2021 Vol. 43, No. 6

Display Method:
S wave envelope synthesis based on different scattering patterns
Jing Yueling, He Lihao, Li Minghui, Zhang Yuxuan, Zheng Siyuan
2021, 43(6): 679-689. doi: 10.11939/jass.20200208
In order to reveal the scattering process of seismic waves in the small scale inhomogeneous medium of the crust and to describe the envelopment broadening phenomenon of seismic waves more accurately, a discrete wave-number method is used to solve the improved seismic wave energy density integral equation based on the multiple anisotropic scattering theory, and the scattering pattern represented by Gaussian autocorrelation function is selected to obtain the S wave energy density envelope. Firstly, we analyzed the contribution of single scattering and multiple scattering to the energy density envelope of S wave. Then, we discussed the effects of absorption coefficient and total scattering coefficient on the synthesis of S wave energy density envelope. Finally, we compared the differences of the energy density envelope of S wave synthesized in different scattering patterns. The results show that: ① The contribution of single scattering and multiple scattering to the seismic wave scattering process is consistent, and for the near earthquakes (hypocentral distance is less than 100 km), the single scattering model can be used to match the S-wave energy density envelope. As the hypocentral distance increases, the multiple forward scattering pattern can approach the total energy density envelope more quickly. ② As the absorption coefficient increases, the amplitude of the direct S wave and the coda wave will decrease. And when the total scattering coefficient increases, the amplitude of the direct S wave will decrease, while the coda wave amplitude of the S wave will increase. ③ In the forward scattering pattern, with the increase of hypocentral distance, the energy density envelope of S-wave appears the peak delay, the envelope is widened, and the attenuation consistency of the coda wave is accelerated.
P-wave velocity changes in the source area before and after two moderate strong earthquakes in Yingjiang, Yunnan in 2014
Cao Ying, Qian Jiawei, Huang Jiangpei
2021, 43(6): 690-705. doi: 10.11939/jass.20200214
The time-lapse tomography method based on double-difference tomography can obtain high-precision seismic wave velocity changes. In this study, we applied this method to the seismic data set that recorded by the Yunnan seismic network to obtain the spatiotemporal changes of P-wave velocity in the source area during the MS5.6 and MS6.1 Yingjiang earthquakes in 2014. The results showed that the spatial and temporal characteristics of the P-wave velocity of the crust at depth around the source area suggest co-seismic velocity reductions. After the MS5.6 earthquake occurred on May 24, the P-wave velocity of the crust dropped slightly. After the MS6.1 earthquake occurred on May 30, the P-wave velocity dropped definitely with an amplitude of about 1%. It showed that the amplitude of the reduction is related to the magnitude of the mainshock. We also reported that P-wave velocity is also related to the the spatio-temporal changes of aftershocks distribution. It is possible that of dynamic and static stresses changes of the aftershocks caused changes of the physical properties in the source area which leads to the velocity change. Finally, about five years after the Yingjiang earthquakes, a healing process is observed. And the rising level did not reach the previous falling level. However, this does not mean that the source area is still in the healing process.
Relocation of earthquakes in Tangshan area in the recent decade
Guo Lei, Sun Lina, Zhou Yi, Wang Ning
2021, 43(6): 706-715. doi: 10.11939/jass.20210029
We relocated 4 874 earthquakes totally which were recorded by Hebei seismic network in Tangshan area during the period between January of 2010 and July of 2020 by using the double-difference relocation method, the double-difference relocation method combined with waveform cross-correlation, and the double-difference tomography method combined with the waveform cross-correlation, respectively. The comparison of the location results by the three relocation methods suggests that the double-difference tomography method combined with the waveform cross-correlation could produce much higher resolution hypocentral parameters than those from the other two methods. As a result, the root-mean-square (RMS) of traveltime residuals were greatly reduced from 0.71 s to 0.27 s. In map view the relocated earthquakes are more concentrated on obvious strips around the faults, and the dominant distribution direction is NE, which is basically consistent with the fault strike in this area. The result is more reasonable in depth distribution with focal depth mainly concentrated in 5−15 km, and the distribution pattern appears as a nearly normal distribution. From the spatial distribution of the epicenters after relocation, it can be seen that there is a small earthquake dense zone with steep dip angle in NNE direction on the east of the Tangshan-Guye fault, indicating that the stress level is high and the tectonic activity is strong in this local area. Therefore it is speculated that there may be a hidden fault at the eastern end of the Tangshan-Guye fault, and the occurrence of earthquake in this region is related to its deep activities.
