长白山天池火山地区的尾波Q值估计

对1999～2005年间长白山天池火山观测站所有数字化波形记录中震级ML≥0.6的94个火山地震事件,利用频率在1.0～16.0Hz范围内的记录,依据S波单次反散射模型测算了长白山天池火山区的尾波Q值,获得尾波Q值与频率的关系为QC=(47±2)f(0.74±0.01)。假定衰减完全由散射损失造成,则估计得到长白山天池火山区的最小平均自由程为17km,且与所用的频率无关,表明天池火山区下方散射体分布密度较大。     

Coda Q Estimates in the Andaman Islands Using Local Earthquakes

The attenuation property of Andaman Island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2.0–4.9, using the single backscattering model. These earthquakes waveforms, recorded on five broadband seismographs sited over the island from north to south during Nov. 2003 to March 2004, have been used to calculate the frequency dependent Coda Q (Q _c ) applying the time domain coda-decay method. The Coda Q, computed at central frequencies from (0.5–12) Hz and five-lapse time windows from 40 to 80 s, progressively increases from 105 f ^0.88 in the north Andaman to 135 f ^0.79 in the south Andaman with an average of 119 f ^0.80. The average Q _c values vary from 75 ± 42 at 0.5 Hz to 697 ± 54 at 12 Hz central frequency for 40 s lapse time window, while for 80 s lapse time window its variation is from 117 ± 38 at 0.5 Hz to 1256 ± 115 at 12 Hz. The Q _c estimated at different lapse times manifests a significant variation from 122f ^0.75 to 174f ^0.73, corresponding to lapse time window lengths of 40 and 80 s, respectively. The variation of Q _c with frequency, lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the Andaman Islands. These observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island.     

A Frequency-dependent Relation of Coda Qc for Koyna-Warna Region, India

—Attenuation properties of the lithosphere around the Koyna-Warna seismic zone is studied by estimating the coda-Q _c from 30 local earthquakes of magnitude varying from 1.5 to 3.8. An average lapse time of 65 sec used in the single scattering model sampled a circular area with an average radius of 114 km. The estimated Q _c values show a frequency-dependent relation, Q _c =169 f?^0.77, and range from 169 at 1 Hz to 1565 at 18 Hz. A comparison of worldwide Q studies reveals that for a large frequency range the Q for active regions is low as compared to that for stable regions. However, South Carolina and Norway are exceptions in that their Q is low in the low frequency range while New England and North Iberia are exceptions as they have a Q value similar to that for active regions like Spain, Turkey, Italy and Garhwal Himalaya (STIH), in the higher frequency range. In contrast to this, the Q for the Koyna-Warna area, which belongs to a stable region, is low in the entire frequency range as compared to the stable regions and similar to the active STIH regions.     

Attenuation of Coda Waves in the Saurashtra Region, Gujarat (India)

The attenuation characteristics based on coda waves of two areas—Jamnagar and Junagarh of Saurashtra, Gujarat (India)—have been investigated in the present study. The frequency dependent relationships have been developed for both the areas using single back scattering model. The broadband waveforms of the vertical components of 33 earthquakes (Mw 1.5–3.5) recorded at six stations of the Jamnagar area, and broadband waveforms of 68 earthquakes (Mw 1.6–5) recorded at five stations of the Junagarh area have been used for the analysis. The estimated relations for the Junagarh area are: Q _c?=?(158?±?5)f^(0.99±0.04) (lapse time : 20?s), Q _c?=?(170?±?4.4)f^(0.97±0.02) (lapse time : 30?s) and Q _c?=?(229?±?6.6)f^(0.94±0.03) (lapse time : 40?s) and for the Jamnagar area are: Q _c?=?(178?±?3)f^(0.95±0.05) (lapse time : 20?s), Q _c?=?(224?±?6)f^(0.98±0.06) (lapse time : 30?s) and Q _c?=?(282?±?7)f^(0.91±0.03) (lapse time : 40?s). These are the first estimates for the areas under consideration. The Junagarh area appears to be more attenuative as compared to the Jamnagar area. The increase in Q _c values with lapse time found here for both the areas show the depth dependence of Q _c as longer lapse time windows will sample larger area. The rate of decay of attenuation (Q ^?1) with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ. A comparison of the coda-Q estimated for the Saurashtra region with those of the nearby Kachchh region shows that the Saurashtra region is less heterogeneous. The obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the Saurashtra region of Gujarat where no such relations were available earlier. These relations are also important for the simulation of earthquake strong ground motions in the region.     

利用Aki模型对山西地区尾波Q值的研究

选取2001年1月至2011年12月山西地震台网记录的ML≥2．0地震的数字地震波资料,运用单次散射Aki模型,计算得到山西地区尾波Q值,拟合Q值对频率的依赖关系。其中,右玉、代县、夏县、东山4个地震台站属于低Q值、高n值,为特质的构造运动活跃地区的尾波性质。     

Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001

Q_C-estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q_c is a function of frequency and increases as frequency increases. The frequency dependent Q_c relations obtained for four lapse-time windows are: Q_c=82 f^1.17 (20–50 sec), Q_c=106 f^1.11 (30–60 sec), Q_c=126f^1.03 (40–70 sec) and Q_c=122f^1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q₀ increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q_c⁻¹ of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.     

