Structure - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2005 by Jeffrey S. Barker) 3. Earth Structure In 1909, a Croatian meteorologist named Mohorovičić observed a precursor to the P wave on a network of seismographs (in those days, meteorological observatories doubled as seismological observatories). This was P n , the head wave that results from the P wave traveling at faster velocity beneath the base of the Crust. Thus, the boundary between the Crust (the outermost layer) and the faster layer underneath (the Mantle ) is called the Moho , which is short for the Mohorovičić discontinuity (a discontinuity is simply an abrupt change, in this case, in seismic velocity). On average, the crust of the Earth is about 30- 35 km thick, but can be as much 70 km beneath mountain ranges, and as little as 5-10 km beneath the oceans. To the right is an example of the kind of data Mohorovičić may have observed. This is from an explosion in Ontario, Canada, recorded on a line of seismometers. [Note that the time scale is "reduced". That is, each trace has been shifted downward in time according to the seismometer's distance from the explosion.] The direct P wave is labeled P, the reflection from the Moho is labeled P M P, and the head wave is labeled P n . Around the 1910s, Beno Gutenberg (at Caltech) and others began observing that the initial P waves in seismograms seemed to fade out at distances beyond around 103°. They could be seen again at about 142°, but delayed in time. We can see this in the travel-time plots of modern data. GEOL 114 1 3. Earth Structure From Barker, Somerville and McLaren, Modeling of Ground- Motion Attenuation in Eastern North America, Electric Power Research Institute report, 1988.
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Recall that waves traveling from faster into slower materials are refracted (the rays are bent) toward vertical. In the 3-dimensional Earth, this results in a lens-like focusing of the waves toward the opposite end, with a "doughnut"-shaped shadow The lack of P waves observed from 103-142° seems to correspond to this doughnut- shaped shadow. In fact, we call this lack of observed P waves the P-wave Shadow Zone . Gutenberg's explanation was that it indicates a lower-velocity material at depth in the Earth. He calculated the depth of this change as about 2900 km, and named the portion inside the Core and the part outside the Mantle . Notice in the observed travel-time data above that the P waves, which reappear beyond 142°, are delayed relative to those before 103 0 . This also suggests that they have passed through some low- velocity material. GEOL 114
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Structure - GEOL 114 The Earth's Dynamic Interior Lecture...

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