2011 Vol. 33, No. 1

Display Method:
2011, 33(1): 1-10.
Temporal variation of vP/vS, vP and vS before and after 2008 Wenchuan earthquake
Wang Linyingup Li Yanrsquo;eup Li Fangup Li GuangpingupZhang TianzhongBao ShuxianSu Jinrongup
2011, 33(1): 1-14.
On May 12, 2008, a great earthquake of MS8.0 occurred on Longmenshan fault, Sichuan, China. With the stress change near-source region before and after the mainshock, would there have been corresponding physical property change of crust media? It became one of the hot questions in the research of Wenchuan earthquake. In this study we collected seismic phase data from January 1, 2001, to May 31, 2010, reported by Sichuan Digital Station Network of China Earthquake Administration (CEA), and the phase data from May 12 to September 30, 2008, reported by the Mobile Digital Seismic Network of Institute of Geophysics, CEA. We processed these data using the Wadati method. Based on the phase arrival times of selected direct wave Pg and Sg we paid special attention to the study of temporal variation of wave apparent velocity, and compared the velocity variation with the temporal variation of wave velocity ratio. To ensure the accuracy and stability of the calculated result, we carefully winnowed the data. Our research result shows that, in Longmenshan fault zone and its surrounding area, there was a 4 years' obvious decrease process of P-and S-wave apparent velocity before the Wenchuan earthquake, and at the time of half year before the earthquake an abrupt return appeared. Different from apparent velocity, the anomalous variation of wave velocity ratio showed a low amplitude decrease and lasted only for more than 2 years. But both wave velocity ratio and apparent velocity decrease returned fast at the time of half year before the mainshock.
Comprehensive analysis on present tectonic stress field in Urumqi region
Li Yingzhen Shen Jun Nie Xiaohong Long Haiyingdiv
2011, 33(1): 15-27.
Urumqi city is located in central part of northern piedmont of Tianshan mountain. The mean direction of principal compressive stress in central section of northern Tianshan deduced from the focal mechanism solutions is around N10deg;E, with a clear feature of gradual change from west to east. In the region of Urumqi city, the direction derived from focal mechanism solutions of moderate-strong earthquakes is N15deg;mdash;20deg;E.The principal stress direction found from fault striation inversion is N17deg;Wmdash;N2deg;E. The stress direction deduced from the above two data sets are basically in the variation range of the principal compressive stress in the central section of northern Tianshan. However, the stress direction inferred from composite fault-plane solutions of small earthquakes within 25 km of the Urumqi seismic station is N40deg;mdash;46deg;E, which shows a deviation of 20deg;mdash;30deg;from that of regional principal compressive stress. Nevertheless, the direction of maximum horizontal compressive stress measured in the boreholes in Urumqi city is around N55deg;E, which displays a further deviation of 10deg;mdash;15deg;, and its deviation from that of regional stress field is up to 30deg;mdash;40deg;. These phenomena indicate that a tension stress in the shallow layers in front part of the compressive fault-fold region has been superposed to the regional NNE-SSW trending compressive tectonic stress and makes the direction of local principal compressive stress turn to NEE-SWW. This is one of the causes why the normal faults, such as the Jiujiawan fault groups, were developed in Urumqi city.
Application of microseismic data to constraining inversion for velocity structure beneath stations in sedimentary area
Wang Weilai Wu Jianping FangLihuacommult
2011, 33(1): 28-38.
The joint inversion of receiver functions and seismic surface wave dispersion has become a common technique to study the velocity structure beneath stations. The results can simultaneously match the two data sets of different resolution so that the non-uniqueness of solution has been effectively suppressed in a certain extent. However, for the present distribution of seismic stations, due to many factors, there are less effective short-period surface wave dispersion data. The horizontal resolutions of seismic tomography are low, so it is difficult to reflect the real dispersion characteristics of the local media near stations. The spatial extents reflected by these surface wave dispersions and receiver functions at different depths are also different, especially in the sedimentary region with complex lateral variations, where receiver functions and surface wave dispersion can not simultaneously achieve a good fitting result. So the constraint data, which can reflect information of media in the same spatial extent, should be used as far as possible for joint inversion. In May of 2009, we laid a small-aperture seismic array for micro-seismic observation near two mobile seismic stations in a sedimentary region on the plateau of northern Hebei province and in North China Plain, and obtained high-frequency phase velocity dispersion curves by spatial autocorrelation method. The micro-seismic dispersion curves are used as constraint on shallow velocity structure for the joint inversion of receiver functions and surface waves in sedimentary region. The inversion result shows that the constraint of high-frequency phase velocity dispersion curves may obviously improve the stability and reliability of the inversion for velocity structure in sedimentary region.
