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Volume 31 Issue 3 June 2018

ISSN 1674-4519 CN 11-5695/P

Adminidrated by: 
China Association of Science and Technology

Sponsored by: 
The Seismological Society of China and Institute of Geophysics, China Earthquake Administration

Editor-in-Chief: Yun-tai Chen

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High-resolution crustal velocity imaging using ambient noise recordings from a high-density seismic array: An example from the Shangrao section of the Xinjiang Basin, China
Gaochun Wang, Xiaobo Tian, Lianglei Guo, Jiayong Yan, Qingtian Lyu
Accepted Manuscript
[Abstract](89) [PDF 0KB](15)
A profile of shallow crustal velocity structure (1–2 km) may greatly enhance interpretation of the sedimentary environment and shallow tectonic deformation. Recent advances in surface wave tomography, using ambient noise data recorded with high-density seismic arrays, have improved the understanding of regional crustal structure. As the interest in detailed shallow crustal structure imaging has increased, dense seismic array methods have become increasingly efficient. This study used a high-density seismic array deployed in the Xinjiang Basin in southeastern China, to record seismic data, which was then processed with the ambient noise tomography method. The high-density seismic array contained 203 short-period seismometers, spaced at short intervals (~ 400 m). The array collected continuous records of ambient noise for 33 days. Data preprocessing, cross correlation calculation, and Rayleigh surface wave phase-velocity dispersion curve extraction, yielded more than 16,000 Rayleigh surface wave phase-velocity dispersion curves, which were then analyzed using the direct-inversion method. Checkerboard tests indicate that the shear wave velocity is recovered in the study area, at depths of 0–1.4 km, with a lateral image resolution of ~ 400 m. Model test results show that the seismic array effectively images a 50 m thick slab at a depth of 0–300 m, a 150 m thick anomalous body at a depth of 300–600 m, and a 400 m thick anomalous body at a depth of 0.6–1.4 km. The shear wave velocity profile reveals features very similar to those detected by a deep seismic reflection profile across the study area. This demonstrates that analysis of shallow crustal velocity structure provides high-resolution imaging of crustal features. Thus, ambient noise tomography with a high-density seismic array may play an important role in imaging shallow crustal structure.
Rupture Imaging of the 25 April 2015 Mw 7.8 Nepal Earthquake from Back-projection of Teleseismic P Waves
Yong Qiu, Qiaoxia Liu
Accepted Manuscript
[Abstract](86) [PDF 0KB](5)
The Mw 7.8 Nepal earthquake of 25 April 2015 had over 8500 fatalities and was the most destructive earthquake in Nepal since the Bihar-Nepal earthquake in 1934. In this study, we imaged the rupture process of this Nepal event by back-projecting the teleseismic P-wave energy recorded at the three regional networks in Alaska, Australia and Europe. The back-projection images of the three subarrays revealed that the Nepal earthquake propagated along the strike in a southeast direction over a distance of ~ 160-170 km with the duration of ~ 50-55 s. The rupture process was found to be a simple, unilateral event with a near constant velocity of 3.3 km/s. The beam power was mainly distributed in the geographic region just north of Kathmandu and the peak intensity for the source time function curve occurred at about 30 s. The earthquake was destructive due to its occurrence at shallow depth (~ 12-15 km) and the fact that the capital lies in a basin of soft sediment. Additionally, the resonance effect for the longer period waves that occurred in the Kathmandu valley led to destructive aggravation, impacting mainly the taller buildings.
Study of crustal thickness and poisson’s ratio of the south of erenhot area by teleseismic receiver function
Lei Jiang, YongHong Duan, YanNa Zhao, Yong Qiu, Cheng Li
Accepted Manuscript
[Abstract](80) [PDF 0KB](9)
The Xing’an Mongolian Orogenic Belt (XMOB) and the northern margin of North China Craton (NCC) have undergone multistage tectonic superimposition and the tectonic evolution is extremely complicated. We collect the teleseismic data of 44 temporary broadband seismic stations deployed in the XMOB and the northern margin of NCC to calculate the P wave receiver functions. The crustal thickness and average crustal ratio as well as the Poisson’s ratios beneath 33 stations are estimated using the H-κ stacking method. The results show: (1) The crustal thickness of the study area ranges from 38.7 to 42.7 km, with an average thickness of 41.2 km. There is a great difference in crustal thickness on both sides of Solonker suture zone. The characteristics of crustal thickness support the geodynamic model that the Paleo-Asian Ocean subducted and closed at the Solonker suture zone. (2) The Poisson’s ratios in the study area are low, ranging from 0.215 to 0.277, with an average value of 0.243, suggesting that the rock composition of the area is dominated by felsic-acid rocks. (3) There exists a negative correlation between the Poisson’s ratio and the crustal thickness. Combined with the lower values of Poisson’s ratio, we speculate that the delamination is the major mechanism in crustal extension and thinning in the study area.

