jl

The seismogram above is vertical earth motion from the 1992 May 21 Lop Nor
PRC 700-1800 kiloton underground nuclear test. It was recorded from a
short period borehole seismometer at 748 meters depth at the OGS Observatory
near Leonard. A direct P arival plus reflected PP and PKKP are marked.
 
The diagram of the earth's cross section below shows the ray paths for the
different P arrivals at Leonard, Oklahoma. The North Geographic Pole is at the
top of the cross section. The pole happens to lie almost at the midpoint of
the great circle path from Lop Nor to Leonard, which is 102.8 great circle
degrees, or 11420 kilometers, or 7100 miles long (i.e., along the earth's
surface, not along the ray paths).

The asterisk on the left represents Lop Nor, The triangle on the right 
represents Leonard. The P wave is the direct wave through the mantle, which
just grazes the core. PP has two P segments through the mantle with one 
reflection at the earths surface, in this instance near the earth's Geographic
North Pole.

The PKKP wave travels through the mantle, is refracted into the outer core,
is than reflected from the mantle-core boundary and passes back through the
outer core, and the mantle. Travel times from the blast are about 13 minutes
and 59.9 seconds for P, 18 minutes and 12.9 seconds for PP, and about 31 
minutes for PKKP. There may be a second branch of PKKP just after the one
marked PKKPdf.

jl

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