Structure under Mount Rainier, Washington, inferred from teleseismic body waves

TLDR

Teleseismic long‑period P waves recorded at the LON station show anomalous particle motion that cannot be attributed to instrument miscalibration or malfunction. By rotating horizontal P waves into the ray direction, deconvolving to remove instrument response, and modeling the data with a dipping high‑contrast interface at 15–20 km depth, the authors explain the observed azimuthally varying tangential component, while Sp arrivals from deep teleseisms reveal a deeper interface at ~145 km depth. The tangential waves attain amplitudes comparable to the radial component, exhibit antisymmetry about a NNE azimuth, and the dipping interface model accounts for most long‑period P‑wave characteristics, though later arrivals suggest additional multiples or scattering, and the deeper interface likely corresponds to the bottom of the upper‑mantle low‑velocity zone.

Abstract

Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.