Observation of high-frequency teleseismicPnon the long-range Quartz profile across northern Eurasia

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 Pn phase from the phase diving deeper into the mantle lithosphere.