Lg Coda Q and its Relation to the Structure and Evolution of Continents: A Global Perspective

—Tomographic maps of Lg coda Q (Q ^c _Lg) variation are now available for nearly the entire African, Eurasian, South American, and Australian continents, as well as for the United States. Q ^c _Lg at 1 Hz (Q ₀) varies from less than 200 to more than 1000 and Q ^c _Lg frequency dependence (<eta>) varies between 0.0 and nearly 1.0. Q ₀ appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region. A plot of Q ₀ versus time since that activity indicates that a single Q ₀-time relation approximates most mean Q ₀ values. Those that deviate most from the trend lay in Australia, the Arabian Peninsula, and the East African rift. The increase in Q ₀ with time may be due to a continual increase in crustal shear wave Q (Q ) caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity. Extrapolated values of Q ^c _Lg at 5 Hz (using Q ₀ and <eta> values measured at 1 Hz and assuming that <eta> is constant in all regions between 1 and 5 Hz) show a similar percentage-wise increase with times that has elapsed since the most recent activity. Other factors that can reduce Q ₀ in continental regions include thick accumulations of sediment (especially sandstone and shale of Mesozoic age and younger), severe velocity gradients at the crust-mantle transition and, possibly, lateral variations in the depth, thickness, and severity of those gradients. Severe and large increases of Q in the mid-crust of some regions can cause relatively large values of <eta>, even if the frequency dependence of Q is small.     

Lg Coda Q Variations across South America and their Relation to Crustal Evolution

Nine broadband seismograph stations in South America have provided 389 recordings of Lg coda with paths that cover most of the continent. Lg coda Q (Q0v) and frequency dependence <(eta)> values at 1 Hz, obtained from these records, were inverted using back-projection tomography to obtain regionalized maps of Q0 and <eta>. The entire western margin of the continent (the active Andean mountain belt) is typified by low Q0 (250–450), whereas broad regions of high Q0 (700–1100) span the central Brazilian shield and contiguous regions to the north and south. Intermediate Q0 (450–700) characterizes the northern Patagonia platform and most of the Atlantic shield. Reduced Q0 in the Atlantic shield may be related to tectonic or igneous activity that occurred during the breakup of Gondwanaland during the Jurassic period. This Q distribution is generally consistent with earlier studies where Q0 was found to be directly proportional to the time that has elapsed since the most recent episode of major tectonic or orogenic activity in any region. Reduced Q0 in the Patagonian platform may, however, be due to young sediments there. Q0 is slightly higher in two portions of the Andean belt (between latitudes 2.0°N and 10.0°S, and between latitudes 24.0°S and 34.0°S) than in other portions of the belt. These variations are consistent with results of earlier studies of body-wave attenuation and heat flow in the Andean mountain belt. Spatial variations of <eta> generally vary inversely with Q0v, being low (0.0–0.2) throughout a broad region centered in the central Brazil shield and extending to the northeastern coast. All surrounding regions except that to the northeast exhibit intermediate to high (0.4–0.8 and possibly higher) <eta> values. Possible biasing of Lg coda Q measurements by proximity to the transition between the South American and Pacific plates was examined using records from a station near that boundary and was found to be small.     

Regional Variation of Lg Coda Q in the Continental United States and its Relation to Crustal Structure and Evolution

—Records from broadband digital stations have allowed us to map regional variations of Lg coda Q across almost the entire United States. Using a stacked ratio method we obtained estimates of Q ₀ (Lg coda Q at 1 Hz) and its frequency dependence, <eta>, for 218 event-station pairs. Those sets of estimates were inverted using a back-projection method to obtain tomographic images showing regional variations of Q ₀ and <eta>. Q ₀ is lowest (250–300) in the California coastal regions and the western part of the Basin and Range province, and highest (650–750) in the northern Appalachians and a portion of the Central Lowlands. Intermediate values occur in the Colorado Plateau (300–500), the Columbia Plateau (300–400), the Rocky Mountains (450–550), the Great Plains (500–650), the Gulf Coastal Plain and the southern portion of Atlantic Coastal Plain (400–500), and the portions of the Central Lowlands surrounding the high-Q region (500–550). The pattern of Q ₀ variations suggests that the United States can be divided into two large Q provinces. One province spans the area from the Rocky Mountains to the Atlantic coast, is tectonically stable, and exhibits relatively high Q ₀?. The other extends westward from the approximate western margin of the Rocky Mountains to the Pacific coast, is tectonically active, and exhibits low Q ₀?. The transition from high to low Lg coda Q in the western United States lies further to the west than does an upper mantle transition for Q and electrical resistivity found in earlier studies. The difference in Q ₀ between the western and eastern United States can be attributed to a greater amount of interstitial crustal fluids in the west. Regions of moderately reduced Q within the stable platform often occur where there are accumulations of Mesozoic and younger sediments. Reduced Q ₀ in the southeastern United States may not be due to anelasticity but may rather be explained by a gradational velocity increase at the crust-mantle boundary that causes shear energy to leak into the mantle.     