Estimation of Coda Wave Attenuation for the National Capital Region, Delhi, India Using Local Earthquakes

Attenuation of seismic waves is very essential for the study of earthquake source parameters and also for ground-motion simulations, and this is important for the seismic hazard estimation of a region. The digital data acquired by 16 short-period seismic stations of the Delhi Telemetric Network for 55 earthquakes of magnitude 1.5 to 4.2, which occurred within an epicentral distance of 100 km in an area around Delhi, have been used to estimate the coda attenuation Q_c. Using the Single Backscattering Model, the seismograms have been analyzed at 10 central frequencies. The frequency dependence average attenuation relationship Q_c = 142f ^1.04 has been attained. Four Lapse-Time windows from 20 to 50 seconds duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of Q_c. The Q_c values show that frequency dependence (exponent n) remains similar at all the lapse time window lengths. While the change in Q₀ values is significant, change in Q₀ with larger lapsetime reflects the rate of homogeneity at the depth. The variation of Q_c indicates a definitive trend from west to east in accordance with the geology of the region.     

Speculation on the causes of continuing seismicity near Koyna reservoir,India

The cause for continuous induced seismicity at Koyna is not well understood. A heuristic model based on various physical parameters observed at Koyna is being proposed to explain the ongoing seismicity. This model contains two essential elements: (i) Intersecting faults near Koyna provide means of stress build-up in response to plate tectonic forces. (ii) The annual reservoir loading cycle and changes in the ground water table perturb this stress build-up by an influx of pore pressure in a fluid infiltrated medium. Hence, the spatial and temporal pattern of the pore prussure distribution and the seismicity will be governed by the location and hydromechanical properties of the faults and fractures. The predictions of the model can be tested by comparing the temporal and spatial pattern of seismicity with the changes in lake level and water table.     

利用Aki模型对宁夏及邻区尾波Q值的研究

利用单次散射的Aki模型,选取2008年1月-2009年12月宁夏地震台网记录的82次ML≥2.0地震的数字地震波资料,计算宁夏及邻区尾波Q值,拟合Q值对频率的依赖关系.全区域数据结果为Q(f)=(212±87.62)·f0.7584±0.19.与国内其它区域相比,本区域Q值较低、对频率依赖性较高.结合区域地震地质构造...     

On the geodetic observations during 1965 to 1969 in the Koyna region, India

Limited reports of repeat triangulation and levelling surveys carried out in the Koyna region by Survey of India, before and after the 1967 Koyna earthquake, are available. We analyse these observations of horizontal and vertical displacements to investigate whether analysis of these observations can provide additional constraints on the source parameters of the 1967 Koyna earthquake. We calculate surface displacements due to coseismic slip on the subsurface rupture of the 1967 Koyna earthquake and also due to the Koyna reservoir water load. We find that the reported displacements are too large to be attributed to the coseismic displacements and/or to the displacements induced by the reservoir water load. We conclude that these observations contain large random and systematic errors.     

Attenuation of P,S, and coda waves in Koyna region,India

The attenuation properties of the crust in the Koyna region of the Indian shield have been investigated using 164 seismograms from 37 local earthquakes that occurred in the region. The extended coda normalization method has been used to estimate the quality factors for P waves and S waves , and the single back-scattering model has been used to determine the quality factor for coda waves (Q _c). The earthquakes used in the present study have the focal depth in the range of 1–9 km, and the epicentral distance vary from 11 to 55 km. The values of and Q _c show a dependence on frequency in the Koyna region. The average frequency dependent relationships (Q = Q ₀ f ⁿ) estimated for the region are , and . The ratio is found to be greater than one for the frequency range considered here (1.5–18 Hz). This ratio, along with the frequency dependence of quality factors, indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of Q _c and in the present study shows that for frequencies below 4 Hz and for the frequencies greater than 4 Hz. This may be due to the multiple scattering effect of the medium. The outcome of this study is expected to be useful for the estimation of source parameters and near-source simulation of earthquake ground motion, which in turn are required in the seismic hazard assessment of a region.     

Source parameters of some significant earthquakes near Koyna dam,India

Seismic moment, stress drop, source radius, and fault dislocation have been determined for nineteen significant earthquakes in the Konya area, India using displacement spectra of shear waves computed from strong-motion accelerograph records. Though the stress-drop shows a definite increasing trend with the seismic moment, its correlations with source radius and corner frequency indicate that a constant stress drop of 170 bars represents a good mean value to describe the source mechanism of the Koyna dam earthquakes. An empirical relationship also has been established between magnitude and seismic moment.     

