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Variation of Seismic Coda Wave Attenuation in the Garhwal Region, Northwestern Himalaya

机译：喜马拉雅西北部加尔瓦尔地区地震尾波衰减的变化

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摘要

Seismic coda wave attenuation ( $ Q_{text{c}}^{ - 1} $ ) characteristics in the Garhwal region, northwestern Himalaya is studied using 113 short-period, vertical component seismic observations from local events with hypocentral distance less than 250 km and magnitude range between 1.0 to 4.0. They are located mainly in the vicinity of the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT), which are well-defined tectonic discontinuities in the Himalayas. Coda wave attenuation ( $ Q_{text{c}}^{ - 1} $ ) is estimated using the single isotropic scattering method at central frequencies 1.5, 3, 5, 7, 9, 12, 16, 20, 24 and 28 Hz using several starting lapse times and coda window lengths for the analysis. Results show that the ( $ Q_{text{c}}^{ - 1} $ ) values are frequency dependent in the considered frequency range, and they fit the frequency power law ( $ Q_{text{c}}^{ - 1} left( f right) = Q_{0}^{ - 1} f^{ - n} $ ). The Q 0 (Q c at 1 Hz) estimates vary from about 50 for a 10 s lapse time and 10 s window length, to about 350 for a 60 s lapse time and 60 s window length combination. The exponent of the frequency dependence law, n ranges from 1.2 to 0.7; however, it is greater than 0.8, in general, which correlates well with the values obtained in other seismically and tectonically active and highly heterogeneous regions. The attenuation in the Garhwal region is found to be lower than the Q c −1 values obtained for other seismically active regions of the world; however, it is comparable to other regions of India. The spatial variation of coda attenuation indicates that the level of heterogeneity decreases with increasing depth. The variation of coda attenuation has been estimated for different lapse time and window length combinations to observe the effect with depth and it indicates that the upper lithosphere is more active seismically as compared to the lower lithosphere and the heterogeneity decreases with increasing depth.
机译：喜马拉雅西北部Garhwal地区的地震尾波衰减（$ Q_ {text {c}} ^ {-1} $）特征是通过使用113个短周期垂直分量地震观测来研究的，该观测来自下震中心距小于250 km的局部事件幅度范围在1.0到4.0之间。它们主要位于喜马拉雅山脉中构造良好的不连续性主边界推力（MBT）和主中央推力（MCT）附近。使用单一各向同性散射法在中心频率1.5、3、5、7、9、12、16、20、24和28 Hz处估计尾波衰减（$ Q_ {text {c}} ^ {-1} $）使用几个开始时间和尾气窗口长度进行分析。结果表明，（$ Q_ {text {c}} ^ {-1} $）值在所考虑的频率范围内与频率相关，并且符合频率幂律（$ Q_ {text {c}} ^ {-1 } left（f right）= Q_ {0} ^ {-1} f ^ {-n} $）。 Q 0 （在1 Hz处的Q c ）估计值从10秒的间隔时间和10秒的窗口长度的大约50变化到60秒的间隔时间和60s窗口长度的组合大约350的变化。频率依赖性定律的指数n在1.2到0.7之间。但是，它通常大于0.8，这与在其他地震活动和构造活动以及高度非均质地区获得的值具有很好的相关性。发现Garhwal地区的衰减低于世界其他地震活跃地区获得的Q c -1 值。但是，它可与印度其他地区媲美。尾声衰减的空间变化表明，异质性水平随深度的增加而降低。已估算了不同时间间隔和窗口长度组合的尾声衰减变化，以观察深度的影响，这表明与下部岩石层相比，上部岩石层在地震活动上更为活跃，并且非均质性随深度增加而降低。

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