Radial S wave velocity structure of the D” region under a spot of the D" region under a spot of the east central Pacific Ocean

Abstract

The S wave radial velocity structure at the base of the mantle was constrained for an area of the east central Pacific Ocean centered at 19°N, 132°W. The decay constant technique was applied in the frequency domain to the S and Sdiff waves of an earthquake in the Tonga Islands recorded in the northeastern United States. A total of 29 different S wave velocity models were explored, including models with a velocity discontinuity and also the Preliminary Reference Earth Model (PREM). As determined from the fit to the decay constant in the frequency domain and to the waveforms in the time domain, PREM was considered the best model. This choice implies that D" at this location is 150 km thick and has a slightly negative velocity gradient with increasing depth. No velocity discontinuity was found at the top of D". The proposed velocity structure is consistent with D" acting as a thermal boundary layer and also with the possible existence of mantle enriched in iron through chemical reactions with the core. The preferred model also implies the existence of slightly elevated temperatures at the base of the mantle. The lack of a discontinuity is consistent with the absence of subduction at this location during the last 180 Myr.