Attenuation of coda waves and Q_c value beneath the Chengdu telemetered seismic network

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Seismic attenuation in Faroe Islands basalts

We analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave‐induced fluid flow within pores and micro‐cracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure.     

1985年云南禄劝地震余震尾波 Q 值的某些研究

建立在用来解释地方震和区域地震的尾波散射模型的基础上,用云南短周期区域台网12个台站的 VGK 地震仪记录的68个禄劝地震的余震序列尾波观测资料,分六个时间段估算了尾波 Q 值.发现在所观测的频率范围内(0.40——1.65Hz),Q 值与频率有关,估算的 Q 值在80——240之间,频率相关值=0.45.对表示散射强度的波源因子 B(fp)的估算结果多数在10-23——10-24量级,低 Q 值地区表现出激发强度高.值得注意的是禄劝地震前后,离震中区较近的台站所测得的 Q 值发生了明显变化,有的变化达2.0倍以上.Q 值随时间的区域变化,这在地震预报中也许有重要的实际意义.此外,测定了余震震源参数.对震源参数间的定量关系的讨论结果给出:IgE=1.59ML+11.335;E=(2.1010-5M0;在3.0ML5Pa,一个显著的特征是应力降与地震震级不存在明显的线性关系.      

A Summary of Attenuation Measurements from Borehole Recordings of Earthquakes: The 10 Hz Transition Problem

—Earthquake seismograms recorded by instruments in deep boreholes have low levels of background noise and wide signal bandwidth. They have been used to extend our knowledge of crustal attenuation both in the near-surface and at seismogenic depths. Site effects are of major importance to seismic hazard estimation, and the comparison of surface, shallow and deep recordings allows direct determination of the attenuation in the near-surface. All studies to date have found that Q is very low in the near-surface (～ 10 in the upper 100 m), and increases rapidly with depth. Unlike site amplification, attenuation at shallow depths exhibits little dependence on rock-type. These observations are consistent with the opening of fractures under decreasing lithostatic pressure being the principal cause of the severe near-surface attenuation. Seismograms recorded in deep boreholes are relatively unaffected by near-surface effects, and thus can be used to measure crustal attenuation to higher frequencies (≥ 100 Hz) than surface recordings. Studies using both direct and coda waves recorded at over 2 km depth find Q to be high (～ 1000) at seismogenic depths in California, increasing only weakly with frequency between 10 and 100 Hz. Intrinsic attenuation appears to be the dominant mechanism. These observations contrast with those of the rapidly increasing Q with frequency determined from surface studies in the frequency range 1 to 10 Hz. Further work is necessary to constrain the factors responsible for this apparent change in the frequency dependence of Q, but it is clearly unwise to extrapolate Q estimates made below about 10 Hz to higher frequencies.     

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

We investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective Q_S/Q_P ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small.     

Crustal attenuation characteristics of S-waves beneath the Eastern Tohoku region,Japan

An inversion method was applied to crustal earthquakes dataset to find S-wave attenuation characteristics beneath the Eastern Tohoku region of Japan.Accelerograms from 85 shallow crustal earthquakes up to25 km depth and magnitude range between 3.5 and 5.5were analyzed to estimate the seismic quality factor Q_s. A homogeneous attenuation model Q_s for the wave propagation path was evaluated from spectral amplitudes, at 24 different frequencies between 0.5 and 20 Hz by using generalized inversion technique. To do this, non-parametric attenuation functions were calculated to observe spectral amplitude decay with hypocentral distance. Then, these functions were parameterized to estimate Q_s. It was found that in Eastern Tohoku region, the Q_s frequency dependence can be approximated with the function 33 f~(1.22) within a frequency range between 0.5 and 20 Hz. However,the frequency dependence of Q_s in the frequency range between 0.5 and 6 Hz is best approximated by Q_s(f) = 36f~(0.94) showing relatively weaker frequency dependence as compared to the relation Q_s(f) = 6 f~(2.09) for the frequency range between 6 and 15 Hz. These results could be used to estimate source and site parameters for seismic hazard assessment in the region.     

Attenuation of coda waves in northern Greece

The single scattering model has been applied for the estimation of codaQ values for local earthquakes that occurred in northern Greece during the period 1983–1989 and recorded by the telemetered network of the Geophysical Laboratory of the University of Thessaloniki. CodaQ estimations were made for four frequency bands centered at 1.5 Hz, 3.0 Hz, 6.0 Hz and 12.0 Hz and for the lapse time windows 10–20 sec, 15–30 sec, 20–45 sec, 30–60 sec and 50–100 sec. The codaQ values obtained show a clear frequency dependence of the formQ _c =Q ₀ f ⁿ , whileQ ₀ andn depend on the lapse time window.Q ₀ was found equal to 33 andn equal to 1.01 for the time window of 10 to 20 sec, while for the other windowsQ ₀ increased from 60 to 129, withn being stable, close to 0.75. This lapse time dependence is interpreted as due to a depth dependent attenuation. The high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity, in agreement with studies in other seismic regions.     

