Q P and Q S in Beijing and Jianchuan, Yunnan areas

Events from the December 1982 Huairou County, Beijing, and the July 1982 Jianchuan, Yunnan earthquake series were recorded at one station in Beijing City and at four stations in Eryuan area, Yunnan, respectively. Dividing the spectra (for P and S waves) from the smaller events in the series by spectra (for corresponding P and S waves) from the larger events in cach region, we have determined the high frequency source spectral decay rate to be ω⁻¹in both cases. Through trial and error method, we are able to determine the appropriate constantQ _P andQ _S that corrects the individual spectra to the proper high frequency decay rate. It is found that aQ _P of 800 and aQ _S of 550 can adequately compensate for the attenuation of the waves in Beijing area. For Jianchuan, Yunnan area the corresponding values are 900 and 400. This subject is supported by Chinese Joint Seismological Science Foundation.     

Research on coda Q distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated theQ-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used. These earthquakes were recorded from January 1, 1989 to December 31, 1990 at 20 stations. This paper shows the variations of the codaQ-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the codaQ-factors in these two years. The results indicate that codaQ-factor depends strongly on the lapse time and frequency. It is assumed that whenQ _C=Q ₀f^η, for the three time windows of 15–30s, 30–60s and 60–90s, the average values ofQ ₀ are 48, 115 and 217; and the average values ofη are 0. 89, 0.91 and 0.74, respectively. Contribution No. 95A0009, Institute of Geophysics, SSB, China. This work is a contract subject 85-04-01-02 of the State Seismological Bureau, China.     

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).     

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 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. Foundation item: Foundation of Verification Researches for Army Control Technology (413290102).     

Research on codaQ distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated theQ-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used. These earthquakes were recorded from January 1, 1989 to December 31, 1990 at 20 stations. This paper shows the variations of the codaQ-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the codaQ-factors in these two years. The results indicate that codaQ-factor depends strongly on the lapse time and frequency. It is assumed that whenQ _C=Q ₀f^η, for the three time windows of 15–30s, 30–60s and 60–90s, the average values ofQ ₀ are 48, 115 and 217; and the average values ofη are 0. 89, 0.91 and 0.74, respectively.     

Fracture characteristics of the 1997 Jiashi, Xinjiang, China, earthquake swarm inferred from source spectra

Broadband P and S waves source spectra of 12 MS5.0 earthquakes of the 1997 Jiashi, Xinjiang, China, earthquake swarm recorded at 13 GDSN stations have been analyzed. Rupture size and static stress drop of these earthquakes have been estimated through measuring the corner frequency of the source spectra. Direction of rupture propagation of the earthquake faulting has also been inferred from the azimuthal variation of the corner frequency. The main results are as follows: ①The rupture size of MS6.0 strong earthquakes is in the range of 10～20 km, while that of MS=5.0～5.5 earthquakes is 6～10 km.② The static stress drop of the swarm earthquakes is rather low, being of the order of 0.1 MPa. This implies that the deformation release rate in the source region may be low. ③ Stress drop of the earthquakes appears to be proportional to their seismic moment, and also to be dependent on their focal mechanism. The stress drop of normal faulting earthquakes is usually lower than that of strike-slip type earthquakes. ④ For each MS6.0 earthquake there exists an apparent azimuthal variation of the corner frequencies. Azimuthally variation pattern of corner frequencies of different earthquakes shows that the source rupture pattern of the Jiashi earthquake swarm is complex and no uniform rupture expanding direction exists.     

大同地震窗P波频散衰减的异常变化特征

通过测量介质品质因子来开展大同窗附近测震台站记录P波衰减频散特征的研究,并分析大同窗近场区和外围区震例前后的变化。对山自皂台记录的2001年以来大同窗地震序列的研究结果表明,近场区3次和外围区6次地震前,介质品质因子显示超出1倍均方差的高值变化特征,震后逐渐恢复到背景起伏状态;与大同窗传统的频次指标相比,介质品质因子的映震效果更好;阳原台的结果表明,仅近场区1次和外围区2次地震前,介质品质因子显示超出1倍均方差的高值变化特征,且有虚报、漏报。所处地质构造位置不同、传播路径介质差异可能是引起上述两个台站附近介质品质因子的地震对应效果明显不同的原因。     

Ground motion attenuation relation for small to moderate earthquakes in Fujian region, China

We collect 1974 broad-band velocity records of 94 earthquakes (ML=2.8~4.9, △=13~462 km) from seven stations of the Fujian Seismic Network from March 1999 to March 2007. Using real-time simulation, we obtain the corresponding acceleration and then adopt different models to analyze the seismic data. As a result, a new attenuation relationship between PGA and PGV of the small and moderate earthquakes on bedrock site in Fujian region is established. The Yongchun earthquake occurred recently verifies the attenuation relationship well. This paper provides a new approach for studying the ground motion attenuation relationship using velocity records.     

