Wednesday, October 11, 2006

 

More recordings of nuclear blasts

From the Oklahoma Geological Survey:

A 700-1800 kiloton blast from 1992. The largest underground blast since 1976 according to that site. The station was 11420 km from the blast.



For comparison the 1995 M6.9 Kobe earthquake (10600 km from the station and 22 km deep). (note that this seismogram is ~1 hr long while the seismogram from the nuclear blast is only ~20 minutes long).



As I learned in the comments from thread at No Se Nada a sesimogram from a blast is dominated by P-waves. I can certainly see the lack of S waves in the recording from the nuclear blast (assuming they were cut from the plot).

Here's an explanation from LLNL:

"Livermore seismologist Bill Walter explains that the differences in seismic P- and S-wave energy provide one method of discriminating explosions from earthquakes. Seismic P waves are compressional waves, similar to sound waves in the air. Shear (S) waves are transverse waves, like those that propagate along a rope when one end is shaken. Because underground explosions are spherically symmetric disturbances, they radiate seismic P waves efficiently. In contrast, earthquakes result from sliding or rupture along a buried fault surface and strongly excite the transverse motions of S waves. Thus, we expect that explosions will show strong P waves and weak S waves and that earthquakes will show weak P waves and strong S waves, as seen in Figure 2."


Here's the figure referred to in that quote:



The LLNL page also has a good discussion of the difficulties in inferring blast yields from seismograms.

UPDATE: I forgot to give a link to the LLNL page.

UPDATE: Lab Lemming has a couplenice posts on this topic.

UPDATE: Posts about the North Korea nuclear test at WG:

Radioactive material detected from North Korea

Guestblog: Teachable Moment: North Korea nuclear test

More recordings of nuclear blasts

North Korea nuclear test

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