四川及邻区Lg尾波的Q值分布特征

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。     

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.     

Tomographic Imaging of Lg and Sn Propagation in the Middle East

?—?Observations based on relatively limited data recorded by sparsely distributed stations have indicated that regional seismic phase propagation (Lg and Sn) is very complex in the Middle East. Accurate characterization of regional seismic wave propagation in this region necessitates the use of a large number of seismic stations. We have compiled a large data set of regional and local seismograms recorded in the Middle East. This data set comprises approximately four years of data from national short-period networks in Turkey and Syria, data from temporary broadband arrays in Saudi Arabia and the Caspian Sea region, and data from GSN, MEDNET, and GEOFON stations in the Middle East. We have used this data set to decipher the character and pattern of regional seismic wave propagation. We have mapped zones of blockage as well as inefficient and efficient propagation for Lg, Pg, and Sn throughout the Middle East. Two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the Middle East.¶We observe evidence of major increase in Lg attenuation, relative to Pg, across the Bitlis suture and the Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. We also observe a zone of inefficient Sn propagation along the Dead Sea fault system which coincides with low Pn velocities along most of the Dead Sea fault system and with previous observations of poor Sn propagation in western Jordan. Our observations indicate that in the northern portion of the Arabian plate (south of the Bitlis suture) there is also a zone of inefficient Sn propagation that would not have been predicted from prior measurements of relatively low Pn velocities. Mapped high attenuation of Sn correlates well with regions of Cenozoic and Holocene basaltic volcanism. These regions of uppermost mantle shear-wave attenuation most probably have anomously hot and possibly thin lithosphere.     

Estimating seismic moments and Lg Q using Lg spectra

A multi-event and multi-station inverse method is presented in the paper to simultaneously estimate the seismicmoments (Mo) and source comer frequencies (fc) of several Jiashi (Xinjiang, China) earthquakes, as well as theapparent Lg Q models for the paths from Jiashi to eight seismic stations (WMQ, AAK, TLG MAKZ, KUR, VOS,ZRN and CHK) in Central Asia. The resultant seismic moments correlate well with the Mo values obtained by Harvard University using the centroid moment tensor (CMT) inversion and the surface-wave magnitudes as well. Afterthe correction by a typical value of average radiation coefficient for regional SV waves, the Mo values from Lgspectral inversion are still close to the corresponding values obtained from CMT inversion. The obtained apparentQOLg values (Lg Q at 1 Hz) are consistent with the tectonic features of corresponding propagation paths. The QoLgvalues are 351±87, 349±86 and 300±27 for the paths from Jiashi to AAK, TLG and MAKZ, respectively. They aresmaller than QoLg values for the paths to KUR, VOS, ZRN and CHK, which are 553±72, 569±58, 550±57 and603±65, respectively. These results agree with the condition that the paths to AAK, TLG and MAKZ mainlypropagate through the mountainous Tianshan area where relatively strong seismic activities and large variations oftopography are exhibited, while the paths to KUR, VOS, ZRN and CHK mainly propagate through the stable areaof Kazak platform. The QoLg value for the path to WMQ is 462±56. This is also in agreement with the conditionthat the path to WMQ is basically along the border area between Tianshan Mountain and Tarim Basin, and alongthis path the variations of topography and crustal thickness are moderate in comparison with that along the path toMAKZ.     

Distribution of Lg coda Q in Xinjiang and its adjacent regions

Introduction Lg wave is usually considered as one of the most prominent phases recorded by short-period or broadband seismometers in the continental area. Therefore, it is often used to determine the magnitude of regional earthquakes, the yield of nuclear explosions (Nuttli, 1986) and the horizon-tal variations of crustal attenuation. A theoretical ray approach shows that Lg is the superposition of higher-mode surface waves propagating in the continental crust (Knopoff et al, 1973; Panza and…     

Distribution of L_g coda Q in the Chinese continent and its adjacent regions

In continent Lg is usually one of the predominant phases recorded by short-period or broad-band seismometers. A ray-theoretical approach shows that Lg wave is the superposition of higher-mode surface waves propagating in the continental crust[1—4]. The g…     

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。     

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.     

