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Coda Q Estimates in the Koyna Region, India 总被引:1,自引:0,他引:1
—The coda Q, Q c ?, have been estimated for the Koyna region of India. The coda waves of 76 seismograms from thirteen local earthquakes, recorded digitally in the region during July–August, 1996, have been analyzed for this purpose at nine central frequencies viz., 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 12.0, 16.0 and 24.0 Hz using a single backscattering model. All events with magnitude less than 3 fall in the epicentral distances less than 60 km and have focal depths which range from 0.86 to 9.43 km. For the 30 sec coda window length the estimated Q c values vary from 81 to 261 at 1.5 Hz and 2088 to 3234 at 24 Hz, whereas the mean values of Q c with the standard error vary from 148 ± 13.5 at 1.5 Hz to 2703 ± 38.8 at 24 Hz. Both the estimated Q c values and their mean values exhibit the clear dependence on frequency in the region and a frequency dependence average attenuation relationship, Q c = 96f 1.09, has been obtained for the region, covering an approximate area of 11500 km2 with the surfacial extent of about 120 km and depth of 60 km.¶Lapse time dependence of Q c has also been studied for the region, with the coda waves analyzed at five lapse time windows from 20 to 60 sec duration with the difference of 10 sec. The frequency dependence average Q c relationships obtained at these window lengths Q c = 66f 1.16 (20 sec), Q c = 96f 1.09 (30 sec), Q c =131f 1.04 (40 sec), Q c = 148f 1.04 (50 sec), Q c = 182f 1.02 (60 sec) show that the frequency dependence (exponentn) remains mostly stationary at all the lapse time window lengths, while the change in Q 0 value is significant. Lapse time dependence of Q c in the region is also interpreted as the function of depth. 相似文献
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Estimation of Coda Wave Attenuation for the National Capital Region, Delhi, India Using Local Earthquakes 总被引:1,自引:0,他引:1
William K. Mohanty Rajesh Prakash G. Suresh A. K. Shukla M. Yanger Walling J. P. Srivastava 《Pure and Applied Geophysics》2009,166(3):429-449
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 Qc. Using the Single Backscattering Model, the seismograms have been analyzed at 10 central frequencies. The frequency dependence
average attenuation relationship Qc = 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 Qc. The Qc 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 Qc indicates a definitive trend from west to east in accordance with the geology of the region. 相似文献
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—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. 相似文献
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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。 相似文献
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Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001 总被引:1,自引:0,他引:1
S. C. Gupta Ashwani Kumar A. K. Shukla G. Suresh P. R. Baidya 《Pure and Applied Geophysics》2006,163(8):1583-1595
QC-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 Qc is a function of frequency and increases as frequency increases. The frequency dependent Qc relations obtained for four lapse-time windows are: Qc=82 f1.17 (20–50 sec), Qc=106 f1.11 (30–60 sec), Qc=126f1.03 (40–70 sec) and Qc=122f1.02 (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, Qc−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. 相似文献
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Guangquan Li Jiafu Hu Haiyan Yang Hong Zhao Lianli Cong 《Pure and Applied Geophysics》2009,166(12):1937-1948
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. 相似文献
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研究区包括安达曼弧、缅甸平原和云南地区三个部分。对安达曼弧7级以上地震与缅甸、云南7级和6.8级地震的群体与个体的相关特征进行了研究,发现安达曼弧、缅甸平原M≥7.0地震与云南M≥6.8地震在群体上有一定相关性,但并非一一相关。云南有4个地震活跃期,但安达曼弧与缅甸平原却只有3个,缺失第3活跃期。研究区强震活动从板缘向板内逐渐减弱,三个地区最大震级分别为8.7级、7.8级和7.7级。安达曼弧与缅甸、云南7级地震的个体相关概率分别为33%和27%,平均30%左右,缅甸与云南的个体相关概率为43%。最后对云南的地震趋势进行了预测。 相似文献
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首先利用叠加谱比法,对成都数字地震台网的12个台站及中国数字地震台网的5个台站所记录的504个宽带垂直分量进行分析,得到与各路径相对应的椭圆内的Lg尾波Q0和η的测量值,再应用反投影技术反演得出四川及邻区Lg尾波Q0和η的分布图像。结果表明:Q0值的分布与构造活动性紧密相关,在所研究的范围内,Q0值在225~400的范围内变化。低的Q0值(225~250)发生在安宁河断裂、龙门山断裂和鲜水河断裂的交汇区域,以及由安宁河断裂和则木河断裂等多条断裂所组成的南北向断裂带;在川东的四川盆地,Q0值有所升高,在250~300的范围内变化;在所研究区域东南部的扬子地台,Q0值呈现出350~400的较高值。叩值在0.4~0.8的范围内变化。绝大部分地区的叩和Q0反向相关。 相似文献