Study of Sediment Nonlinearity Using Explosion Data in the Mississippi Embayment

Strong and weak motion data from the Mississippi Embayment Seismic Excitation Experiment (ESEE) were analyzed for signatures of nonlinear site responses. This experiment was performed jointly by the University of Memphis and U. S. Geological Survey in October 2002, by detonating two explosions of 2500 and 5000 lbs. Intrinsic and scattering Q estimates (Q_I and Q_S) from the coda of the strong motion data were found to be very low compared to previously determined Q values of P- and Rayleigh waves of weak motion data from the same explosions. The Q_I estimates from P-wave late coda of the strong motion data are less by more than 100 at 3 Hz and by more than 200 at 10 Hz, compared to the P-wave Q values determined from the weak motion data by Langston et al (2005). Also, Q_I determined from the late coda of strong motion Rayleigh-wave data is less by more than 200 at 0.5 Hz and by more than 50 at 3.0 Hz, compared to Q values determined from Rayleigh-wave weak motion data. A resonance peak spectral amplitude of the early part of a strong motion seismogram is shifted to lower frequencies compared to that from a later part of the same seismogram. Spectral amplitude ratios between transverse and vertical components of the strong motion data are degraded between frequencies 2 and 10 Hz for P waves, and less than 4 Hz for Rayleigh waves compared to the weak motion transverse to vertical spectral ratio. All these are signatures of nonlinear site responses during strong ground motion. This study proves the non-transportability of weak motion attenuation results to estimate ground motion from a future large earthquake that may take place in areas like the New Madrid Seismic zone.     

Scattering and anelastic attenuation of seismic energy in northern Greece

The relative contribution of scattering (Q _s ^–1 ) and intrinsic (Q _i ^–1 ) attenuation to the totalS-wave attenuation for the frequencies of 1.5, 3.0, 6.0 and 12.0 Hz has been studied by applying the radiative energy transfer theory, Data of local earthquakes which occurred in northern Greece and were recorded by the permanent telementered network of the Geophysical Laboratory of the University of Thessaloniki have been used. The results show that in this area the scattering attenuation is dominant over all frequencies while intrinsic attenuation is significantly lower. The estimatedQ _s ^–1 andQ _i ^–1 values have frequency dependences off ^–0.72 andf ^–0.45, respectively. The frequency dependence ofQ _s ^–1 is the same as that of the codaQ _c ^–1 , obtained by applying the single scattering model, which probably implies that the frequency dependence of the coda wave attenuation is attributed to the frequency dependence of the scattering attenuation.Q _c ^–1 values are very close to scattering attenuation for short lapse times, (10–20 sec), and intermediate between scattering and intrinsic attenuation for the longer lapse times, (50–100 sec). This difference is explained as the result of the depth-dependent attenuation properties and the multiple scattering effects.     

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

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.     

广西龙滩水库库区地震尾波衰减特征

按照高信噪比和计算要求,从 2006—2013 年广西龙滩水库地震台网的10个子台记录到的龙滩库区近场数字地震波形中挑选出 3 822 条三分向记录,经滤波和消除环境噪声后,采用Sato单次散射模型,计算了地震波传播路径上的尾波Q （ f ）值,对Q （ f ）与频率f之间的关系进行了拟合,并在此基础上分析讨论了库区尾波衰减的时空特征. 结果显示,龙滩库区总体呈低Q₀值和依赖性指数η较高的特点,这两个参数值均低于浙江珊溪水库,与其地震活动程度高于后者的事实相一致。尾波Q值的空间分布显示,周围小震密集分布的台站和历史地震活跃地区的台站的Q₀值低于相对于周围小震分布稀疏的台站的Q₀值。库区尾波衰减参数与尾波采样体之间的关系显示,采样深度越深,尾波参数Q₀值越低,因此认为该区域可能存在深部高衰减层。衰减参数随时间的变化过程表明,蓄水后地震尾波衰减参数仍主要反映了库区原有地壳内部介质的非均匀性和地震波吸收特性,库区地壳介质特性无显著变化,但局部敏感点有所调整。      

