A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific
A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific
复制标题
太平洋下地幔的强负剪切速度梯度和横向变化
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
T. Lay
中科院分区：
文献类型：
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作者：
J. Ritsema;E. Garnero;T. Lay
介绍了一种约束核幔边界以上地震剪切速度结构的新方法，该方法同时模拟了SH-SKS走时差分、SV/SKS振幅比和Sdiff波型。利用北美记录的许多西南太平洋深源地震的数据，将这一方法应用于中太平洋以下的下地幔。我们分析了这种源-接收器几何结构的90个宽带和248个数字化模拟记录。SH-SKS时间变化性很强，比标准参考模型预报大10个S，表明研究区存在横向变化的低剪切速度。然而，走时并不限制低速区的深度范围或速度梯度。SV/SKS振幅比和SH波对径向剪切速度剖面很敏感，当与SH-SKS时间同时分析时，平均剪切速度降低3%，仅限于地幔最低190±50公里处。对于中东部太平洋地区(ML)，我们的首选模式有一个很强的负梯度(在2700公里深度，速度相对于初步参考地球模式(PREM)下降0.5%，在2891公里深度，速度下降3%)，而在2000-2700公里深度有轻微的速度下降(比PREM低0-0.5%)。需要显著的小尺度(100-500公里)剪切速度不均匀(0.5%-1%)来解释不同时间和幅度比中的散射。
A new approach for constraining the seismic shear velocity structure above the core-mantle boundary is introduced, whereby SH-SKS differential travel times, amplitude ratios of SV/SKS, and Sdiff waveshapes are simultaneously modeled. This procedure is applied to the lower mantle beneath the central Pacific using da.ta from numerous deep-focus southwest Pacific earthquakes recorded in North America. We analyze 90 broadband and 248 digitized analog recordings for this source-receiver geometry. SH-SKS times are highly variable and up to 10 s larger than standard reference model predictions, indicating the presence of laterally varying low shear velocities in the study area. The travel times, however, do not constrain the depth extent or velocity gradient of the low-velocity region. SV/SKS amplitude ratios and SH waveforms are sensitive to the radial shear velocity profile, and when analyzed simultaneously with SH-SKS times, rnveal up to 3% shear velocity reductions restricted to the lowermost 190±50 km of the mantle. Our preferred model for the central-eastern Pacific region (Ml) has a strong negative gradient (with 0.5% reduction in velocity relative to the preliminary reference Earth model (PREM) at 2700 km depth and 3% reduction at 2891 km depth) and slight velocity reductions from 2000 to 2700 km depth (0–0.5% lower than PREM). Significant small-scale (100–500 km) shear velocity heterogeneity (0.5%–1%) is required to explain scatter in the differential times and amplitude ratios.