川滇地区Lg波Q值层析成像研究

本研究联合使用川滇地区36个台站(云南区域数字台网中22个台站,四川区域数字地震台网中14个台站)记录的644个帆3.5～6.0地震共7468条波形数据,开展川滇地区Lg波Q值层析成像研究.研究中以2.8～3.6km/s的传播速度窗选取Lg波波段,采用平移窗方法计算Lg波垂直向观测位移谱.我们首先利用Lg波观测谱反演得到了研究区的平均QLg(f),通过棋盘测试方法确定将研究区划分为0.5°×0.5°的网格,用平均值QLg-1作为每个网格的初始值,分别反演了0.5Hz,1Hz,2Hz,5Hz等多个频率的QLg(f)分布.     

Coda Q Estimates in the Koyna Region, India

—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.     

Mapping Crustal Heterogeneity Using Lg Propagation Efficiency Throughout the Middle East, Mediterranean, Southern Europe and Northern Africa

—?In this paper we describe a technique for mapping the lateral variation of Lg characteristics such as Lg blockage, efficient Lg propagation, and regions of very high attenuation in the Middle East, North Africa, Europe and the Mediterranean regions. Lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These applications can give significantly biased results if the Lg phase is reduced or blocked by discontinuous structure or thin crust. Mapping these structures using quantitative techniques for determining Lg amplitude attenuation can break down when the phase is below background noise. In such cases Lg blockage and inefficient propagation zones are often mapped out by hand. With our approach, we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of Lg. The visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest. We trace Lg rays for each event/station pair, which is simply the great circle path, and attribute to each cell a value equal to the maximum value of the Lg/P-coda amplitude ratio for all paths traversing that particular cell. The resulting map, from this empirical approach, is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone. This map can then be used to screen out events with blocked Lg prior to performing Q tomography, and to avoid using Lg-based methods of event identification for the CTBT in regions where they cannot work.¶For this study we applied our technique to one of the most tectonically complex regions on the earth. Nearly 9000 earthquake/station raypaths, traversing the vast region comprised of the Middle East, Mediterranean, Southern Europe and Northern Africa, have been analyzed. We measured the amplitude of Lg relative to the P-coda and mapped the lateral variation of Lg propagation efficiency. With the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of Lg propagation. We observe that the propagation characteristics of Lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region. For example, clear strong Lg arrivals are observed for paths crossing the stable continental interiors of Northern Africa and the Arabian Shield. In contrast, weakened to absent Lg is observed for paths crossing much of the Middle East, and Lg is absent for paths traversing the Mediterranean. Regions that block Lg transmission within the Middle East are very localized and include the Caspian Sea, the Iranian Plateau and the Red Sea. Resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations. Lg propagation is best resolved within the Middle East where regions of crustal heterogeneity on the order of 100?km are imaged (e.g., South Caspian Sea and Red Sea). Crustal heterogeneity is resolvable but is poorest in seismically quiescent Northern Africa.     

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.     

Coda Q c Attenuation and Source Parameter Analysis in Friuli (NE Italy) and its Vicinity

—?The digital data acquired by 16 short-period seismic stations of the Friuli-Venezia-Giulia seismic network for 56 earthquakes of magnitude 2.3–4.7 which occurred in and near NE Italy have been used to estimate the coda attenuation Q _c and seismic source parameters. The entire area under study has been divided into five smaller regions, following a criterion of homogeneity in the geological characteristics and the constrains imposed by the distribution of available events. Standard IASPEI routines for coda Q _c determination have been used for the analysis of attenuation in the different regions showing a marked anomaly in the values measured across the NE border between Friuli and Austria for Q ₀ value. A large variation exists in the coda attenuation Q _c for different regions, indicating the presence of great heterogeneities in the crust and upper mantle of the region. The mean value of Q _c (f) increases from 154–203 at 1.5?Hz to 1947–2907 at 48?Hz frequency band with large standard deviation estimates.¶Using the same earthquake data, the seismic-moment, M ₀, source radius, r and stress-drop, Δσ for 54 earthquakes have been estimated from P- and S-wave spectra using the Brune's seismic source model. The earthquakes with higher stress-drop (greater than 1?Kbar) occur at depths ranging from 8 to 14?km.     

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.     

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。