S-wave velocity structure of upper and middle crust beneath the Three Gorges reservoir dam and adjacent region
Li QiangZhao XuupCai Jinrsquo;anupLiu Ruifenguploans.colucash
2011, 33(1): 39-50.
A 3-D S-wave velocity structure of upper and middle crust beneath the Three Gorges reservoir dam and in adjacent region was reconstructed by means of the seismic tomography using the absolute Sg-arrivals from the earthquakes observed by the Three Gorges seismic network. Main research results of the crustal structure are as follows.①The lithology of the Huangling anticline is highly solidified, with uniformity and integrity. Its buried depth is not less than 14 km. ②The rock strata of the Zigui basin are consistent with that of its marginal region, so their velocity structures are the same. The depth distribution and velocity structure characteristics of the Zigui basin and its adjacent areas depended on the intensity of the tectonic deformation.③The partially molten substances, located in upper and middle crust and intruded from upper mantle,mainly distribute beneath the Zigui basin, as well as nearby the Peishi region.④The crystalline basement was constructed inhomogeneously, its top and bottom boundary, and the basement thickness and corresponding velocity structure, show significant regional variation.⑤The velocity gradient zone is generally consistent with the gravitational gradient zone.
An analysis on comparative geothermal observations under different dynamic background
Zhao Gang He Anhuaup Ma Wenjuanup Chen Minuploans.com sh advance salucashadv
2011, 33(1): 51-61.
By analyzing 6 month comparative geothermal observations in 3 wells at Shahe, Wenquan and Wusu stations, respectively, we conclude that the data obtained from the six observational instruments in these stations are reliable. The observations at the same depth of the Shahe and Wenquan well show that the overall trend and pattern, and even details, of the observed temperature variations are the same, and the correlation coefficient of these observations is close to 1. In the Wusu well the observation curves at the two depths of 250 m and 700 m are overall the same, but some details of the observations are deferent due to the sensors being at different depth.This article summarizes the specific operation method of installing two sets of digitalthermometers in the same well for comparative observation, and solves the difficulty in excluding the instrument trouble when abnormal observations appear.
Discussion on appearance of surface fractures along the rear-range of Longmenshan mountain during 2008 MS8.0 Wenchuan earthquake
Xie XinshengJiang Wali Feng Xiyingcom sh advance
2011, 33(1): 62-81.
The information of surface fractures at eleven investigated sites along the Wenchuan-Maoxian fault zone in the rear-range of Longmengshan mountain are presented in this paper. The investigation sites include, from south to north, Yusidong village of Wolong town in Wenchuan county, Niuping of Gengda town, Jinbo village,Zhangpai village and Zuwan village of Caopo town, Gaodongshan village of Miansi town, Linggang village of Yulong town, Qipangou village of Weizhou town, Qiaoqiaozhai village of Maoxian county, to Malianggou gully of Fengyi town, with a total length of 100 km. Although most surface fractures are located at the top or on the slope of the mountain, with the same strike as the mountain trend, there are also a few surface fractures stretching on terraces of rivers, with the fracture trends perpendicular to or obliquely intersecting the rivers. The northwestern side of most fracrures among the 11 sites are uplifted. Vertical displacements are mostly 20mdash;40 centimeters. Among them, the strike of surface scarp on the top of Zoumaling range is perpendicular to the trend of Qipangou gully in the Weizhou town of Wenchuan county, with the scarp on north-western hanging wall being just opposite to the Minjiang river. On the wall of this fault scarp straight smooth frictional mirror surface and striations with 57deg;plunge-angle can be seen, indicating a displacement with dextral strike-slip component. Here we would query whether these fracture surfaces were formed solely by gravitational slide or probably caused by co-seismic displacement along Wenchuan-Maoxian fault during the Wenchuan earthquake. Note that, among 11 investigated sites, there are 6 sites with land scarps, among which two surface fractures are located in fault trough on mountain tops. This shows that the surface fractures on mountain tops during Wenchuan earthquake could have been formed by tectonic activity. Besides, surface dislocation profiles on 5 sites are described in the paper. The three main fault belts in Longmenshan mountain have distinct dislocated geomorphology during Late Quaternary. We think it shows that the three fault belts did not move following the frontward style of Mesozoic tectonic motion, but were synchronal activity during the same events of Late Quaternary with different active intensity. The distribution of small-displacement of surface fractures along the Longmengshan rear-range formed by Wenchuan earthquake is probably the reflection of such synchronal activity. Investigation in this paper would be helpful in calling for more attention to surface fracture survey along the rear-range of Longmenshan mountain during Wenchuan earthquake, facilitating an overall assessment of the Longmengshan tectonic zone activity during the Wenchuan earthquake.
