Estimating seismic moments and Lg <Emphasis Type="Italic">Q</Emphasis> using Lg spectra

A multi-event and multi-station inverse method is presented in the paper to simultaneously estimate the seismic moments (M ₀) and source corner frequencies (f _c) of several Jiashi (Xinjiang, China) earthquakes, as well as the apparent 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 M ₀ values obtained by Harvard University using the centroid moment tensor (CMT) inversion and the surface-wave magnitudes as well. After the correction by a typical value of average radiation coefficient for regional SV waves, the M ₀ values from Lg spectral inversion are still close to the corresponding values obtained from CMT inversion. The obtained apparent Q _0Lg values (Lg Q at 1 Hz) are consistent with the tectonic features of corresponding propagation paths. The Q _0Lg values are 351±87, 349±86 and 300±27 for the paths from Jiashi to AAK, TLG and MAKZ, respectively. They are smaller than Q _0Lg values for the paths to KUR, VOS, ZRN and CHK, which are 553±72, 569±58, 550±57 and 603±65, respectively. These results agree with the condition that the paths to AAK, TLG and MAKZ mainly propagate through the mountainous Tianshan area where relatively strong seismic activities and large variations of topography are exhibited, while the paths to KUR, VOS, ZRN and CHK mainly propagate through the stable area of Kazak platform. The Q _0Lg value for the path to WMQ is 462±56. This is also in agreement with the condition that the path to WMQ is basically along the border area between Tianshan Mountain and Tarim Basin, and along this path the variations of topography and crustal thickness are moderate in comparison with that along the path to MAKZ.     

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.     

基于克里金方法的区域震相走时标定技术研究div.

在台网比较稀疏的情况下,台站走时标定是提高低震级事件定位、识别能力的重要手段．为了提高稀疏台网的定位精度,首先利用标定事件和IASPEI1991走时表以及中国大陆走时表,计算了初至P波和Lg波到AAK,MAKZ,NIL,TLG以及WMQ等5个台站的走时残差;接着,采用非平稳贝叶斯克里金方法和走时残差数据构造上述台站的走时校正曲面;最后,通过加入和不加入走时校正定位一系列位置准确的发生在新疆地区的地震事件评估克里金走时校正的有效性．结果表明,克里金走时校正能够较大地提高稀疏台网的定位精度,同时有效地缩小误差椭圆的面积．      

Lg Wave Attenuation in the Isparta Angle and Anatolian Plateau (Turkey)

We estimate Lg wave attenuation using local and regional seismic phases in the Isparta Angle and the Anatolian Plateau (Turkey). The Isparta Angle (IA) is a tectonically active zone forming the boundary between the African Plate and the Anatolian Plateau, and is currently undergoing N–S extensional deformation. The Anatolian Plateau contains many intra-continental faults including the North Anatolian Fault Zone and the East Anatolian Fault Zone as well as the Menderes Massif. A large waveform data set was compiled from a variety of local and regional seismic networks including 121 digital seismic stations (broad-band and short period) between 1999 and 2008 spanning the IA, the Anatolian Plateau and Azerbaijan. The data set was used to determine the nature of Lg wave propagation and characterize the nature of seismic attenuation within the crust of these regions. Lg waveforms were used to calculate the frequency-dependent Lg-Q _o and Lg- $ \eta $ . A wide range of Lg-Q _o values was obtained between ~52 ± 6 and 524 ± 227. Low Lg-Q _o values (~90–155) are calculated towards the north of IA, Iskenderun Gulf and its vicinity, Bingöl-Karl?ova, Izmit and its vicinity. Lg-Q _o values are especially low (<90) along the Menderes Massif and the Aksehir-Simav Fault Zones. This may be due to intrinsic attenuation of Lg associated with the partially molten crust and young volcanism. The high Lg-Q _o values (~350) are probably caused by the crust not being subject to large amounts of extensional deformation like the Antalya Gulf and apparently being thick enough to support Lg propagation. Relatively higher values along the border of this subduction zone and plate boundary might be related to the Taurus Mountain belts and Bitlis-Zagros Suture Zone. The lateral frequency dependency Lg- $ \eta $ is also consistent with high tectonic activity in this region.     

