中文摘要：

上地幔速度结构是影响Pn渡几何衰减特征的主要因素，本文利用合成理论地震图方法，研究了壳一幔过渡带速度梯度对Pn波几何衰减特征的影响．结果表明，Pn渡的衰减特征对上地幔的速度梯度十分敏感，如：1000km处Pn波的振幅，在相当于将均匀上地幔经展平变换后的等效速度梯度（O．0013s^-1）模型下，约为在半无限空间上叠加一层的简单模型下的10倍；当震中距大于某一距离后，具有速度梯度模型下的Pn波振幅随震中距衰减的速度明显小于无速度梯度的半无限空间模型下Pn波振幅的衰减速度，速度梯度越大，衰减速度越慢，在某一距离范围内，振幅甚至有增大的趋势；当存在速度梯度时，Pn波的几何衰减随频率而变化，上述结果对深入理解Pn波在区域震中距范围内的衰减规律提供了理论依据．

英文摘要：

Pn geometric spreading is a dominate reason for Pn attenuation. This report gives a theoretical study of the dependence of Pn geometric spreading on the velocity gradient in the upper mantle. The results show that Pn attenuation is very sensitive to the velocity gradient in the upper mantle. For example, the Pn amplitude at 1000 km is a factor of 10 larger for a model with a weak velocity gradient（equlvalent to the earth-flattening transformation of a homogeneous upper mantle） than a model consisting of layer over an infinite halfspace, and the Pn amplitude increases after a critical distance for linear gradient models. These results are similar to those of Sereno （1989）. Also, Pn geometric spreading is frequency-dependent for an upper mantle model with a simple linear velocity gradient. The results are important because most methods for estimating Pn attenuation require assumptions regarding geometric spreading.