青海玉树单台地震序列的尾波Q值研究

利用玉树单台记录到的2006年7月19日玉树Ms5．4级地震序列的数字地震波形资料，计算了不同频率下该序列尾波Q值的变化特征，为今后将数字地震资料应用到地震预报工作中提供了参考依据。     

Coda Q before and after the eruptions of 13 November 1985, and 1 September 1989, at Nevado del Ruiz Volcano, Colombia

 Coda Q^–1 was calculated at Nevado del Ruiz Volcano (NRV) before and after two phreatomagmatic eruptions (November 1985, September 1989) and for a period of stability (May 1987) using a functional form for coda derived from a single scattering model (Sato 1977). Substantial changes were found before and after the eruptions. The highest value of Q^–1 was found during the November 1985 eruption, an intermediate value for the September 1989 eruption, and the lowest value for May 1987. It seems that the changes in coda Q^–1 at NRV have a still-unknown relationship with the volume or magnitude of the eruption. A relatively strong frequency dependence was found for all periods. Also Q^–1 clearly changed with time, suggesting that the scattering was strong for the eruption of November 1985 and decreased for the eruption of September 1989, and that the intrinsic absorption probably increased. This suggests the possibility that crystallization is taking place in the NRV magma. The clear change of coda Q^–1 before and after the eruptions at NRV also suggests the possibility that coda Q^–1 is a premonitory tool of activity at this volcano. Received: 25 October 1996 / Accepted: 21 January 1998     

西昌地区地方震尾波衰减Q值

基于单次散射模型的尾波动功率谱分析法,利用西昌遥测台网地震波实时处理系统记录的２４个地震数字化波形资料,计算了西昌地区地球介质对应于１３个不同的ＱＣ值,在１．０－２０．０Ｈｚ频率范围内,以幂函数ＱＣ＝ＱＣｆ＾ｎ拟合ＱＣ值随的变化关系,其中Ｑ０值在４３．０－８２．４之间,ｎ值在０．２４－０．９４之间,平均值分别为６６．２和０．５３。     

利用超声尾波观测1.5 m长岩石断层黏滑实验的波速变化

在实验室内利用超声尾波观测大尺度(1.5 m)岩石断层的黏滑过程.利用基于尾波干涉的观测方法,我们获得了高达10~(-6)的相对波速变化的观测精度,这相当于~10 kPa的应力变化.利用高精度的测量,我们获取两种不同加载速率下(1μm·s~(-1),10μm·s~(-1))黏滑过程三个阶段(恢复、加载和滑动)基于波速变化的特征量.我们更进一步获取了断层失稳阶段波速变化的时空演化过程.最后讨论了该观测方法需要改进的地方.以上研究结果表明作为一种对现有实验观测手段的有益补充,利用超声尾波观测实验室大尺度岩石断层的动力学过程是可行的.     

南天山西段地震尾波Q值研究

基于新疆地震台网的数字地震波形资料,采用Aki的地震尾波单次散射模型,选取了南天山西段10个台站周围60km范围内M_S≥2.0的397次地震事件计算并得到了不同台站的尾波Qc值。结果显示,南天山西段尾波Q_0值平均值在140~390之间,其中喀什台和乌恰台Q_0值最低;频率相关系数η的平均值在0.62~1.00之间,拟合平均Q值与对应频率之间的关系结果为Q=261.2±110.8f~(0.80±0.28),表明该地区是以低Q_0高η值为特征的构造活跃区。     

单裂缝不同传播方向Krauklis波振幅变化的实验研究

裂缝广泛分布于地壳岩石中并具有多尺度的特点,研究地震波在裂缝介质中的传播规律具有重要的科学意义和应用价值.含流体的裂缝中传播的Krauklis波(也曾被称为慢流体波、裂缝波、斯通利导波等)近年来逐渐成为地球物理学界的研究热点之一.本研究利用地震物理模型技术研究横波在单裂缝样品中传播时随传播方向变化的响应特征,在实验室用环氧树脂为基质构建模型样品,一个为不含裂缝的参考样品,一个为含直径30 mm、厚度0.2 mm的薄币状单裂缝的模型,利用0.5 MHz换能器测试了横波在模型中传播时透射信号的特征.实验结果显示在单裂缝样品中透射横波发生了波形的畸变和振幅降低,振幅谱显示透射信号的主频明显降低,在初至横波之后为具有一定能量的尾波.对尾波进行时频分析,发现在方位角较小时尾波能量较弱而方位角较大时尾波能量较强,尾波中信号的主频在0.1 MHz左右,与前人理论和实验研究结果一致.     