Lg magnitude scale in the mainland of China

The data of short-period seismograms had been collected widely in the mainland area of China not including Xinjiang and Tibet. The physical quantities of Lg wave are determined respectively in the five subregions. The group velocities of priminary arrival and maximum amplitude of Lg wave are equal to 3.54±0.02 km/s and 3.30±0.05 km/s, respectively. The periods of Lg waves are between 0.2s to 1.2s, averaging 0.7s. The γ-values of Lg waves in the five subregions are equal to 0.0034±0.0001 km⁻¹ for East, 0.0031±0.0004 km⁻¹ for Southwest, 0.0027±0.0004 km⁻¹ for Northeast, 0.0022±0.0001 km⁻¹ for South, and 0.0021±0.0002 km⁻¹ for Northwest subreqion, respectively. The average γ-value for the five subregions, γ=0.0027±0.0006 km⁻¹. The relations among the amplitude ratioH/Z, the station correctionD _z andD _h of amplitudes, and among them and station site condition are discussed. The subregional magnitude calibration functions ofm _Lg had been established according to each regional γ-value. From these, the unified magnitude calibration function of Chinese mainland not including Xinjiang and Tibet was given by

QLg tomography in Gujarat,Western India

We propose a novel Lg attenuation tomography model (Q_Lg tomography) for the state of Gujarat, Western India, using earthquake data recorded by the Gujarat Seismic Network, operated by the Institute of Seismological Research in Gandhinagar. The waveform dataset consist of 400 3-component recordings, produced by 60 earthquakes with magnitude (M_L) spanning from 3.6 to 5.1, recorded at 60 seismic stations having epicentral distances spanning between 200 and 500 km. Spectral amplitude decays for Lg wave displacement were obtained by generalized inversion at 17 frequencies spanning between 0.9 and 9 Hz. Lg wave propagation efficiency was measured by Lg/Pn spectral ratio categorizing as efficient ratio ≥6 for 86%, intermediate ratio of 3–6 for 10% and inefficient ratio <3 for 4% paths of total 400 ray paths. The earthquake size and quality of waveform recorded at dense network found sufficient to resolve lateral variation of Q_Lg in Gujarat.Average power-law attenuation relationship obtained for Gujarat as Q_Lg(f) = 234f^0.64, which corresponds to high attenuation in comparison to peninsular India shield region and other several regions around the world. Q_Lg tomography resolves the highly attenuating crust of extremely fractured Saurashtra region and tectonically active Kachchh region. The Gujarat average attenuation is also lying in between them. The low attenuation in Cambay and Narmada rift basins and extremely low attenuation in patch of Surendranagar area is identified. This study is the first attempt and can be utilized as pivotal criteria for scenario hazard assessment, as maximum hazard has been reported in highly attenuating tectonically active Kachchh region and in low attenuating Cambay, Narmada and Surendranagar regions. The site and source terms are also obtained along with the Q_Lg inversion. The estimated site responses are comparable with observed local geological condition and agree with the previously reported site amplifications at the same sites. The source terms are comparable with local magnitude estimated from Network. The M_w (Lg) is nearly equivalent to M_L (GSN) and the slight differences are noted for larger magnitude events.     

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.     

The revisedm Lg scale in six eastern provinces of China

The influence of the four different methods of measuring Lg amplitude, and the selection of different geometrical attenuation coefficient ζ-values (=5/6 or 1) on the determination ofγ value of Lg wave are discussed.γ=0.0034±0.0001km^?1 (when ζ=5/6) for six eastern provinces is redetermined. The revised magnitude calibration function ofm _Lg (mxh),q _E (Δ)=(5/6)logΔ+0.00147Δ+1.81 is deduced.     

<Emphasis Type="Italic">Lg</Emphasis> Body-Wave Magnitude Scaling for the Continental Margin around Korea and Japan

Regional body-wave magnitude scalings are essential for quantification of small and moderate-size earthquakes that are observed only up to regional distances. Crustally-guided shear waves, Lg, develop stably at regional distances in continental crusts and are minimally influenced by the source radiation patterns. Lg body-wave magnitude scalings, m_b(Lg),m_b(Lg), are widely used for assessment of sizes of regional crustal events. The m_b(Lg)m_b(Lg) scaling has rarely been tested in continental margins where Lg waves are significantly attenuated due to abrupt lateral variation of crustal structures. We test the applicability of m_b(Lg)m_b(Lg) scaling to the eastern margin of the Eurasian plate around the Korean Peninsula and Japanese islands. Both third-peak and root-mean-square (rms) amplitudes of Lg vary significantly according to the crustal structures along raypaths, causing apparent underestimation of m_b(Lg).m_b(Lg). Implementation of raypath-dependent quality factors (Q) allows accurate estimation of m_b(Lg),m_b(Lg), retaining the transportability of m_b(Lg)m_b(Lg) in the continental margin around Korea and Japan. The calibration constants for an rms-amplitude-based m_b(Lg)m_b(Lg) scaling are not determined to vary by region in the continental margin due to complicated crustal structures. The calibration constants are determined to be distance-dependent. Both the third-peak-amplitude-based and rms-amplitude-based m_b(Lg)m_b(Lg) scalings yield accurate magnitude estimates when raypath-dependent quality factors are implemented.     

