We use waveform inversion to infer high-resolution 3-D models of the S-velocity perturbation δV_S and the anisotropy parameter δξ in the lowermost 400 km of the mantle beneath the northern Pacific. Our inferred models show three prominent features: (i) a widespread positive δV_S layer with ~100 km thickness that exists ~250 km above the core-mantle boundary (CMB), which could be due to a bridgmanite to post-perovskite phase transition related to the D″ discontinuity; (ii) distinct positive -δV_S anomalies with positive δξ resolved from 100 km to more than 400 km above the CMB, which could be the subducted Izanagi, Farallon, and Telkhinia slabs; (iii) a vertically continuous low-δV_S anomaly with negative δξ at the edge of the subducted slab, which we interpret as an upwelling plume induced by slab sinking. We infer the Clapeyron slope of the post-perovskite phase transition in the lowermost mantle beneath the northern Pacific to be 10.2 ± 1.3 MPa/K, based on the δV_S gradient as a function of depth for the inferred 3-D V_S structure. Our estimated large positive Clapeyron slope at the CMB suggests vigorous convection in the lowermost mantle.

我们使用波形反演来推断北太平洋下方地幔最低 400 公里处的S 速度扰动δV _S和各向异性参数δξ的高分辨率 3-D 模型。我们推断的模型显示了三个突出的特征：（i）在核-地幔边界（CMB）上方约 250 公里处存在一个约 100 公里厚的广泛的正δV _S层，这可能是由于桥式锰矿到后钙钛矿相变与 D″不连续有关；(ii)具有正δξ 的明显正 - δV _S异常在CMB上方100公里至400多公里处解析，可能是俯冲的Izanagi、Farallon和Telkhinia板块；(iii)在俯冲板片边缘具有负δξ的垂直连续低δV _S异常，我们将其解释为由板片下沉引起的上涌羽流。我们根据推断的 3-D V _S结构的δV _S梯度作为深度的函数，推断北太平洋下方最下地幔中钙钛矿后相变的 Clapeyron 斜率为 10.2 ± 1.3 MPa/K 。我们估计在 CMB 的大正 Clapeyron 斜率表明最下层地幔有强烈的对流。