A single-station coda solution for source, attenuation and site factors

Based on the empirical formulation formed from coda observations, the digital waveforms from 33 local events with magnitude M _L ranging between 2.1 and 3.5, recorded at Changli station of Beijing Telemetered Seismographic Network from 1989 to 1991, are used to compute coda Q for the Changli region and the source factors of all earthquakes by single-station coda method. Furthermore, assuming a certain source model, we have obtained the station site frequency response and source spectra, as well as source parameters such as corner frequencies, seismic moments and stress drops and so on. Their variations with time are monitored before and after larger earthquakes. Because the coda method can more effectively reduce the influence of source radiation pattern and a particular propagation path than direct wave method, more data can be used and more accurate results can be obtained, which provided a possible approach to study the source properties and reveal the variation of source parameters before larger earthquakes. Contribution No. 97A0107, Institute of Geophysics, SSB, China.     

Three-dimensional Q structure in Jiashi earthquake region of Xinjiang

3-D S-waveQ structure in Jiashi earthquake region is inverted based on the attenuation of seismic waves recorded from earthquakes in this region in 1998 by the Research Center of Exploration Geophysics (RCEG), CSB, and a rough configuration of deep crustal faults in the earthquake region is presented. First, amplitude spectra of S-waves are extracted from 450 carefully-chosen earthquake records, called observed amplitude spectra. Then, after instrumental and site effect correction, theoretical amplitude spectra are made to fit observed amplitude spectra with nonlinear damped least-squares method to get the observed travel time overQ, provided that earthquake sources conform to Brune’s disk dislocation model. Finally, by 3-D ray tracing method, theoretical travel time overQ is made to fit observed travel time overQ with nonlinear damped least-squares method. In the course of fitting, the velocity model, which is obtained by 3-D travel time tomography, remains unchanged, while onlyQ model is modified. When fitting came to the given accuracy, the ultimateQ model is obtained. The result shows that an NE-trending lowQ zone exists at the depths of 10–18 km, and an NW-trending lowQ zone exists at the depths of 12–18 km. These roughly coincide with the NE-trending and the NW-trending low velocity zones revealed by other scientists. The difference is that the lowQ zones have a wider range than the low velocity zones. Foundation item: Joint Seismological Science Foundation of China (957-07-414) and State Key Basic Research Development and Programming Project (95-13-02-02). Contribution No. RCEG200105, Research Center of Exploration Geophysics, China Seismological Bureau.     

Estimation of coda and shear wave attenuation in the volcanic area in SE Sabalan Mountain,NW Iran

The quality factors of coda and shear waves have been estimated for the SE Sabalan Mountain, geothermal region in northwestern Iran. We have analyzed 65 local earthquakes with magnitude of 2.8 to 6.1 and 2.8 to 5 for shear and coda wave quality factor estimation, respectively. These events were recorded on five stations installed by Building and Housing Research Center Network. Coda normalization and Spectral decay methods have been used to estimate the frequency dependence attenuation relation for shear wave, and single back-scattering method for coda waves. We have observed that the coda normalization method has supplied significantly higher Q _S values as compared to the spectral method. The results show that, in general, Q values are significantly smaller for the entire frequency range as compared to tectonically active areas and are close to the values for volcanic areas.     

Inversion of source parameters and attenuation of the Tangshan aftershocks

Source parameters and characteristics of regional attenuation of Tangshan aftershocks are studied by using digital records of Tangshan aftershocks. An inversion method of P wave spectra to reduce influence on the ambiguity in the estimates of parameters by the usual spectrum analysis method is developed. By testing with digital simulation data and applying to actual data, it is confirmed that the method is usable. Source parameters of the Tangshan Luanxian area are obtained by using records of 35 earthquakes at 5 stations.Q values of P wave and high frequency decay rate γ of source spectrum at 5 stations are obtained. TheQ values range from 408 to 847, and the mean value is 520; whiley ranges from 1.54 to 3.22, and the mean value is 2.41. In the studies of spectra of the micro-earthquakes in the Luanxian area, that stress drop increases with increasing earthquake moment is found. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 430–441, 1991.     

Study on the sources of some deep focus earthquakes in Northwest Pacific region

The sources for thirteen deep focus earthquakes ofm _b≥5.5 andh>400 km in Northwest Pacific region were studied using waveform fitting and shear fracture source model. The source parameters were obtained as follows: focal depth, faulting plane, slip direction, rupture velocity, rupture length, rupture direction and scalar moment tensor. It was found that all these earthquake sources can be interpreted as shear faulting and have simple source time functions. The strike direction of faulting plane for most deep focus earthquakes coincides with that of the subduction zone, especially in the deep part of the subduction zone, it results in the tendency of reducing the dip angle of the subduction zone. The multiple point source model was also used to study the source process. The waveform fitting is better than the shear fracture model, but the general rupture direction which coincides with that from unilateral shear source model can not be obtained from the multiple point source model. This study is supported by the National Science Foundation of China and the Chinese Joint Seismological Science Foundation.     

