Gross errors in upper‐mantle discontinuity topography from underside reflection data
Gross errors in upper‐mantle discontinuity topography from underside reflection data
复制标题
来自底面反射数据的上地幔不连续地形的粗略误差
DOI：
10.1111/j.1365-246x.1997.tb00949.x
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发表时间：
1997
影响因子：
2.8
通讯作者：
John Van Decar
中科院分区：
文献类型：
--
作者：
F. Neele;Han de Regt;John Van Decar
摘要
由于使用几何光学将观测到的旅行时与反射点附近不连续面的深度变化联系起来，由长周期底面反射(PDP或SDS波)的走时变化得到的上地幔不连续地形的当前模型可能包含粗略误差。如果不连续面深度的变化在横向尺度上小于数据中菲涅尔区的大小，则几何光学的使用是无效的。几何光学没有考虑走时面底反射的大尺寸和复杂结构，在用于反演时可能会引入虚假结构。俯冲岩石圈附近反射的合成长周期PDP波形的例子表明，来自预期的板片内部不连续的小尺度深度变化的散射导致复杂的波形变化。对于15-20s的PDP波，这些小尺度深度变化(尺度长度为4)小于菲涅耳区(约20°)，但大于420公里深度(约150-200公里)的PDP波长。合成波形被处理以获得PDP走时，而PDP走时又利用几何光学转换为反射点的视不连续深度。表面不连续深度变化不仅与从实际数据获得的变化具有相同的量级，而且还显示出与当前模型中观察到的特征非常相似的伪影。结果表明，俯冲带附近670公里不连续面的大尺度(波长约1000公里)向下翘曲可以解释为俯冲板块内部和附近的单个小尺度(波长数百公里)的不连续面的长波偏转。
SUMMARY
Currnet models of upper-mantle discontinuity topography derived from traveltime variations of long-period underside reflections (PdP or SdS waves) may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point. The use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the Fresnel zone in the data. Geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions. Examples of synthetic long-period PdP waveforms for reflections near subducting lithosphere show that scattering from expected small-scale depth variations of the discontinuities inside slabs causes complex waveform variations. For a 15–20s PdP wave these small-scale depth variations (with a scale-length of 4) are smaller than the Fresnel zone (roughly 20°) but larger than the PdP wavelength at 420 km depth (about 150–200 km). The synthetic waveforms are processed to obtain PdP traveltimes, which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics. The apparent discontinuity-depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models. It is shown that large-scale (wavelengths of the order of 1000 km) downwarping of the 670-km discontinuity near subduction zones derived from underside reflection data can be explained as the long-wavelength manifestation of a single, small-scale (wavelengths of several hundreds of kilometres) deflection of the discontinuity inside and near subducting slabs.