Body wave amplitude patterns and upper mantle attenuation variations across North America
Body wave amplitude patterns and upper mantle attenuation variations across North America
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北美各地的体波振幅模式和上地幔衰减变化
DOI：
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发表时间：
1981
期刊：
影响因子：
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通讯作者：
D. Helmberger
中科院分区：
文献类型：
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作者：
T. Lay;D. Helmberger
总结。美国WWSSN台站记录的南美深部事件的P波和SH波的宽带幅度研究提供了对上地幔衰减变化的限制。由于源的复杂性、辐射模式和源结构，选择这些事件是为了将污染降至最低。短周期和长周期带的普遍特征是SH振幅与P振幅呈现相同的区域变化，短周期的区域变化相似，但相对于长周期的变化增强。东海岸和落基山脉各省之间的振幅具有一定的系统性区域差异，在墨西哥湾沿岸和中西部台站的短周期和长周期都有所增强，这可能是由于厚厚的沉积层接收器结构所致。由于个别台站的详细接收器结构知之甚少，我们使用了区域幅度模式是由上地幔衰减的横向变化引起的工作假设。同时考虑了常数Q和频率相关的衰减算符，用短周期和长周期频段的相对幅度和波形来估计常数Q模型中At*的可接受范围，或描述频率相关模型中吸收带高频滚落的参数r的可接受范围。时间域模拟表明，在1-10s周期范围内的短周期P和SH幅度和波形并不明确地要求随频率变化而衰减，从而允许源频率成分和长周期吸收带幅度的不确定性。短周期P波幅可在aT=0.5 S的范围内拟合，或在r的范围内(0.001~0.25s)拟合，任一单参数变化均符合一般的短周期SH波幅行为。用于确定适合于5-20s频段的t*和可能的r值的绝对值的技术被应用于所选数据。从深部地震到北美的传播路径的t*=0.8S和小7，到南部非洲台站的路径有较小的t*或较大的r1。长周期幅度变化不能用频率相关来解释
Summary. A broad band amplitude study of P- and SH-waves from deep South American events recorded at WWSSN stations in the United States provides constraints on upper mantle variations in attenuation. The events were selected to minimize contamination due to source complexity, radiation pattern, and source structure. The prevalent feature in the short- and longperiod bands is that the SH amplitudes show the same regional variations as the P amplitudes, with the regional variations of short periods being similar, but enhanced relative to the variations at long periods. The amplitudes show some systematic regional difference between the East Coast and Rocky Mountain provinces, and both short and long periods are enhanced at the Gulf Coast and midwestern stations, probably due to thick sedimentary bed receiver structure. Since detailed receiver structures for individual stations are poorly known, we used the working hypothesis that the regional amplitude patterns are caused by lateral variations in upper mantle attenuation. Both a constant Q and a frequency-dependent attenuation operator were considered, with the relative amplitudes and waveforms in short- and long-period bands used to estimate the acceptable range of At* in the constant Q model, or the acceptable range of the parameter r, , which describes the high-frequency roll-off of the absorption band in the frequency-dependent model. Time domain modelling indicates that the short-period P and SH amplitudes and waveforms in the 1-10s period range do not unambiguously demand frequency-dependent attenuation, allowing for the uncertainty in source frequency content and long-period absorption band amplitude. The short-period P amplitudes can be fit with a range in At; = 0.5 s or a range in r, from 0.001 to 0.25s, with either single parameter variation consistent with the general short-period SH amplitude behaviour. A technique for determining the absolute value of t* and possible r, values appropriate for the 5-20s band was applied to selected data. A value of t* = 0.8 s and small 7, were found for travel paths from the deep events to North America, and lower t* or large r,, values were found for paths to southern African stations. Longperiod amplitude variations cannot be explained by frequency-dependent