Saturday, June 6, 2009

Comment on explosions and fault plane solutions

Image from Wikipedia

I haven't yet figured out how to make the Comments in this blog visible without having to first click on the Comments blog. If you know how to do so, please clue me in.

Anyway, good question from my last entry, if you didn't see it: "Is there any way you'd consider sharing a higher-resolution version of that figure (say, big enough for a PowerPoint) with me? I'd like to use if for educational purposes (specifically in explaining the foundations of beach ball diagrams)."

I usually provide links to the text and figures I shamelessly borrow from other sources. So the best resolution you're gonna get is to go back to the source I used. In many cases, you can get a better resolution of the figures I post by clicking on the figure itself, and a better copy will open in a new window. Of course, I'd encourage you to retain the reference to the source if you re-use the figures in your own presentations.

One of the classic ways to distinguish seismic waves emanating from earthquake vs. seismic waves original from blasts, including underground nuclear tests, is that the energy from explosions is all directed radially outwards, while the initial pulse of energy from earthquakes is compressional in some quadrants, but dilatational in others. The global distribution of these compressional and dilatational first motions of seismic waves can be used to infer the orientation of the fault plane on which the earthquake occurred, and also whether the sense of motion is normal, reverse, or strike slip. This kind of analysis is known as a first motion study, fault plane solution, or focal mechanism solution. So earthquakes and explosions will yield quite distinct fault plane solutions.

I found a couple of decent resources if you want to learn more about earthquakes and fault plane solutions. There is a very nice online tutorial from Arild Andresen, University of Oslo. The Wikipedia page on focal mechanism look ok, and has two links at the end to a pdf file with more technical material for the geologist, and a cool page from the University of Bristol which lets you construct your own fault plane diagram.

But for your classroom demonstration, you have to see if you can find a beach ball that has four quadrants, not six. Faulty analogy, perhaps?

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