Propagation of Lg waves in the North Australian Craton: Influence of crustal velocity gradients
Propagation of Lg waves in the North Australian Craton: Influence of crustal velocity gradients
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发表时间：
1991-04
影响因子：
3
通讯作者：
J. R. Bowman;B. Kennett
中科院分区：
文献类型：
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作者：
J. R. Bowman;B. Kennett
摘要澳大利亚北领地Tennant Creek附近三次大地震的余震为研究北澳大利亚克拉通的区域地震相提供了一个独特的机会。三台便携式数字地震仪在震源区附近运行，以控制地震位置，七台记录仪在田纳特克里克和南部430公里处的爱丽斯斯普林斯地震台阵之间形成了一条线。在主震后两个月的一周部署期间，记录了11次足够大的地震以供分析。沿着这条3.7公里/秒的剖面观测到的LG波的群速度处于正常范围的高端，适合于稳定的大陆内部。我们通过测量作为距震源距离的函数的幅度衰减，并假定几何扩展的常见r−5/6关系，来估计离散频带中的衰减。然而，这一假设导致了与频率相关的Q值：Q=(230±36)f(0.66±0.12)，对于稳定的大陆地区来说，这些值是异常低的。表观Q与其他观测结果也不一致，例如高LG群速度、利用标准衰减关系对澳大利亚其他地区进行的M L估计的正偏差、澳大利亚中部大地震的有感区域大小，以及在我们的剖面附近收集的地震反射数据的高频含量。根据地震折射工作得出的一些澳大利亚北部的地壳模型显示，在30到55公里之间有一个梯度带，而不是尖锐的莫霍面。因此，我们利用用波数积分方法计算的合成地震记录，研究了壳幔转变处的梯度带对Lg振幅衰减的影响。模拟结果表明，存在陡峭下界的地壳波导层中的部分S波能量可以在有梯度的情况下泄漏出去，从而增强衰减。此外，由于梯度区焦散的形成，也存在聚焦效应，这不能用单一的解析振幅校正来表示。
Abstract Aftershocks of three large earthquakes near Tennant Creek in the Northern Territory of Australia provide a unique opportunity to study regional seismic phases in the North Australian Craton. Three portable digital seismographs were operated near the source zone to provide control on earthquake locations, and seven recorders formed a line between Tennant Creek and the seismic array at Alice Springs, which is 430 km to the south. Eleven earthquakes of sufficient size for analysis were recorded during a one week deployment two months following the main shocks. The group velocity observed for Lg waves along this profile of 3.7 km / sec is at the high end of the normal range and appropriate for a stable continental interior. We estimate attenuation in discrete frequency bands by measuring the decay of amplitude as a function of distance from the source and assuming the common r −5/6 relationship for geometrical spreading. However, this assumption leads to frequency dependent Q values: Q = (230 ± 36) f (0.66 ± 0.12) that are abnormally low for a stable continental region. The apparent Q is also not consistent with other observations, such as the high Lg group velocity, the positive bias in M L estimates obtained for other parts of Australia using standard attenuation relations, the size of felt areas for large central Australian earthquakes, nor with the high frequency content of seismic reflection data collected near our profile. Some crustal models for northern Australia derived from seismic refraction work show a gradient zone between 30 and 55 km rather than a sharp Moho discontinuity. We therefore investigate the effect of gradient zones at the crust-mantle transition on the decay of Lg amplitudes using synthetic seismograms calculated with the wavenumber integral method. The modeling suggests that some of the S -wave energy that would be trapped in a crustal wave guide with a sharp lower boundary can leak out in the presence of a gradient and so enhance the attenuation. In addition, there are focusing effects due to the formation of caustics from the gradient zones which cannot be represented by a single analytic amplitude correction.