Lg coda Q0 value and its relation with the tectonics in Chinese mainland and adjacent regions

We have collected 432 vertical component records from 45 stations of new CENC （China Earthquake Network Center） in Chinese mainland and adjacent regions. These records were used to calculate Q0 （Q at 1Hz） and η values of Lg coda from each station by the stack spectral ratio （SSR） method. Then the tomography method was applied to obtaining lateral variation of Q0 and η values in Chinese mainland and adjacent regions. The result indicates that Q0 value varies between 150 and 600 in the studied areas. Yunnan, southwest Sichuan, and northwest Myanmar show the lowest Q0 value （Q0〈240） and the crust of these regions is characterized by complicated crack and strong hydrothermal activity. The highest Q0 value （Qo〉510） exists in the border of southern Mongolia, Alxa and Ordos block. The η value varies between 0.45 and 0.75 in Chinese mainland and its adjacent regions.     

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.     

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.     

Distribution of Lg coda Q in Xinjiang and its adjacent regions

In this study, we collected 1 156 broadband vertical components records at 22 digital seismic stations in Xinjiang region, ürümqi station, and 7 stations in the adjacent regions during the period of 1999–2003. The records were firstly processed by the stacked spectral ratio method to obtain Q ₀ (Q at 1 Hz) and the frequency correlation factor η corresponding to each path. Based on the results, the distribution images of Q ₀ and η in 1°×1° grids for Xinjiang region were gained by the back-projection technique. The results indicate that Q ₀ is high (300–450) in the Tarim platform and marginal Siberian platform, while Q ₀ is low (150–250) in the southern regions as west Kunlun fold system and Songpan-Ganzi fold system. In the northern regions as Junggar fold system and Tianshan fold system, Q ₀ is also low (250–300) and η varies between 0.5 and 0.9. Foundation item: National Natural Science Foundation of China (49974012) and Joint Seismological Science Foundation of China (604004).     

华北地区Lg尾波衰减研究--Lg尾波Q的测量

利用1989～1998年间发生在中国华北地区ML≥3.0级的地震事件,华北遥测台网以及数字化台网记录到的信噪比较高的Lg尾波时间序列,经过滤波、消除背景噪声等预处理,采用单台记录叠加频谱比法,得到传播路径上Lg尾波Q0（Q=Q0fη, Q0是1Hz的Q值,η是Q值与频率相关系数）,进而获得该地区Q0和η的空间分布. 测量结果表明,华北地区地壳Lg尾波Q0值分布存在显著的横向不均匀性,这种不均匀性与该地区地壳的速度分布基本一致,表现为与该地区地表地质特征明显的一致性,即传播路径位于太行山隆起和燕山隆起区为高Q0值分布;传播路径位于冀中坳陷和黄骅坳陷等平原区表现为低Q0值;传播路径位于隆起区和坳陷区二者之间,其Q0值也介于二者之间,说明华北地区的地壳结构没有很大的阶跃式变化而使波导遭到破坏,Lg尾波Q0值基本上反映了地壳内介质滞弹性的固有衰减.     

Lapse time and frequency-dependent attenuation characteristics of coda waves in the Northwestern Himalayas

We analyzed the local earthquakes waveform recorded on a broadband seismic network in the northwestern Himalayan Region to compute lapse time and frequency dependence of coda Q (Q _c). The observed Q _c values increase with increasing lapse time at all frequency bands. The increase in Q _c values with lapse time is attributed to an increase in Q _c with depth. This implies that attenuation decreases with increasing depth. The approximate radius of medium contributing to coda generation varies from 55 to 130 km. By comparing the Q _c values with those from other regions of the world, we find that they are similar to those obtained from tectonically active regions. The estimated Q _c values show a frequency-dependent relationship, Q _c = Q ₀ f ⁿ , where Q ₀ is Q _c at 1 Hz and n represents degree of frequency dependence. They represent the level of heterogeneity and tectonic activity in an area. Our results show that northwest Himalayas are highly heterogeneous and tectonically very active. Q ₀ increases from 113 ± 7 to 243 ± 10 and n decreases from 1.01 ± 0.05 to 0.85 ± 0.03 when lapse time increases from 30 to 70 s. As larger time window sees the effect of deeper part of the Earth, it is concluded that Q ₀ increases and n decreases with increasing depth; i.e., heterogeneity decreases with depth in the study area.     

