【摘要】 PKP震相包含了下地幔底部P波速度结构的重要信息.中国地震台网(CSN)台站记录到的南美洲地震的PKP波的射线,对中太平洋下的D″层有很好的采样.本研究采用这些PKP波的AB和DF两个分支的走时差,研究了中太平洋下地幔底部P波速度的小尺度变化.AB-DF的走时差减小了上地幔横向不均匀性的影响,而对下地幔底部P波速度的横向变化十分敏感.与此同时,AB-DF的走时差也减小了地震定位误差的影响,消除地震发震时间测定误差的影响.本研究的结果表明,在中太平洋的地幔底部存在着大范围的AB-DF走时正残差,也即低速异常区,这可能是太平洋下超大地幔热柱的源处.观测到的P波速度异常的空间分布总体上与Grand通过层析成像得到的CMB的S波的速度异常相一致,并在变化的幅度上很好地相关,P波速度的扰动值(在D″层大约为2%)是Grand速度模型中的S波异常的36%.这一结果有助于太平洋下超大地幔热柱的结构和性质的进一步研究.更多还原

【Abstract】 Seismic PKP arrivals are sensitive to P velocity structure in the lowermost mantle.Earthquakes in South America recorded at China Seismographic Network(CSN) provide excellent samples of the lowermost mantle underneath the Central Pacific.Here we use differential travels between the AB and DF branches of PKP waves to constrain the small-scale variation of P waves beneath the Central Pacific.The use of differential AB-DF times reduces the biases of errors of earthquake locations and origin time and heterogeneity of the upper mantle but is sensitive to the structure in the lowermost mantle.Our results show large positive AB-DF differential time residuals across large areas of the lowermost mantle beneath the central Pacific,which may indicate the source region of the "Pacific Super Plume".The observed PKP residuals consist with the predictions for Grand’s tomographic S model.Our inferred P velocity variation(which is about 2% in the bottommost 200 km D″ layer) is about 36% of the S velocity variation.However,we observe significant variations in travel-time residuals both inside the Central Pacific Plume and near its boundaries.The results have implications for understanding the structure and nature of the Pacific Super Plume.更多还原