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Rupture model of the 2013 MW 6.6 Lushan (China) earthquake constrained by a new GPS data set and its effects on potential seismic hazard
Rumeng Guo, Yong Zheng, Faqi Diao, Xiong Xiong, Jiao Xu
2018, 31(3): 117 -125   doi: 10.29382/eqs-2018-0117-1
[Abstract](275) [FullText HTML](213) [PDF 1175KB](14)
Vertical records are critically important when determining the rupture model of an earthquake, especially a thrust earthquake. Due to the relatively low fitness level of near-field vertical displacements, the precision of previous rupture models is relatively low, and the seismic hazard evaluated thereafter should be further updated. In this study, we applied three-component displacement records from GPS stations in and around the source region of the 2013 MW6.6 Lushan earthquake to re-investigate the rupture model. To improve the resolution of the rupture model, records from both continuous and campaign GPS stations were gathered, and secular deformations of the GPS movements were removed from the records of the campaign stations to ensure their reliability. The rupture model was derived by the steepest descent method (SDM), which is based on a layered velocity structure. The peak slip value was about 0.75 m, with a seismic moment release of 9.89 × 1018 N·m, which was equivalent to an MW6.6 event. The inferred fault geometry coincided well with the aftershock distribution of the Lushan earthquake. Unlike previous rupture models, a secondary slip asperity existed at a shallow depth and even touched the ground surface. Based on the distribution of the co-seismic ruptures of the Lushan and Wenchuan earthquakes, post-seismic relaxation of the Wenchuan earthquake, and tectonic loading process, we proposed that the seismic hazard is quite high and still needs special attention in the seismic gap between the two earthquakes.
Fault geometrical model of Dujiangyan section in Longmenshan fault zone
Hu-Rong Duan, Sheng-Lei Chen, Run Li, Quan-Chao Yan
2018, 31(3): 126 -136   doi: 10.29382/eqs-2018-0126-2
[Abstract](151) [FullText HTML](87) [PDF 956KB](12)
The focal mechanism solution on the seismic fault plane can reflect the geometric and kinematic characteristics of faults, and it is an important way to further study the fine structure of fault plane. From the focal mechanism solution of the earthquakes around the Dujiangyan fault in Longmenshan fault zone, we derived the average dip angle of Dujiangyan fault is 45.1° based on the seismic moment tensor theory. In order to refine the fault geometry structure, this paper decomposed it into multiple sub-fault planes along the length and width of the fault plane and forms a number of models A13, B13, A23a, A23b, A23c, B23a, B23b and B23c, then calculated the sub-fault’s dip of each model. In order to clarify exactly which one of the fault models is closest to the real fault model, the fault slip was carried out for each model in turn, then compared the surface displacement of each model with GPS observations. The results show that B23c model with high dip in shallow and small dip in deep is the best model, the lengths of each sub-fault of Dujiangyan fault from south to north are 33 km, 21 km and 46 km, respectively. When the depth of the fault bottom is about 11 km, the dip angles are 70.56°, 67.41° and 45.55°. When the depth of the fault bottom is about 30 km, The fault dip angles are 44.55°, 29.18° and 44.25°.
Time-history responses on the surface by regularly distributed enormous embedded cavities: Incident SH-waves
Mehdi Panji, Saeed Mojtabazadeh Hasanlouyi
2018, 31(3): 137 -153   doi: 10.29382/eqs-2018-0137-3
[Abstract](499) [FullText HTML](185) [PDF 2298KB](103)
The time-history responses of the surface were obtained for a linear elastic half-plane including regularly distributed enormous embedded circular cavities subjected to propagating obliquely incident plane SH-waves. An advanced numerical approach named half-plane time-domain boundary element method (BEM), which only located the meshes around the cavities, was used to create the model. By establishing the modified boundary integral equation (BIE) independently for each cavity and forming the matrices, the final coupled equation was solved step-by-step in the time-domain to obtain the boundary values. The responses were developed for a half-plane with 512 cavities. The amplification patterns were also obtained to illustrate the frequency-domain responses for some cases. According to the results, the presence of enormous cavities affects the scattering and diffraction of the waves arrived to the surface. The introduced method can be recommended for geotechnical/mechanical engineers to model structures in the fields of earthquake engineering and composite materials.
Analytic wave series solution of out-of-plane (SH) waves diffraction by an almost semi-circular shallow cylindrical hill
Chunyang Ji, Vincent Lee
2018, 31(3): 154 -165   doi: 10.29382/eqs-2018-0154-4
[Abstract](347) [FullText HTML](243) [PDF 5016KB](21)
A closed-form wave equation analytic solution of two-dimensional scattering and diffraction of out-of-plane (SH) waves by an almost semi-circular shallow cylindrical hill on a flat, elastic and homogeneous half space is proposed by applying the discrete Fourier series expansions of sine and cosine functions. The semi-circular hill problem is discussed as a special case for the new formulated equation. Compared with the previous semi-circular cases solutions, the present method can give surface displacement amplitudes which agrees well with previous results. Although the proposed equation can only solve the problem of SH-waves diffracted by almost semi-circular shallow hills, the stress and displacement residual amplitudes are numerical insignificantly everywhere. Moreover, the influences of the depth-to-width ratio (a parameter defined in this paper to evaluate the shallowness of the topography of hills) on ground motions are presented and summarized. The limitations and errors of truncation from Graf’s addition theorem and Fourier series equations in the present paper are also discussed.
Dynamic equivalent soil characteristics identification using earthquake records
Asskar Janalizadeh Choobbasti, Saman Soleimani Kutanaei
2018, 31(3): 166 -173   doi: 10.29382/eqs-2018-0166-5
[Abstract](247) [FullText HTML](196) [PDF 825KB](19)
Techniques for soil property estimation can be categorized into two main groups, in-situ and laboratory methods. Previous investigations indicated that strong ground motions record provides a very useful tool to estimating the in-situ characteristics of soil. The main objective of the present work is to utilize the particle swarm optimization algorithm (PSOA) integrated with linear site response method to obtain the equivalent soil profile characteristics from the available surface and bedrock earthquake motion records. To demonstrate the numerical efficiency and the validity of this approach, the procedure is validated against an available case. Then this procedure is utilized to identify the soil properties profiles of the site by using strong ground motions data recorded during the Bam earthquake of December 26, 2003. The magnitude and PGA of Bam earthquake were MW 6.6 and 0.8 g respectively.
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