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.     

Sedimentary Basins and the Blockage of Lg Wave Propagation in the Continents

v--vThe phenomenon of "Lg blockage," where Lg is strongly attenuated by crustal heterogeneities, poses a serious problem to CTBT monitoring because Lg is an important seismic phase for discrimination. This paper examines blockage in three continental regions where the Lg blockages may be caused by large, enclosed sedimentary basins along the propagation path. The Barents Sea Basin blocks Lg propagation across the Barents Sea from the Russian nuclear test sites at Novaya Zemlya to Scandinavian stations. Also, "early Lg" waves are observed in Sn codas on NORSAR, NORESS, and ARCESS recordings of Novaya Zemlya explosions where direct Lg is blocked. Early Lg waves may have resulted from Sn-to-Lg mode conversion at the contact between the Barents Basin and the Kola Peninsula. The Northern and Southern Caspian Sea Basins also block Lg waves from PNEs and earthquakes, perhaps due to thick, low-velocity, low-Q sediments replacing the granitic layer rocks in the crust. Lg blockage has also been observed in the Western Mediterranean/Levantine Basin due to low-Q sediments and crustal thinning. A "basin capture" model is proposed to explain Lg blockage in sedimentary basins. In this model, shear waves that reverberate in the crust and constitute the Lg wave train are captured, delayed, and attenuated by thick, low-velocity sediments that replace the "granitic" layer rocks of the upper crust along part of the propagation path. Sn waves, which propagate below the basin, would not be blocked and in fact, the blocked Lg waves may be diverted downward into Sn waves by the low velocity sediments in the basin.     

单台Lg尾波Q_(Lg)~c的测量研究

将单台记录叠加频谱比法（简称SSR法）应用于震中距小于500km的区域地震Lg尾波为lHz下的值,n是与频率f有关的指数)的提取之中·为了检验方法的稳定性和可靠性,我们选用两组发生在不同地区的地震记录．一组是1989-10-18大同主震（ML＝5．9）的15个余震,北京遥测地震台网记录到的信噪比较高的Lg尾波序列。另一组是秦岭造山带数字流动地震观测台网记录到的1995年甘肃平凉两次地震信噪比较高的Lg尾波序列．每一组地震的震源参数基本相同,即它们到某一特定台站的路径几乎相同,得到的　可以认为是不受震源影响的地震一台站路径上的平均衰减效应·两区域　的测量结果表明：（1）尽管两个地震观测台网所用观测仪器不同,每个台站的观测条件不同,所记录的地震也不尽相同,但对Q₀和,的拟合结果线性程度都比较高,说明SSR法是稳定的;（2）同一震群的不同地震到某一特定台站测得的结果基本一致,即Q₀值及其平均值大都在其标准差范围之内,说明SSR法是可靠的。（3）对同一地震不同方位各台站的Lg尾波测量结果可以看出,穿过不同构造区的Q₀值存在明显差...     

Lg Coda Q and its Relation to the Geology and Tectonics of the Middle East

—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.     

An inversion of site response and Lg attenuation using Lg waveform

Based on spectral ratio method, a joint inversion method was used to obtain parameters of Lg wave attenuation and site response. The inversion method allows simple and direct (two-parameter) determination of Lg wave attenua- tion and site response from sparse spectral data, which are not affected by radiation pattern factor and different response of same instrument after geometrical spreading. The method was used successfully for estimating site re- sponse of stations of Zhejiang Seismic Network and measuring Lg wave attenuation. The study is based on 20 earth- quakes occurred in northeast of Taiwan with magnitude MS5.0~6.7 and 960 seismic wave records from 16 stations in Zhejiang area from 2002 to 2005. The parameters of site response and Lg attenuation were calculated with a fre- quency interval of 0.2 Hz in the range of 0.5 Hz to 10 Hz. Lg wave attenuation coefficient corresponding to U-D, E-W and N-S components are γ ( f )=0.00175 f 0.43485, γ ( f )=0.00145f 0.48467 and γ ( f )=0.0021f 0.41241, respectively. It is found that the site response is component-independent. It is also found that the site response of QIY station is significant above the frequency of 1.5 Hz, and that the site response of NIB station is low for most frequency