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—Regional seismograms were collected to image the lateral variations of Lg coda Q at 1 Hz (Q 0?) and its frequency dependence <(eta)> in the Middle East using a back-projection method. The data include 124 vertical-component traces recorded at 10 stations during the period 1986–1996. The resulting images reveal lateral variations in both Q 0 and <eta>. In the Turkish and Iranian Plateaus, a highly deformed and tectonically active region, Q 0 ranges between about 150 and 300, with the lowest values occurring in western Anatolia where extremely high heat flow has been measured. The low Q 0 values found in this region agree with those found in other tectonically active regions of the world. Throughout most of the Arabian Peninsula, a relatively stable region, Q 0 varies between 350 and 450, being highest in the shield area and lowest in the eastern basins. All values are considerably lower than those found in most other stable regions. Low Q values throughout the Middle East may be caused by interstitial fluids that have migrated to the crust from the upper mantle, where they were probably generated by hydrothermal reactions at elevated temperatures known to occur there. Low Q 0 values (about 250) are also found in the Oman folded zone, a region with thick sedimentary deposits. <eta> varies inversely with Q 0 throughout most of the Middle East, with lower values (0.4–0.5) in the Arabian Peninsula and higher values (0.6–0.8) in Iran and Turkey. Q 0 and <eta> are both low in the Oman folded zone and western Anatolia. 相似文献
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William Menke Hannah Abend Dalia Bach Kori Newman Vadim Levin 《Surveys in Geophysics》2006,27(6):603-613
The December 26, 2004 Sumatra–Andaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes to occur since global monitoring began in the 1890s. Its seismic moment was M
0 = 1.00 × 1023–1.15 × 1023 Nm, corresponding to a moment-magnitude of M
w
= 9.3. The rupture propagated from south to north, with the southerly part of fault rupturing at a speed of 2.8 km/s. Rupture propagation appears to have slowed in the northern section, possibly to ∼2.1 km/s, although published estimates have considerable scatter. The average slip is ∼5 m along a shallowly dipping (8°), N31°W striking thrust fault. The majority of slip and moment release appears to have been concentrated in the southern part of the rupture zone, where slip locally exceeded 30 m. Stress loading from this earthquake caused the section of the plate boundary immediately to the south to rupture in a second, somewhat smaller earthquake. This second earthquake occurred on March 28, 2005 and had a moment-magnitude of M
w
= 8.5. 相似文献
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Along the east coast of the Andaman Islands, abundant detrital chromian spinels frequently occur in black sands at the confluence of streams meeting the Andaman Sea. The mineral chemistry of these detrital chromian spinels has been used in reconstructing the evolutionary history of the Andaman ophiolite. The chromian spinels show wide variation in compositional parameters such as Cr# [= Cr/(Cr + A1) atomic ratio] (0.13–0.91), Mg# [= Mg/(Mg + Fe2+) atomic ratio] (0.23–0.76), and TiO2 (<0.05–3.9 wt%). The YFe3+[= 100Fe3+/(Cr + A1 + Fe3+) atomic ratio] is remarkably low (usually <10 except for south Andaman). The ranges of chemical composition of chromian spinels are different in each locality. The spinel compositions show very depleted signatures over the entire island, which suggests that all massifs in the Andaman ophiolite were affected under island‐arc conditions. Although the degree of depletion varies in different parts of the island, a directional change in composition of the detrital chromian spinels from south to north is evident. Towards the north the detrital chromian spinels point to less‐depleted source rocks in contrast to relatively more depleted towards the south. The possibilities to explain this directional change are critically discussed in the context of the evolution of Andaman ophiolite. 相似文献