2. (8 pts). The travel-time curve for Pdiff (sometimes labeled just Pa), the P wave that diffracts along the core-mantle boundary, directly tells us the P-wave velocity at the base of

the mantle. The Pdiff travel-time curve is linear, with a ray parameter p = dT/dA = Temb/vcmb, where remb is the radius of the core-mantle boundary, and Vemb is the velocity at the base of the mantle. a) Measure the ray parameter (in s/degree) for the Pdiff phases shown in the record section of Figure P3.4 (attached). Pdiff is the one prominent phase seen on each seismogram. How does this value compare to the slope of the Pdiff travel-time curve in Figure 3.5-4? (Incidentally, I published a paper in 2004 using seismograms from these stations, which were part of MOMA, a PASSCAL experiment spanning the eastern US that predated EarthScope. Station CCM is in Missouri, and station HRV is in Massachusetts. My paper focused on surface waves, not core diffractions.) b) Convert the Pdiff ray parameter to s/radian, and find the velocity at the base of the mantle. c) Using values of dT/dA from Figure 3.5-4 for Pdiff and Sdiff, find the average ratio of P to S velocity (Vp/Vs) in the mantle. d) Compare the travel-times of PcP and ScS at zero epicentral distance for an earthquake at the surface (0 km depth). What is the average ratio of P to S velocity in the mantle (Vp/Vs)?