Study on earthquake focal mechanism and seismogenic structure of the Santai MS4.7 earthquake in 2013
Liu Suihai, Lü Jinxiong, Zhao Xueqin, Zhang Gang, Wen Mingru
2021, 43(6): 716-729. doi: 10.11939/jass.20200201
Santai MS4.7 earthquake occurred in Mianyang city, Sichuan Province on February 19, 2013. The epicenter located in an intersection area which combined the basement fracture in central Sichuan basin, the Mianyang-Santai-Dazu fracture and the Pujiang-Santai-Bazhong fracture. Based on the earthquake catalogue, seismic phase report and waveform data, we relocated the earthquake sequence, and solved the focal mechanism of the main earthquake by joint inversion of local and teleseismic waveforms, and also studied the seismogenic structure of the Santai MS4.7 earthquake according to the field geological survey. The spatial distribution direction of the relocated aftershock sequence is NW-SE and it highly fitted with the nodal plane I of the focal mechanism, the seismogenic structure of the earthquake can be considered as strike NW-SE, dip NE, and rupture plane is nearly vertical, and the seismogenic fracture was dextral strike-slip dislocation by the proximately horizontal confining pressure in the direction of nearly NS. During the field survey, any fracture marks nor surface ruptures can be obviously found, hence the seismic structure is concluded as a buried fault. The focal depths are 21.6 and 19 km respectively through the relocation and joint inversion of local and teleseismic waveforms. The focal depth reflects that the location of the source area is in the low-velocity layer which is in the bottom of the upper-crust. This earthquake is caused during the adjusted and changed process of the tectonic stress field caused by Wenchuan earthquake, and it is the result of the reactivation of the high-angle fault concealed in the Presinian crystalline substrate in middle-block of Sichuan basin.
The dynamic mechanical response of the fault under different water injection schedules
Zhu Aiyu, Sun Zihan, Jiang Changsheng, Chen Shi, Zhang Dongning, Cui Guanglei
2021, 43(6): 730-744. doi: 10.11939/jass.20210137
Water injection used in industry can lead to the activation of existing faults and have induced many destructive earthquakes. Therefore, it is of great significance to study the dynamic response of faults under water injection to explore the mechanism of induced earthquakes. The poroelastic spring-slider model calculates the fault stability under three kinds of classical water injection schedules(ascending, rapidly ascending, descending and intermittent)using poroelastic coupling numerical simulation. The results show that, with the continuous injection of fluid, the pore pressure inside the fault will go through three stages: slow rise, rapid rise, and stable rise. For different water injection schedules, the three stages are not fully reflected, and the forms are different; under the same water injection schedule, the smaller the reservoir permeability is, the greater the pore pressure near the wellhead is, the smaller the pore pressure at fault is, and the greater the difference of pore pressure between the two is; the larger the value is, the easier the earthquake will be induced. The value is negatively correlated with the fluid pressure of injected reservoir fluid but positively correlated with the change rate of fluid pressure; the critical stiffness increases rapidly in the early stage due to the increase of the change rate of pore pressure and decreases in the later stage due to the influence of pore pressure. The rapid rising and falling water injection schedule greatly increase the possibility of inducing earthquake in the early stage of water injection. The intermittent water injection schedule causes a large change of necessary stiffness in the late stage of water injection, which increases the possibility of inducing an earthquake. This study can provide the quantitative scientific basis for the risk assessment of water injection-induced earthquakes and reduce the possibility of water injection-induced earthquakes.
The conjunct effects of ISO and CLVD sources in underground explosions on the spectral null in Rg waves
Wang Xuliang, Jin Ping, Zhu Haofeng, Xu Henglei, Xu Xiong
2021, 43(6): 745-752. doi: 10.11939/jass.20200197
The compensated linear vector dipole (CLVD) source is usually induced by underground explosions and could lead to a null in the Rg spectra. There is a theoretical scale between the null frequency and the depth of the CLVD source. Therefore we can estimate the depth of underground explosions with the null frequency. However, this relationship is based on simply considering the CLVD source while the conjunct effects of ISO and CLVD sources are neglected. This study researches the conjunct effects of ISO and CLVD sources in underground explosions on the spectral null in Rg waves by numerical calculation of the Rg spectra with seismology theory of surface waves. The results indicate that the null in mixed signals is quite different from the results of single CLVD source. Thus the null frequency of Rg waves is not simply corresponding to the theoretical value in practical underground explosions and it is improper to estimate the depth of explosions by directly applying the theoretical formula.
Study on 2D in-plane HVSR simulation and application with transverse inhomogeneous body scattering
Ba Zhenning, Zhang Enwei, Liang Jianwen, Rong Mianshui
2021, 43(6): 753-767. doi: 10.11939/jass.20200177
In order to analyze the significant influence of lateral inhomogeneity of site on horizontal-to-vertical spectral ratio (HVSR) curves, the diffuse field approach proposed by Sánchez-Sesma et alwas adopted to simulate the HVSR curves of 2-D sediment topography by calculating the imaginary part of Green’s functions of total wave field. The imaginary part of Green’s functions was solved by the dynamic stiffness matrix and in-plane inclined Green’s functions based on the indirect boundary element method (IBEM). The HVSR curves of 2-D sediment topographies and corresponding 1-D layered half-space were compared, the influences of sediment topography shapes and the relative position of calculation points on the HVSR curve were discussed in detail. The results show that the effect of impedance ratio between inside and outside materials of sediment topography on HVSR is the most significant; With the increase of the impedance ratio and the slopes of the interface on the sediment side, the frequencies of the first peak of HVSR curves increase significantly, which can be up to 3.3 times of the corresponding layered half-space results, simultaneously, platform emerges on HVSR curves; Amplitudes of HVSR curves in high frequency band increase with the decrease of distances from the calculation points to the sediment boundary. According to the results obtained in this study, the HVSR method can be used to preliminarily determine the place where local sediment topography exists. From this aspect, the cost of regional geophysical investigation can be reduced visibly via HVSR method.
