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G377 Lecture 7

1, the seismic record of the N. KoreaNuke test on 10/9/2006;

2, seismic wave energy partitioning atan interface

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Vernons preliminary interpretation:

I am guessing the first two phases are P phases, Possibly Pn and Pg. The Pn phase is traveling just

beneath the Moho at approx 8 km /sec, giving a distance from station MDJ of 424 km for the origin time

reported. The Pg phase, closely following Pn is traveling at approx 6-6.5 km/sec and is confined to a

crustal path. More intense scattering in the crust compared to the upper mantle makes the Pg coda a

little better developed and more complex than the Pn coda. Lg (multiply critically reflected SmS phases

in the crust or Rayleigh/Love modes confined to the crust wi ll arrive about 70 sec after Pn and appear

around 01:37, best developed on the North and South components of MDJ. Since the P phases look so

much better developed than Lg, this definitely looks like an explosion. The ratio Pg/Lg is commonly

used as a discriminant for explosions versus earthquakes, with Pg/Lg higher for explosions. There is

a suggestion of a longer period fundamental mode Rayleigh wave (Rg phase) around 01:37:20. The Rg

observation is not surprising for a near surface event, which should be good in exciting a fundamentalmode Rayleigh wave. The Rg phase should be visible at MDJ, provided there are no substantial

blocking structures between the event and the station, i.e., if the crust is relatively uniform in

thickness between N. Korea and MDJ.

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Rayleigh waves

Love waves(Image courtesy of European Center for

Geodynamics and Seismology)

Surface waveA surface wave is a seismic

seismic wave that is trappednear the surface of the earth.

Rayleigh waveA Rayleigh wave is a seismic

surface wave causing theground to shake in an ellipticalmotion, with no transverse, orperpendicular, motion.

Love waveA Love wave is a surface wavehaving a horizontal motion thatis transverse (or

perpendicular) to the directionthe wave is traveling.

http://earthquake.usgs.gov/image_glossary/seismic_wave.htmlhttp://earthquake.usgs.gov/image_glossary/seismic_wave.html

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Cartoon showing the basic concept of the Lg-wave simulation. Lg-wave is a

guided wave that can be trapped in the crustal wave-guide and propagated for

thousands of kilometers. It is a useful phase for investigating the seismic sources,as well as character the path effects. Various types of wave-guide features will

affect the propagation of the Lg-wave. Some of these features shown in this figure

including irregular topography, uneven Moho discontinuity, scatterings of various

scales through the wave-guide, intrinsic and scattering attenuations, etc. Elastic

screen method has been designed to handle Lg-wave propagations in realisticcrustal wave-guides.

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The Moment Tensor Representation of Seismic Sources

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(McCamy et al, 1962)

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Literature Reading:

Three important papers on the energy partitioning at an interface:

1, Keith McCamy, Robert P. Meyer, and Thomas J. Smith: Generally

applicable solutions of Zoeppritz' amplitude equations, BSSA, 52:923-955, 1962.

2, R. D. Tooley, T. W. Spencer, and H. F. Sagori, Reflection and

transmission of plane compressional waves, Geophysics, 30(4), 552-570, 1965.

3, L. R. Denham, On: Reflection and transmission of plane

compressional waves by R. D. Tooley, T. W. Spencer, and H. F.

Sagori, (Geophysics, 30, 552, April, 1965). Geophysics, 49, 2195,

1984. (a correction of Tooley et als 1965 paper of one figure).

The pdf files of these 3 papers are attached.