Intrinsic and Scattering Seismic Attenuation in Southwestern Anatolia

The intrinsic dissipation and scattering attenuation in southwestern (SW) Anatolia, which is a tectonically active region, is studied using the coda waves. First the coda quality factor (Q_c) assuming single scattering is estimated from the slope of the coda-wave amplitude decay. Then the Multiple Lapse Time Window (MLTW) analysis is performed with a uniform earth model. Three non-overlapping temporal data windows are used to calculate the scattered seismic energy densities against the source-receiver distances, which, in turn, are used to calculate separate estimates of the intrinsic and scattering factors. In order to explore the frequency dependency, the observed seismograms are band pass-filtered at the center frequencies of 0.75, 1.5, 3.0, 6.0 and 12.0. The scattering attenuation (Q_s⁻¹) is found lower than the intrinsic attenuation (Q_i⁻¹) at all frequencies except at 0.75 Hz where the opposite is observed. Overall the intrinsic attenuation dominates over the scattering attenuation in the SW Anatolia region. The integrated energy curves obtained for the first energy window (i.e., 0–15 s) are somewhat irregular with distance while the second (i.e., 15–30 s) and third (i.e., 30–45 s) data windows exhibit more regular change with distance at most frequencies. The seismic albedo B₀ is determined as 0.61 at 0.75 Hz and 0.34 at 12.0 Hz while the total attenuation factor denoted by L_e⁻¹ changes in the range 0.034–0.017. For the source-station range 20–180 km considered the scattering attenuation is found strongly frequency dependent given by the power law Q_s⁻¹ = 0.010*f^−1.508. The same relations for Q_i⁻¹, Q_t⁻¹ (total), Q_c⁻¹ and (expected) hold as Q_i⁻¹ = 0.0090*f^−1.17, Q_t⁻¹ = 0.019*f^−1.31, Q_c⁻¹ = 0.008*f^−0.84 and respectively. Compared to the other attenuation factors Q_c⁻¹ and are less dependent on the frequency.     

小湾水库近区地震波非弹性衰减和场地响应研究

利用小湾电站水库诱发地震监测台网11个子台自2005年5月21日开始观测至2006年9月记录的数字波形资料,采用Atkinson方法反演了小湾水库附近区域的非弹性衰减,得到该地区Q值与频率的关系是Q(f)=225.8f0.332。另外,用Moya方法反演了小湾台网11个子台的场地响应,得到了43个地震的震源参数,并对所得结果进行了初步讨论。     

影区地震速报定位的改进

影区地震的定位是成都遥测台网速报经常遇到的难点问题之一。本文分析了成都遥测台网记录中出现的影区地震的震相Ｓ、Ｌｇ特征，总结出识别它们的方法，并发现增用Ｓ波参加速报定位，常常可提高定位精度，达到规范要求。对于某些影区地震，可用ＳｃＳ震相测定其深度。     

Erratum: Comparison of Seismic Dispersion and Attenuation Models: Stud. Geophys. Geod., 46(2002), 293-320

Studia Geophysica et Geodaetica -     

地震灾害评估软件烈度衰减系数研究

河北省地震应急指挥技术系统建成后,地震灾害评估软件中烈度衰减系数采用经验参数,计算出的烈度范围与实际地震存在一定偏差。通过收集河北省及周边地区地震烈度数据,采用回归分析法得到新的烈度衰减系数,并通过软件验证,其计算的烈度范围比采用原系数计算的烈度范围更接近实际情况。     

云南通海及其周边地区地震烈度衰减关系研究

选用云南通海及其周边地区的地震烈度资料,研究了该区地震震源深度对烈度衰减的影响;通过在烈度衰减模型中加入震源深度参数,计算了通海及其周边地区的地震烈度衰减关系。同时,研究了1970年通海7.8级地震的等震线图,给出等震线长、短轴数据的处理方法。     

Inelastic Attenuation of Seismic Wave and Site Response near the Xiaowan Reservoir

Based on digital seismograms recorded by 11 stations of the Xiaowan Reservoir-induced Earthquake Monitoring Network from May 21, 2005 to September 2006, we calculated the inelastic attenuation near Xiaowan reservoir using Atkinson method. We got the relation of Q value to frequency as Q（f） = 225.8 f^0332. Using the Moya method we got the site response of the 11 stations of Xiaowan Network and source parameters of 43 earthquakes. The source parameters were discussed briefly.     

云南地区地震动衰减特性的研究

根据发生在云南地区震级大于3.0级的地震资料,采用最小二乘法对该地区的峰值加速度、加速度反应谱、峰值速度、峰值位移、均方根加速度等多个地震动参数进行回归分析。结果表明,该地区的地震动衰减关系的形态与前人的研究基本一致,但水平向差异较大;长周期部分的离散性较大,需进一步收集资料进行研究。     

台基对地震波、干扰信号的选择性抑制及其抗干扰品质的实验研究

基于地震波及干扰信号在覆盖层与基岩分界面的衰减差异实验,分析了基岩类台基对地震波及干扰信号的选择性抑制的机理,提出台基抗干扰品质的概念。在此基础上,以昭通地震台的抗干扰改造为例,给出利用多套地震仪测定台基抗干扰品质的方法。     

江西及邻区地震烈度衰减关系研究

收集江西及邻区的地震烈度等震线资料,采用椭圆长、短轴联合衰减模型和近、远场补点与长轴可转向方法,建立了该地区的地震烈度衰减关系并进行了验证,结果表明,该衰减关系符合该地区历史和近代地震震害分布的地域性特点,可进一步应用于地震安全性评价、震害预测、损失快速评估等研究.     