Characteristics of Arias intensity and Newmark displacement of strong ground motion in Lushan earthquake
Li Xuejing, Xu Weijin, Gao Mengtan
2021, 43(6): 768-786. doi: 10.11939/jass.20200180
The characteristics of spatial distribution, attenuation and correlation of ground motion parameters are important research contents in engineering seismology. In this paper, based on the ground motion records of Lushan earthquake, we study the spatial distribution and attenuation characteristics of Arias intensity and Newmark displacement as well as their correlation with other ground motion parameters, respectively. The results show that the spatial distribution of Arias intensity is related with the spatial distribution of seismic faults and the direction of earthquake rupture. Arias intensity has a good correlation with PGA. Furthermore, the site conditions have a significant effect on the correlation between the two: for the same PGA, the softer the site condition, the greater the Arias intensity. In addition, magnitude is also an important factor affecting the correlation between Arias intensity and PGA: with the same PGA, the greater the magnitude, the greater the Arias intensity. Newmark displacement has a good correlation with both PGA and Arias intensity, among which the correlation with Arias intensity is stronger, and the correlation coefficient can reach above 0.94. The research in this paper also shows that the existing models cannot describe the attenuation characteristics of Arias intensity and Newmark displacement of Lushan earthquake well, which indicates the particularity of Lushan earthquake in both the duration and rupture process. The particularity of Lushan earthquake reveals that the seismic geological and tectonic environment in Western China is significantly different from that in other regions. Therefore, the prediction equations of ground motion parameters suitable for earthquakes in Western China should be studied. The research results of this paper have important scientific significance and application value for us on both understanding the characteristics of ground motion and the prediction and prevention of earthquake disaster in China.
Problems and suggestions on site classification
Chi Mingjie, Li Xiaojun, Chen Xueliang, Ma Shengjie
2021, 43(6): 787-803. doi: 10.11939/jass.20200152
The physical meaning of site classification is not clear in the current seismic design code for buildings, at the same time, the boundary of site classification is easy to cause the divergence of design ground motion parameters. For the above problems and deficiencies, some suggestions are given. To solve the problem that the physical meaning of site classification is not clear, on the basis of the current site classification method, according to the site classification index such as the covering layer thickness and the equivalent shear wave velocity, the sites are classified by two-level: the first level classification is consistent with the current one, which classifies sites based on the fundamental period of the site and the thickness of the overburden layer; the second level classification further considers the degree of hardness of the geotechnical medium based on the first level classification, and sub-classification according to equivalent shear wave velocity. Based on the current research on seismic disaster and seismic motion characteristics of thick soft site, combined with the development of long-period constructions, the site classification is expanded from the original four categories to five categories, at the same time, the boundary of each classification, especially the boundary of class II, III and IV sites, is limited from the original open type, which can effectively avoid the problem of divergence of design ground motion parameters caused by site classification. The related research results can provide reference for site classification and determination of design ground motion parameters.
Postseismic deformation observation,mechanism and lithospheric rheology of the central and northern Tibetan Plateau after the 2001 MW7.8 Kunlun earthquake:Insights and challenges
Zhao Dezheng, Qu Chunyan, Shan Xinjian, Zhang Guohong, Li Yanchuan, Gong Wenyu, Song Xiaogang
2021, 43(6): 804-816. doi: 10.11939/jass.20210058
The 2001 MW7.8 Kokoxili earthquake was the largest earthquake in the central and northern part of the Tibetan Plateau in the recent half century. The large coseismic stress disturbance caused by the coseismic rupture drives and controls the significant postseismic deformation following this major earthquake. A decade of geodetic measurements documented large-spatial-scale, long-time-span and time-dependent postseismic deformation and their different evolution processes, and the geodetic measurements also revealed the complex fault kinematics characteristics, friction properties along fault strike and lateral heterogeneity of lithospheric rheological properties/structure in north-central Tibetan Plateau. In this paper, we briefly summarize postseismic deformation observations and their spatiotemporal characteristics of the Kokoxili earthquake based on InSAR time-series and GPS observations on a decadal scale. Particularly, the spatiotemporally dense InSAR observations are deemed as an important supplement to the postseismic GPS observations in this tectonic area. We summarize the models of large-scale postseismic deformation and the revealed postseismic deformation processes, various dynamic mechanisms and their relationships. Finally, we summarize the scientific understanding and unsolved scientific problems associated with the 2001 Kokoxili earthquake in the past 20 years: On the one hand, it is necessary to continuously observe and study the large-scale surface deformation of the Kunlun fault; On the other hand, the postseismic deformation model should be updated continuously to deepen our understanding of the earthquake cycle deformation of the Kunlun fault, the control of regional tectonics on earthquake cycle deformation, and the spatiotemporal evolution mechanism of complex fault movement in this region.