Bulk Sediment Qp and Qs in the Mississippi Embayment, Central United States
Bulk Sediment Qp and Qs in the Mississippi Embayment, Central United States
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DOI：
10.1785/0120050047
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发表时间：
2005-12
影响因子：
3
通讯作者：
C. Langston;P. Bodin;C. Powell;M. Withers;S. Horton;W. Mooney
中科院分区：
文献类型：
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作者：
C. Langston;P. Bodin;C. Powell;M. Withers;S. Horton;W. Mooney
我们利用爆炸P波和瑞利波的光谱距离衰减估计了美国中部密西西比河河口厚的松散沉积物的纵波和横波非弹性衰减系数。沉积物捕获的P波（Psed）在10赫兹下可观测到80公里的范围，在海湾北部5000磅的钻孔爆炸中可观测到130公里外的1赫兹瑞利波。4赫兹的瑞利波在距离3公里远的地方可以看到一个较小的50磅的爆炸。对这两种波的群速和幅距衰减的分析表明，对于与频率无关的围堰沉积物，Qp平均为100，Qp为200。Q估计中的散射来自多个p波混响和瑞利波模式的干扰。衰减模型是自洽的，因为它与利用推断出的q值对合成地震图进行分析得到的模型是一致的。推断出的Qp和Qs值比以前的估计高3倍以上，这意味着坝口的松散沉积物不会显著减弱小应变地震地面运动。这些估计代表了沉积物Q的下限，因为在波形数据中观察到明显的散射，这有助于波幅的距离衰减。较高的Q值还意味着，对于浅层地震或大地震在沉积物中破裂所辐射的表面波，堤防未固结的沉积物将形成有效的导波，产生高振幅、长持续时间的波列，这应在地震危险性评估中加以考虑。
We have estimated P-wave andS-wave anelastic attenuation coefficients for the thick, unconsolidated sediments of the Mississippi embayment, central United States, using the spectral distance decay of explosion P and Rayleigh waves. The sediment-trapped P wave, Psed, is observed to ranges of 80 km at 10 Hz, and 1-Hz Rayleigh waves are observed out to 130 km from a 5000-lb borehole explosion in the northern part of the embayment. Rayleigh waves of 4 Hz are seen to distances of 3 km from a smaller 50-lb explosion. Analysis of the group velocity and amplitude- distance decay of both waves yields an average Qs of 100 and Qp of 200 for embay- ment sediments that are independent of frequency. Scatter in the Q estimates comes from interference of multiple P-wave reverberations and Rayleigh-wave modes. The attenuation model is self-consistent in that it is the same as obtained by the analysis of synthetic seismograms using the inferred Q-values. Inferred Qp and Qs values are more than three times higher than previous estimates and imply that unconsolidated sediments of the embayment do not significantly attenuate small-strain earthquake ground motions. These estimates represent a lower bound to Q of the sediments since significant scattering is observed in the waveform data that contributes to the distance decay of wave amplitude. Higher Q values also imply that the unconsolidated sedi- ments of the embayment will form an efficient wave guide for surface waves radiated from shallow earthquakes or large earthquakes that rupture into the sediments, pro- ducing high-amplitude, long-duration wave trains that should be considered in earth- quake hazard assessments.