Dispersion of seismic waves by a causal approach

Basic ideas of the causal approach to wave propagation in random media are first overviewed. This approach appeals from the outset to the linearity, causality, and passivity of the effective medium and is therefore particularly simple from the conceptual viewpoint. The energy analysis and the Kramers-Kronig relations play the major role in this method, which does not resort to ensemble averaging.Then the dispersion of plane wave propagation in randon media is evaluated by extending Wu's results on attenuation induced by scattering. These results are particularly suitable for seismic waves, for which the so-called mean-field approach may not provide adequate modeling. The presence of intrinsic losses is also incorporated. The analysis also includes the case of propagation of a small-amplitude discontinuity.     

Short Communication: Comments on Comparison of Seismic Dispersion and Attenuation Models by Ursin B. and Toverud T., Stud. Geophys. Geod., 46, 293-320

Studia Geophysica et Geodaetica -     

Modeling of Wave Dispersion Using Continuous Wavelet Transforms

In the estimate of dispersion with the help of wavelet analysis considerable emphasis has been put on the extraction of the group velocity using the modulus of the wavelet transform. In this paper we give an asymptotic expression of the full propagator in wavelet space that comprises the phase velocity as well. This operator establishes a relationship between the observed signals at two different stations during wave propagation in a dispersive and attenuating medium. Numerical and experimental examples are presented to show that the method accurately models seismic wave dispersion and attenuation.     

Seismic Attenuation Computed from GLIMPCE Reflection Data and Comparison with Refraction Results

—Instantaneous frequency matching has been used to compute differential t* values for seismic reflection data from the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) experiment. The differential attenuation values were converted to apparent Q ^?1 models by a fitting procedure that simultaneously solves for the interval Q ^?1 values using non-negative least squares. The bootstrap method was then used to estimate the variance in the interval Q ^?1 models. The shallow Q ^?1 structure obtained from the seismic reflection data corresponds closely with an attenuation model derived using instantaneous frequency matching on seismic refraction data along the same transect. This suggests that the effects of wave propagation and scattering on the apparent attenuation are similar for the two data sets. The Q ^?1 model from the reflection data was then compared with the structural interpretation of the reflectivity data. The highest interval Q ^?1 values (>0.01) were found near the surface, corresponding to the sedimentary rock sequence of the upper Keweenawan. Low Q ^?1 values (<0.0006) are found beneath the Midcontinent rift’s central basin. In addition to structural interpretation, seismic attenuation models derived in this way can be used to correct reflection data for dispersion, frequency and amplitude effects, and allow for improved imaging of the subsurface.     

Attenuative Dispersion of <Emphasis Type="Italic">P</Emphasis> Waves and Crustal <Emphasis Type="Italic">Q</Emphasis> in Turkey and Germany

We have measured group delays of the spectral components of high-frequency P-waves along two portions of the North Anatolian Fault Zone (NAFZ) in Turkey and in a region of southern Germany. Assuming that the observed dispersion is associated with attenuation in the crust and that it can be described by a continuous relaxation model, we obtained Q and the high-frequency relaxation times for those waves for each of the three regions. Individual P-wave Q values exhibit large scatter, but mean values in the NAFZ increase from about 25 to 60 over the distance range 5–90 km. Mean Q values are somewhat higher in the eastern portion of the NAFZ than in the western portion for measurements made at distances between 10 and 30 km. P-wave Q values in Germany range between about 50 and 300 over the hypocentral distance range 20–130 km. In that region we separated the effects of Q for basement rock (2–10 km depth) from that of the overlying sediment (0–2 km depth) using a least-squares method. Q varies between 100 and 500 in the upper 8–10 km of basement, with mean values for most of the distance range being about 250. Q in the overlying sediments ranges between 6 and 10. Because of large scatter in the Q determinations we investigated possible effects that variations of the source-time function of the earthquakes and truncation of the waveform may have on Q determinations. All of our studies indicate that measurement errors are relatively large and suggest that useful application of the method requires many observations, and that the method will be most useful in regions where the number of oscillations following the initial P pulse is minimized. Even though there is large scatter in our Q determinations, the mean values that we obtained in Turkey are consistent with those found in earlier studies. Our conclusions that Q is significantly higher in the basement rock of Germany than in the basement rock of Turkey and that Q is lower in western Turkey than in eastern Turkey are also consistent with results of Q studies using Lg coda.     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

不同频带数字化测震对比

通过对蒙城地震台不同频带数字化测震仪器记录的波形及震级进行对比分析,得出不同频带仪器记录不同地震事件的特点及其优越性,有利于提高数字化资料分析的精度.在进一步保证蒙城地震台观测资料的完整性和提高全频带数字化测震观测质量的同时,为蒙城地球物理野外观测研究站的科学研究提供有力数据支持.     

Attenuation of S wave in the crust of Ordos massif

Introduction Located between Qingzang (Qinghai-Xizang) Plateau that highly uplifts due to compression and North China plain that badly subsides due to crustal extension, the Ordos massif has both the basic attributes of stretching tectonic region of North China marginal basin and the features of marginal shear-compression zone of Qingzang Plateau. It has a length of about 600 km in the N-S direction and a width of about 400 km in the W-E direction. Its geological structure is very simple…     

四川盆地地区介质衰减特征研究

利用四川数字地震台网记录的地震波形资料,采用遗传算法联合反演方法研究了四川盆地地区的介质非弹性衰减模型。结果表明,在500km震源距范围内,研究区非弹性衰减Q值随频率f的关系为Q(f)=217.8f0.816。这一结果为在四川盆地地区进一步开展震源谱和强地面运动的模拟等研究工作奠定了基础。