Attenuation models inferred from intraplate earthquake recordings

Strong-motion recordings at 87 sites from 56 different intraplate earthquakes from North America, Europe, China and Australia have been used through a two-step regression analysis to develop new attenuation models for peak ground acceleration, and for pseudo-relative velocity for frequencies of 0.25, 0.5, 1.0, 2.0, 5.0 and 10.0 Hz, all for 5 per cent of critical damping. The estimates are obtained along with an analysis of residuals and scatter. A similar regression analysis has been performed also for Fourier spectra of acceleration, in which case the coefficient for the anelastic term has been interpreted in terms of a frequency dependent quality factor Q. The resulting Q-model shows a strong frequency sensitivity with values around 600–700 at 1 Hz, around 2000 at 10 Hz and around 5200 at 25 Hz. These PGA, PSV and Q results depend, however, on the underlying assumption for geometrical spreading, in particular for low frequencies.     

云南地区散射衰减、吸收衰减及尾波衰减的综合研究

利用云南省地震局遥测台网记录的地震波形资料,采用多流逝时间窗分析方法,得到了云南地区不同台站的散射衰减和吸收衰减,并通过单次散射模型得到了不同台站的尾波衰减.研究结果显示,云南地区的散射衰减、吸收衰减、总衰减和尾波衰减空间分布以金沙江—红河断裂带为界,均表现出西部低,东部高的特征.云南地区地震波衰减以吸收为主,2~4 Hz、4~8 Hz、8~16 Hz三个频带各台站的吸收衰减Q^－1_i均大于散射衰减Q^－1_c,1~2 Hz除个别台站的吸收衰减明显小于散射衰减,其它台站均大于或接近散射衰减.尾波衰减Q^－1_c在1~2 Hz稍大于吸收衰减而小于总衰减,在2~4 Hz、4~8 Hz、8~16 Hz与总衰减接近,个别台站大于总衰减Q^－1_t.云南地区散射衰减与频率的关系为Q^－1_s=0.0138f^－1.61,吸收衰减与频率的关系为Q^－1_i=0.0119f^－0.86,总衰减与频率的关系为Q^－1_t=0.0247f^－1.1,尾波衰减与频率的关系为Q^－1_c=0.0129f^－0.71.与世界其它地区相比,云南地区的散射衰减处于中等偏低的水平,吸收衰减处于中等偏高的水平.     

Ambiguity Between the Residual Geometrical Spreading and Scattering Attenuation

By introducing a residual geometrical spreading factor, a new model was proposed in recent papers of Morozov to improve the estimates of seismic Q, and some published seismic Q observations were reinterpreted under the framework of the new model. We found that in these papers the definitions about the residual geometrical spreading and seismic scattering attenuation were conceptually confusing, and physically impossible negative values of seismic Q may arise in the new model. We argue that the estimates of the residual geometrical spreading in the new model are influenced by seismic scattering. Thus, the correlation between the residual geometrical spreading and tectonic activity, and the observation of temporal variations of the residual geometrical spreading and apparent attenuation, as reported by Morozov, are not surprising.     

Attenuation of coda waves in the Northeastern Region of India

Coda wave attenuation quality factor Qc is estimated in the northeastern region of India using 45 local earthquakes recorded by regional seismic network. The quality factor Qc was estimated using the single backscattering model modified by Sato (J Phys Earth 25:27–41, 1977), in the frequency range 1–18 Hz. The attenuation and frequency dependence for different paths and the correlation of the results with geotectonics of the region are described in this paper. A total of 3,890 Qc measurements covering 187 varying paths are made for different lapse time window of 20, 30, 40, 50, 60, 70, 80, and 90 s in coda wave. The magnitudes of the analyzed events range from 1.2 to 3.9 and focal depths range between 7 and 38 km. The source–receiver distances of the selected events range between 16 and 270 km. For 30-s duration, the mean values of the estimated Qc vary from 50 ± 12 (at 1 Hz) to 2,078 ± 211(at 18 Hz) for the Arunachal Himalaya, 49 ± 14 (at 1 Hz) to 2,466 ± 197 (at 18 Hz) for the Indo-Burman, and 45 ± 13 (at 1 Hz) to 2,069 ± 198 (at 18 Hz) for Shillong group of earthquakes. It is observed that Qc increases with frequency portraying an average attenuation relation for the region. Moreover, the pattern of Qc^− 1 with frequency is analogous to the estimates obtained in other tectonic areas in the world, except with the observation that the Qc^− 1 is much higher at 1 Hz for the northeastern region. The Qc^− 1 is about 10^− 1.8 at 1 Hz and decreases to about 10^− 3.6 at 18 Hz indicating clear frequency dependence. Pertaining to the spatial distribution of Qc values, Mikir Hills and western part of Shillong Plateau are characterized by lower attenuation.     