Monitoring Contact State of Laboratory Fault with Coda Transmission Waves

We monitored the amplitude changes of coda transmission waves around 500kHz across the frictional interface of a simulated 1.5-meter-long fault during normal stress holding test. We find that the amplitude of coda transmission waves increases with the logarithm of stationary contact time. Localized increase amounted to a level ranging from 4% to 16% along the fault is observed during the 1-hour experiment. We discuss that the frictional strength at mesoscopic scale, which is related to the amplitude of coda transmission waves, is responsible for the phenomenon. Combining the reported method with other complementary approaches will enhance the understanding of fault mechanism either at laboratory or on-site applications.     

Scattering and intrinsic attenuation in Cairo metropolitan area using genetic algorithm

A total number of 46 local earthquakes (2.0≤ML≤4.0) recorded in the period 2000–2011 by the Egyptian seismographic network (ENSN) were used to estimate the total (Q_t⁻¹), intrinsic (Q_i⁻¹) and scattering attenuation (Q_sc⁻¹) in Cairo metropolitan area, Egypt. The multiple lapse time window analysis (MLTWA) under the assumption of multiple isotropic scattering with uniform distribution of scatters was firstly applied to estimate the pair of L_e⁻¹, the extinction length inverse, and B₀, the seismic albedo, in the frequency range 3–24 Hz. To take into account the effect of a depth-dependent earth model, the obtained values of B₀ and L_e⁻¹ were corrected for an earth structure characterized by a transparent upper mantle and a heterogeneous crust. The estimated values of Q_t⁻¹, Q_sc⁻¹ and Q_i⁻¹ exhibited frequency dependences. The average frequency-dependent relationships of attenuation characteristics estimated for the region are found to be: Q_t⁻¹=(0.015±0.008)f ^{(−1.02±0.02)}, Q_sc⁻¹=(0.006±0.001)f ^{(−1.01±0.02)}, and Q_i⁻¹=(0.009±0.008)f ^{(−1.03±0.02)}; showing a predominance of intrinsic absorption over scattering attenuation. This finding implies that the pore-fluid contents may have great effect on the attenuation mechanism in the upper crust where the River Nile is passing through the study area. The obtained results are comparable with those obtained in other tectonic regions.     

2010年广西凌云与凤山交界震群尾波Q值特征研究

2010年6月28日至8月初, 广西凌云与凤山交界地区出现一次罕见的震群活动。 利用震区附近8个近距离台站所记录的数字地震波形, 按照高信噪比和计算要求, 挑选出此次震群中74次M_L>1.5地震可用于计算地震尾波的共179条三分向波形记录, 采用Sato单次散射模型经滤波以及消除环境噪声后, 计算了地震尾波传播过程途经区的品质因子Q(f)值; 拟合Q(f)值与频率f之间的关系, 得到研究区Q(f)=(44.32±23.56)f^0.88±0.17; 并对震中区及其邻区地震尾波衰减的时间与空间特征进行了探讨。 通过分析震区及其附近尾波衰减参数与尾波采样体之间的关系, 认为该区域可能存在浅部高衰减层。 此次震群是在低应力水平条件下发生的, 震中区Q₀值较其他台站高, 其地震尾波衰减较慢, 介质非均匀性水平相对较低, 震中区为低衰减区域, 介质较为稳定。     

Inferred attenuation from site effect-correctedT phases recorded on the island of Hawaii

Coda site amplification factors are used to eliminate the site effect from records of three circum-PacificT phases recorded by the Hawaiian Volcano Observatory (HVO) network on the island of Hawaii. ObservedT-phase amplitudes across the island generally decrease with increasing distance from the conversion point where acoustic waves in the SOFAR channel become seismic waves propagating through the crust. However, the decay of the observedT-phase signal across the island is not regular in regions of dense station coverage, in particular, the Kilauea caldera region. We divide the maximum observedT-phase amplitudes at a given station by the coda site amplification factor obtained for the same station and frequency band (3.0Hz); the distribution of these amplitudes reveals a smooth pattern over the entire island. The distance over which the site effect-correctedT-phase amplitude decreases by one-half, combined with the apparent velocity of propagation ofT phases across the island, allows for an approximate determination of near-surfaceQ over much of the island of Hawaii. We found a region of lowQ in the Kilauea summit area (Q≈30) and east rift zone (Q≈60) with considerably higherQ in the Kaoiki and northern portions of the island (Q≈150 to 200). The lowQ values obtained in the Kilauea summit region and east rift zone are significantly lower than estimates of codaQ in the same region, suggestingT phases may be sampling the earth's near-surface properties.