Lateral Variations of Lg Coda Q in Southern Mexico

—Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.9 ≤m _b ≤ 6.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and/or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q ₀ (Q at 1?Hz) and the frequency dependence parameter η estimates averaged on the frequency interval of 0.5 to 2?Hz for the regions and the three components considered are: i) Guerrero region 173 ≤Q¯ ₀≤ 182 and 0.6 ≤Q¯ ₀ ≤ 0.7, ii) Oaxaca region 183 ≤Q¯ ₀ ≤ 198 and 0.6 ≤Q¯ ₀ ≤ 0.8, iii) Michoacan-Jalisco region 187 ≤Q¯ ₀ ≤ 204 and 0.7 ≤Q¯ ₀ ≤ 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 313 ≤Q ₀≤ 335 and η = 0.9. ¶The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. ¶The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan-Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q ^?1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0?Hz approximately on the horizontal component. Deeper (h > 50?km) events yield lower values of Q ^?1 than shallower events. For frequencies higher than 1.0?Hz no clear dependence of Q ^?1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.     

重庆荣昌地区尾波Q_C值特征

基于Sato单次散射模型,利用2004年1月-2007年9月重庆市台网记录的荣昌县及其附近地区48个ML≥2.0地震波形资料,计算了该区域尾波QC值,并对QC值与频率之间的依赖关系进行了拟合.结果表明该区为QC值低值、对频率强依赖性区域.另对QC值分三个时间段进行研究,显示QC值变化量不大,结果较稳定,说明在研究时段内该区域地下介质结构无明显变化.     

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.     

江苏地区尾波Q值特征研究

基于Aki单次散射模型,利用江苏数字地震台网记录的地震数字波形资料开展了江苏地区尾波Q值特征研究。结果表明,江苏地区总体上属于高Q值地区,但不同地区存在明显差异,总体呈现出南高北低的特点。本文对台站相对密,地质构造特点明显、尾波Q0计算结果较为集中的四个区域分区进行了研究,获得了江苏四个区域的Q值与频率的特征关系。苏南至长江口和茅山断裂带附近为江苏的高Q值地区,苏中至沿岸为江苏的低Q值地区,苏北连云港及附近处于Q值中等水平地区。对照江苏四个Q值不同分区的构造及地震活动分析,Q值、地壳运动和地震活动性之间存在一定的相关关系。     

浙江及邻区尾波Q值研究

基于单次散射(Aki)模型,利用2000年至今浙江及安徽台网记录的37次ML3.0以上地震的数字地震波资料,计算了浙江及邻近省区平均尾波Q值,并拟合了Q值对频率的依赖关系数据。结果为:Q(f)=212±33.5f0.62±0.11,与国内其它区域相比,本区域为Q值高值、对频率低依赖性区域。认为本研究区内构造活动弱,地震活动水平低。本文就Q值对频率、尾波窗长等的依赖性特征及影响Q值计算的噪声水平、滤波器、数据采样等因素进行了深入分析。     

L g Coda Q Variation across the Myanmar Arc and its Neighboring Regions

Regional seismograms were collected to image the lateral variations of L _g coda Q at 1 Hz (Q ₀ ) and its frequency dependence η across Burma and its neighboring regions. The data include 660 vertical-component traces recorded at 39 stations. The resulting image indicates that L _g coda Q, at a frequency of 1 Hz, varies between 100 and 500. Lowest Q values (< 200) lie in the Three rivers (the Jinshajiang River, Nujiang River, and Lancangjiang River) area of Southwest China. Relatively low Q values (200–250) are found in the Himalayan region and the eastern Burma highland. Higher L _g coda Q values (> 250) are found in the eastern Indian block. From the L _g coda Q tomography, we found that (1) The Sagain fault acts as a rough boundary between the eastern Indian plate and the Three rivers area of the Eurasia plate; (2) near the eastern Himalayan syntaxis, higher Q value appears in the background of relatively low Q (which may be the consequence of the northward intrusion of the Assam block of the Indian plate into the southern Qinghai-Tibet plateau.     

Comparison of Sesimic Body Wave and Coda Wave Measures of Q

—Measurements of seismic attenuation (Q ^?1) can vary considerably when made from different parts of seismograms or using different techniques, particularly at high frequencies. These discrepancies may be methodological, or may reflect earth processes. To investigate this problem, we compare body wave with coda Q ^?1 results utilizing three common techniques i) parametric fit to spectral decay, ii) coda normalization of S waves, and iii) coda amplitude decay with lapse time. Q ^?1 is measured from both body and coda waves beneath two mountain ranges and one platform, from recordings made at seismic arrays in the Caucasus and Kopet Dagh over paths ≤ 4° long. If Q is assumed frequency independent, spectral decay fits show Q _s and Q _coda near 700–800 for both mountain paths and near 2100–2200 for platform paths. Similar values are determined with the coda normalization technique. However, frequency-dependent parameterizations fit the data significantly better, with Q _s ?(1 Hz) and Q _coda?(1 Hz) near 200–300 for mountain paths and near 500–600 for platform paths. Lapse decay measurements are close to the frequency-dependent values, showing that both spectral and lapse decay methods can give similar results when Q has comparable parameterizations. Above 6 Hz, coda measurements suggest some enrichment relative to body waves, perhaps due to scattering, but intrinsic absorption appears to dominate at lower frequencies. All approaches show sharp path differences between the Eurasian platform and adjacent mountains, and all are capable of resolving spatial variations in Q.