Empirical attenuation of ground-motion spectral amplitudes in southeastern Canada and the northeastern United States
Empirical attenuation of ground-motion spectral amplitudes in southeastern Canada and the northeastern United States
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DOI：
10.1785/0120030175
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发表时间：
2004-06-01
影响因子：
3
通讯作者：
Atkinson, GM
中科院分区：
文献类型：
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作者：
Atkinson, GM
利用1990 ~ 2003年发生在加拿大东南部和美国东北部的186次m（N）2.5-5.6级地震的1700个数字地震图，建立了一个地震数据库。对数据库进行最大似然回归分析，以确定剪切窗口、运动的垂直和水平分量、频率为0.2至20 Hz的傅立叶频谱振幅衰减模型。傅立叶振幅遵循铰链三线性衰减模型。在距震源70 km范围内，傅立叶谱振幅衰减为R-1.3（R为震源距）。从70到140公里有一个过渡区，因为直达波被强的临界后反射加入，在那里衰减被描述为R+0.2;在这个范围内，对于低频，频谱振幅实际上随着距离的增加而增加。在140公里以外，衰减可用R-0.5很好地描述，对应于二维的几何扩展。对于大于1 Hz的频率，区域品质因数的相关模型可以表示为Q = 893 f（032）。在较宽的频率范围（0.2-20 Hz）内，Q可以通过多项式表达式更好地建模：log Q = 3.052 - 0.393 log f + 0.945（log f）（2）- 0.327（log f）（3）。多项式表达式适应Q值在接近1Hz处最小（约1000）并且在较低和较高频率处上升的观察。利用已知震源深度的地震子组，得到了描述震源深度对振幅及其衰减影响的谱振幅模型的校正因子，衰减模型与早期研究（Atkinson和Mereu，1992）中用有限数据确定的衰减模型相似，但扩大的数据库表明近源振幅衰减更快，Q值更高。衰减模型用于回放衰减效应，以确定数据库中每个地震的视震源谱，从而确定矩震级（M）和Brune应力降。这些事件的矩震级在2.5到5之间。对于M的事件，应力降随力矩大小而增加
A database of 1700 digital seismograms from 186 earthquakes of magnitude m(N) 2.5-5.6 that occurred in southeastern Canada and the northeastern United States from 1990 to 2003 was compiled. Maximum-likelihood regression analysis of the database was performed to determine a model for the attenuation of Fourier spectral amplitudes for the shear window, for the vertical and horizontal component of motion, for frequencies from 0.2 to 20 Hz. Fourier amplitudes follow a hinged trilinear attenuation model. Fourier spectral amplitudes decay as R-1.3 (where R is hypocentral distance) within 70 km of the source. There is a transition zone from 70 to 140 km as the direct waves are joined by strong postcritical reflections, where the attenuation is described as R+0.2; spectral amplitudes actually increase with distance in this range, for low frequencies. Beyond 140 km, the attenuation is well described by R-0.5, corresponding to geometric spreading in two dimensions. The associated model for the regional quality factor for frequencies greater than 1 Hz can be expressed as Q = 893f(032). Q can be better modeled over a wider frequency range (0.2-20 Hz) by a polynomial expression: log Q = 3.052 - 0.393 log f + 0.945 (log f)(2) - 0.327 (log f)(3). The polynomial expression accommodates the observation that Q values are at a minimum (about 1000) near 1 Hz and rise at both lower and higher frequencies. Correction factors for the spectral amplitude model that describe the effects of focal depth on the amplitudes and their attenuation are developed using the subset of events with known focal depth.The attenuation model is similar to that determined from an earlier study with more limited data (Atkinson and Mereu, 1992), but the enlarged database indicates more rapid near-source amplitude decay and higher Q. The attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database and hence determine moment magnitude (M) and Brune stress drop. The events have moment magnitude in the range from 2.5 to 5. Stress drop increases with moment magnitude for events of M