On the nature of P n
On the nature of P n
复制标题
关于 P n 的性质
DOI：
10.1029/2000jb900061
复制
发表时间：
2000
影响因子：
--
通讯作者：
K. Fuchs
中科院分区：
文献类型：
--
作者：
M. Tittgemeyer;F. Wenzel;K. Fuchs
在许多折射地震剖面上都观察到Pn相，在地震记录中很常见。它们的速度通常在7.8至8.2公里每秒。用于解释这些观测结果的经典射线理论表明，上地幔速度梯度为正。然而，从岩石学和岩石物理资料来看，岩石圈地幔中没有广泛分布的正速度梯度。在高温和高压下的实验室速度测量表明，只有在非常低的热流值（≤40 mW m - 2）下，速度梯度才为正。较高的热流导致负梯度。因此，与观测结果相反，上地幔的岩石学模型将Pn观测限制在前寒武纪盾和旧台地。我们通过考虑包含随机速度波动叠加在正或负速度梯度上的介质来克服这一矛盾。在这两种情况下，这些结构通过广角散射波产生Pn相。仅0.5-1%的短波随机速度波动叠加在负速度梯度上就足以产生Pn相。因此，这意味着观测到的Pn波并不一定是正的上地幔速度梯度。对于橄榄岩上地幔，这种大小的波动可以用其矿物学成分相对比例的轻微变化来解释。各向异性很可能导致推断出的波动。
Pn phases are observed along many refraction seismic profiles and are common in earthquake records. Their velocities usually range from 7.8 to 8.2 km s−1. Classical ray theory used to interpret these observations implies a positive upper mantle velocity gradient. However, a wide spread positive velocity gradient in the lithospheric mantle is not expected from petrological and petrophysical data. Laboratory velocity measurements at elevated temparatures and pressures suggest positive velocity gradients only for very low heat flow values (≤40 mW m−2). Higher heat flow causes negative gradients. Consequently, petrological models of the upper mantle would restrict Pn observations to Precambrian shields and old platforms, contrary to observations. We overcome this contradiction by considering media that contain random velocity fluctuations superimposed on positive or negative velocity gradients. In both cases, these structures generate Pn phases by wide-angle scattered waves. Short-wavelength random velocity fluctuations of only 0.5–1% superimposed on negative velocity gradients are sufficient for generating Pn phases. Consequently, this implies that an observed Pn wave does not necessitate a positive upper mantle velocity gradient. For a peridotitic upper mantle, fluctuations of this size can be explained by slightly varying the relative proportions of its mineralogical constituents. Anisotropy is likely to contribute to the inferred fluctuations.
影响因子：
3.9
作者：
CHRISTENSEN, NI;MOONEY, WD
通讯作者：
MOONEY, WD