Ground motion acceleration criterion for judging landslide induced by the 2008 Wenchuan earthquake
Wang Xiuying Nie Gaozhongup Wang Songuploans.com sh advance same daylucashadv
2011, 33(1): 82-90.
By analyzing strong ground motion records obtained from the 2008 Wenchuan MS8.0 earthquake, a ground motion acceleration parameter, named Maximum Critical Acceleration (MCA), is given in this paper. Analyzed result shows that the parameter MCA is more relevant to earthquake-induced landslide than PGA, and by using it we may solve the problems encountered in using PGA criterion to judge whether landslide can be triggered during earthquake. The MCA attenuation relation is built based on the data from the Wenchuan earthquake in Longmenshan mountain area. The appraised landslide area following the attenuation relation well fits the actual landslide distribution in the Wenchuan earthquake-hit area, indicating that this parameter is suitable for evaluating earthquake-induced landslide. Comparing MCA directly with slope critical acceleration may determine whether landslides can be triggered or not, simplifying the judging process of earthquake-induced landslide. The conclusion is that this parameter can be applied to the earthquake-induced landslides evaluation as an earthquake triggering factor.
A study on input energy spectrum in consideration of site effect and design earthquake classification
Wang Decai Ye Xianguoup,  Chang Leiloans.com sh advance same day a hreflucashadv
2011, 33(1): 91-102.
Establishing a simplified applicable design input energy spectrum is the basis of applying the energy method to practical engineering design and verification. This paper has chosen 694 horizontal seismic records on classⅠ,ⅡandⅢ sites, and analyzed energy spectrum with different classification of design earthquakes and site classes. The correlation between 12 different ground motion parameters and the energy spectrum is examined, and a ground motion parameter among them having strong relation with ground motion input energy is selected. Based on the provisions of fortification intensity and security standard in the present code, a new parameter of classification of design earthquake is put forward and the corresponding design input energy spectrum for damping ratio of 5% of the elastic single degree of freedom system is built. Furthermore, some valuable recommendations for using the energy-based design method and selection of input ground motion are proposed.
On standard response spectrum for nuclear power plant based on strong earthquake ground motions
Qin Fengupup, Xu Longjunup Xie Lililoans.com sh advlucashadv
2011, 33(1): 103-113.
Based on accelerograms on rock site from the Wenchuan earthquake, analyses of peak ground motion parameters and standard response spectra for nuclear power plant are made. The authors paid attention to the fact that the ratio (PGA/PGV) could be affected by PGA and epicenter distance, which is taken into account in developing standard response spectrum. This paper verified that such affection would significantly influence the shape of the deduced standard response spectrum. It is also validated that the vertical PGA on rock site may be much greater than 2/3 of the horizontal PGA. Compared to the pseudo-velocity spectra, the standard spectra are smaller in the long period region. Two new methods are presented to fit the mean-plus-one-standard-deviation spectra more aptly than conventional method.
HHT analysis on seismic response of a structure model tested on shake table
Shi ChunxiangupupLi HushengupupLuo Qifengup2upShi Weixingup2up
2011, 33(1): 114-119.
Hilbert-Huang transform (HHT) is a novel data analysis method which can be used to process nonlinear, non-stationary data and is very adaptive. A 45-storey RC frame structure and its shake table test model were briefly introduced. The structural response records under different seismic intensities were analyzed with HHT method, and the time-frequency-amplitude and Hilbert marginal spectrum were obtained. The result shows that Hilbert spectrum can clearly present the energy distribution with time and frequency. Most energy of Hilbert spectrum is much more concentrated in the definite range of time and frequency. The Hilbert spectrum can grasp main characteristics of the signal. Hilbert marginal spectrum curve and natural vibration frequency at peak amplitude vary with different seismic intensity. The same trend of all figures shows that Hilbert marginal spectrum can be used in the safety assessment of civil engineering structures.
2011, 33(1): 120-121.