Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States

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Low-velocity zone atop the 410-km seismic
discontinuity in the northwestern United States

• Teh-Ru Alex Song, Don V. Helmberger and Stephen
  P. Grand
• Nature 427, 530-533, 2004

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Outline

• Introductionmotivationsstudy area
• Methods and Resultsupper-mantle-distance
  waveformsmodelling results
• Discussion and Conclusions

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410-km seismic discontinuity

• Due to the phase change from a- to b-olivine.
• Topography due to temperature and the positive
  Clapeyron slope of the phase change.
• Most previous studies use 410-SS-precursors that
  only give large scale (1000 km) variations.
• The receiver function method can reveal short
  length scale (100-300 km) variations.
• The trade-off between velocity and topography is
  a problem.
• This study combines 410 refracted waves and
  scattered waves (receiver functions).

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Profile A
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Profile B
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Synthetics at dists of 21-23 degs
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Profile C, D, E, and F (21-24 degs)
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Receiver functions
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Discussion and Conclusions

• A LVZ on top of the 410. Vs drops 5 and
  thickness varies from 20 to 90 km with rapid
  lateral variation.
• The area extends from southwestern Oregon to the
  northern BR.
• The spatial extent coincides with an anomalous
  compositions of overlying volcanism
  (low-potassium, high-alumina olivine theoleiites
  or HAOT), which is linked to extensional
  tectonics in the back-arc regime.
• The LVZ is believed to be a compositional anomaly
  (not thermal).
• Possibly due to partial melt and water content
  linked to prior subduction of the Farallon plate .