have more than 3 or 4 syllables.
ReplyDeleteThis is a good start, but needs to be simplified to avoid difficult scientific terms. I thought that your oral discussion was nice, although it takes a few minutes to comprehend your English, perhaps slow down a little bit and stop at the end of your sentences for emphasis and to give your audience time to understand what you just said.
ReplyDelete