Seismic-refraction measurements in the Northwest Pacific Basin

Seismic-refraction measurements made on Shatsky rise in the Northwest Pacific basin show crustal layering appreciably different from that in the basin on either side. Beneath the crestal zone of the rise and its broad western flank at water depths less than 5 km, oceanic basement (layer 2) is about 3–6 km thick and contains two refracting layers, 4.7 and 5.5 km/sec, overlying the main crustal layer (layer 3) of velocity averaging 7.0 km/sec. Pronounced crustal thickening beneath the rise is associated mainly with a 7.3- to 7.8-km/sec layer intermediate between layer 3 and the upper mantle; the thickness of layer 3 is approximately normal. The depth to mantle is at least 22 km near the crestal zone and perhaps even greater below the crest where comparatively shorter range profiles did not determine it. Additional measurements between two large seamounts of the Emperor seamount chain, on the eastern flank of a low rise in the basin floor, also indicate two layers within layer 2, but with velocities different from those found beneath Shatsky rise. Here, layer 2 is 2–4 km thick and composed of material with velocities of 3.7 and 6.2 km/sec. Below these layers the crust seems normal, except that the average thickness of layer 3 is somewhat lower than regional average.