Time-space evolution characteristics of abrupt variation of wave velocity ratio in the seismogenic process of recent strong earthquakes in Yunnan area

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Q P andQ S in Beijing and Jianchuan,Yunnan areas

Events from the December 1982 Huairou County, Beijing, and the July 1982 Jianchuan, Yunnan earthquake series were recorded at one station in Beijing City and at four stations in Eryuan area, Yunnan, respectively. Dividing the spectra (for P and S waves) from the smaller events in the series by spectra (for corresponding P and S waves) from the larger events in cach region, we have determined the high frequency source spectral decay rate to be ω^?1in both cases. Through trial and error method, we are able to determine the appropriate constantQ _P andQ _S that corrects the individual spectra to the proper high frequency decay rate. It is found that aQ _P of 800 and aQ _S of 550 can adequately compensate for the attenuation of the waves in Beijing area. For Jianchuan, Yunnan area the corresponding values are 900 and 400.     

Stochastic ground-motion simulation of two Himalayan earthquakes: seismic hazard assessment perspective

The earthquakes in Uttarkashi (October 20, 1991, M _w 6.8) and Chamoli (March 8, 1999, M _w 6.4) are among the recent well-documented earthquakes that occurred in the Garhwal region of India and that caused extensive damage as well as loss of life. Using strong-motion data of these two earthquakes, we estimate their source, path, and site parameters. The quality factor (Q _β ) as a function of frequency is derived as Q _β (f) = 140f ^1.018. The site amplification functions are evaluated using the horizontal-to-vertical spectral ratio technique. The ground motions of the Uttarkashi and Chamoli earthquakes are simulated using the stochastic method of Boore (Bull Seismol Soc Am 73:1865–1894, 1983). The estimated source, path, and site parameters are used as input for the simulation. The simulated time histories are generated for a few stations and compared with the observed data. The simulated response spectra at 5% damping are in fair agreement with the observed response spectra for most of the stations over a wide range of frequencies. Residual trends closely match the observed and simulated response spectra. The synthetic data are in rough agreement with the ground-motion attenuation equation available for the Himalayas (Sharma, Bull Seismol Soc Am 98:1063–1069, 1998).     

Temporal variation of body wave attenuation as related to two earthquake doublets in W. Greece

In this research four earthquakes, which are considered to compose two earthquake doublets, recorded prior to and after a magnitudeM _s =5.0 and a magnitudeM _s =4.8 event in W. Greece are used to derive temporal variations ofP-wave attenuation in the region.Spectral ratios at four stations are computed and the results indicated a variation (increase) ofQ of the order of 15–20%. To confirm that this variation is not an artifact due to changes of the source parameters, a smaller event which occurred at the same hypocentre was used as a Green function and the deconvolution proved that the earthquakes of one of the doublet possessed the same source parameters.The outcome of this research verifies the possible role of the migration of fluids in the crust in the focal region and their effect on the attenuation of seismic waves.     

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.     

Attenuation of high-frequency earthquake waves in South America

Earlier attenuation studies for the South American continent indicate that for Sn and Lg waves there is low attenuation in the shield region east of the Andes, whereas in the west of South America, with some exceptions, there is high attenuation. Those studies, however, were nonquantitative. In this study theQ ₀ (1-Hz values) for Lg waves for South America are presented, based on a scattering model ofAki (1969) as extended byHerrmann (1980) for the coda waves of shallow local and near-regional earthquakes. The results of the codaQ method are compared with those obtained byNuttli's method (1973). TheseQ ₀ values are in good agreement with the apparentQ ₀ of Lg waves obtained by the latter method. The data were obtained from over 100 local and regional earthquakes recorded by 12 WWSSN stations throughout continental South America. They provided a range of frequencies from 0.4 to 1.4 Hz, for which the frequency dependence ofQ was investigated by assumingQ=Q ₀(f/f ₀). The observed data indicate that the tectonic region of western South America is characterized by lowQ ₀ and a large frequency-dependent factor , the values ranging from 150 to 350 and 0.4 to 0.7, respectively. TheQ ₀ values increase in the shield region east of the Andes, but frequency dependence decreases. The average crustalQ ₀ for north and central Argentina ranges from 420 to 580, and ranges from 0.2 to 0.3. TheQ ₀ is larger in the Brazil region, ranging from 580 to 980, and varies from 0.0 to 0.2. In the lower-attenuation region of eastern South America higher values of attenuation correlate with greater thickness of the sedimentary layers.     

Distribution of<Emphasis Type="Italic">L</Emphasis><Subscript>g</Subscript> coda<Emphasis Type="Italic">Q</Emphasis> in the Chinese continent and its adjacent regions

785 traces of vertical components from shallow earthquakes recorded by 10 CDSN (Chinese Digital Seismographic Network) stations and 5 GSN (Global Seismographic Network) stations were collected to study the attenuation characteristics ofL _g coda in the Chinese continent and its adjacent regions. The records were processed first using the stack spectral ratio method to obtain the average values ofQ ₀ (Q at 1Hz) and η, the frequency dependence, ofL _g coda in the ellipses corresponding to the paths. The back-projection technique was then employed to obtain the tomographic maps ofQ ₀ and η values, and the distribution of their errors. Results indicate that in the studied areaQ ₀ varies between 200 and 500. The lowest value ofQ ₀ exists in the Yun-nan-Tibetan region, while the highest value ofQ ₀ occurs in the southern edge of Siberian platform. η varies between 0.3 and 0.8. For most part of the studied area η varies inversely withQ ₀.