Step 1: Nomenclature.

Figure 1. Stacks of SS (left) and PP (right) precursors derived from over 30,000 long-period seismograms. The SS and PP reference phases are aligned at zero time. Amplitudes are shown relative to the reference phase, with positive amplitudes in blue and negative amplitudes in red. The precursors are underside reflections off the 410- and 660-km discontinuities that arrive several minutes before the main phase. Shearer and Flanagan, Science.

This lab, Rays through the Earth, is largely based on Chapter 4 of Introduction to Seismology by Peter Shearer. Therefore, you will probably need to read this chapter and review your lecture notes to be able to answer all the questions.

The different layers in the Earth, such as, crust, mantle, outer core, inner core, and the two different types of body wave types (P and S), results in a large number of possible ray geometries. Each ray geometry results in a distinct arrival and such arrivals are called seismic phases. Seismologists use a standardized set of names for the different phases. In short:
P:P wave in the mantle
K:P wave in the outer core
I:P wave in the inner core
S:S wave in the mantle
J:S wave in the inner core
c:reflection off the core-mantle boundary (CMB)
i:reflection off the inner-core boundary (ICB)
Review section 4.8 of your book, Introduction to Seismology by Peter M. Shearer, or your lecture notes, to figure out how the naming scheme works. Then anwer the following questions:

1. Draw a cross section through the Earth and indicate the ray paths of SS, ScSScS, PP, PcPPcP, PKP, P410P, and S660S. Explain why P410P and S660S arrive before the main phases PP and SS, respectively.

2. Start program Seismic Wave (Start->Programs->voorlichting->Seismic Wave). Set off the August 1999 Turkey earthquake and follow the wave fronts. The figure on the left shows the projections of the wave front on the surface, the figure on the right shows a cross section through the Earth. At the top you can see the seismogram, the recordings of the different phases. Run the program several times and try to follow individual phases. How does the core influence the wave field?



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