Abstract
Short-period, three-component recordings of the seismic wave field
of Peaceful Nuclear Explosions (PNEs) on long-range profiles are
used to determine the fine structure of the mantle lithosphere. By
analyzing the frequency content of the recorded phases and applying
different band-pass filters to the data, it is possible to divide
the wave field into two distinctly different constituents: low-frequency
body waves traveling along Fermat paths (first arrivals) and the
high-frequency teleseismic (or long-range) Pn phase traveling with
a group velocity of 8.1 km/s and accompanied by a long, incoherent
coda. This high-frequency teleseismic Pn phase is observable from
about 750 km, where it separates from the faster first arrival, to
the maximum recording distance of 3145 km. It is recorded from shots
at different locations and appears to be almost unaffected by the
major tectonic feature along the profile, the Urals. The frequency
spectrum of this Pn phase contains more high-frequency energy (up
to 12 Hz) than first arrivals that penetrate deeper into the upper
mantle. The teleseismic high-frequency Pn arrival has a remarkable
coda, which is incoherent between closely spaced stations. The coda
duration is dependent on the component of motion, being shortest
on the vertical and longest on the transverse component. We propose
a velocity model that is characterized by a zone extending from the
crust-mantle boundary to a depth of about 100 km. This zone has randomly
distributed, spatially anisotropic velocity fluctuations. We propose
that these velocity heterogeneities are stretched in the horizontal
direction. This zone forms a scattering waveguide that confines the
high-frequency teleseismic Pn. There are indications that below this
Pn waveguide, either the scale of the velocity fluctuations or the
Q factor changes. This is expressed in a separation of the teleseismic
P n phase from the phase diving deeper into the mantle lithosphere.
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