Separation of high‐frequency P N phases and mantle refracted P phases at distances between 6° and 18° in the western Pacific by ocean bottom seismograph array

Observations of high‐frequency (HF) Pn phases by an oceanbottom seismometer (OBS) array were made at distances between 6° and 18° in the north‐western Pacific. At this distance range, mantle‐refracted P phases arrive earlier than HF‐Pn phases. These two phases are successfully separated for near surface earthquakes by applying appropriate filtering, since the dominant frequencies of HF‐Pn phases are higher than 6 Hz and the frequencies of mantle‐refracted P phases are rather low, about 3 Hz. The apparent velocities of HF‐Pn phases in this distance range are estimated to be from 8.3 to 8.1 km/sec or even lower, while those of mantle‐refracted P waves are from 8.4 to 8.6 km/sec. The reported value of 8.33 km/sec for Hf‐Pn phases appears to be an average of both HF‐Pn and mantle‐refracted P phases. Therefore, these observations suggest that a proposed additional HF‐Pn wave guide for the higher velocity of 8.33 km/sec is questionable.