Estimating permeability from field measurements of seismic attenuation in fractured chalk

Broadband (100–4000 Hz) cross‐hole seismic data have been acquired at a borehole test site where extensive hydrological investigations have previously been performed, including in situ estimates of permeability. The rock type is homogeneous chalk and fractures and bedding planes have been identified from well logs. High values of seismic attenuation, Q= 22 ≤ 27 ≤ 33, were observed over a 10 m depth interval where fracture permeability values of 20–50 darcy had been recorded. An attempt has been made to separate the attenuation due to scattering and intrinsic mechanisms. The estimated values of intrinsic attenuation, Q= 31 ≤ 43 ≤ 71, have been reproduced using a number of current theories of seismic‐wave propagation and fluid‐flow‐induced seismic attenuation in cracked and fractured media. A model that considers wavelength‐scale pressure gradients is the preferred attenuation mechanism. Model parameters were obtained from the hydro‐geological and seismic data. However, we conclude that it is not possible to use seismic Q to measure rock permeability remotely, principally because of the inherent uncertainties arising from model parameterisations.     

岩石非均匀性对超声衰减的影响及其修正

在充分考虑了岩石的非均匀性引起的Rayleigh散射对衰减测量值的影响后，通过有关理论和计算，将岩石的本征衰减和散射加以区分，对原先以均匀介质为基本假定的频谱比法进行了修正，改进后的频谱比法可在较宽的频率范围内进行Q值计算，避免了传统频谱比法在不同频段上得出不同的Q值的不确定性问题，同时，我们还引进并改进了频率移动法，我们将传统和改进的频谱比法，以及频率移动法应用于同一套实验数据，计算结果符合较好。     

用近震尾波估算昆明及其周围地区的Q值和地震矩

本文应用推广了的Aki尾波理论模式,忽略震源频谱含量的影响,考虑到仪器系统和地球Q-滤波器对尾波频散的影响,计算了f_p-t^*和k-t^*独立的两套尾波响应理论量板。在假定Q与频率有Q（f）=Q₀（f/f₀）ⁿ关系的基础上,用云南区域台网短周期地震仪器记录的小震尾波资料,估算了昆明周围地区频率约1.0Hz的短周期波的地壳Q₀值。结果表明,该区的Q₀值在150-220之间,平均为191;频率相关值η不是常数,多数为0.2-0.3。这些结果与用尾波估算川滇交界地区的结果接近,表明云南地区的地壳Q₀值与美国西部地区类似,是属于低Q₀值地区。 此外,用尾波与地震矩建立的关系,提出用尾波估算地震矩的一种可能方法,讨论了该区震级与地震矩的经验关系。     

Attenuation in the uppermost inner core from PKP recordings at African seismological stations

The attenuation factor Q_P at the top of the inner core is evaluated by using the amplitude spectral ratio of PKPdf and PKPbc phases observed at African stations (BGCA mostly), from strong deep earthquakes in the Pacific Ocean area. The maximum depth of penetration of the PKPdf phase into the inner core (IC) is roughly 377 km, and the sampled region of IC is centered beneath the Southern Indian Ocean. The derived mean value of Q_P is 249 ± 31 (95% confidence level) in the frequency range 0.2–2 Hz, where no frequency dependence of attenuation has been reliably observed. By using Student’s t-test, we show that the value is statistically significantly different (with a probability greater than 95%) from other mean values of Q derived by using the same method, for both the western (180 °W to 40 °E) and eastern (40 °E to 180 °E) hemispheres of the IC. The decrease of Q with the radius of the turning point (denoted by r^TP), according to Q_P = 840 − 0.62 r^TP, has a moderate statistical support (the R-squared value is 38%). A slightly increase of Q as a function of the angle of the PKPdf path within the inner core with respect to the Earth’s spin axis is observed, in agreement with various investigations performed in the time domain. However, the value of the anisotropy, if any, is suggested to be around 3%.     

Strongly Frequency-Dependent Intrinsic and Scattering Q for the Western Margin of the Sichuan Basin, China, as Revealed by a Phenomenological Model

Through applying the energy-flux model to teleseismic P-wave data, seismological scattering Q and intrinsic Q for the western margin of the Sichuan basin in southwestern China are estimated separately. The results exemplify the frequency-dependent Q values in the seismic frequency band. The scattering Q is heavily frequency-dependent and reaches a minimum at ~1.5 Hz. The intrinsic Q is nearly invariant for frequencies above 2 Hz and rapidly increases at lower frequencies. A major advantage of the energy-flux model is that it is a phenomenological model based on the conservation of energy and requires no geometrical spreading correction. The results are good counterexamples against the claim of a frequency-independent seismological Q.