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Structural characteristics of Yanyuan basin deduced from geophysical data and it dynamic implication
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doi: 10.11939/jass.20200119
Abstract:
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2021, 43(4): 397-409.
doi: 10.11939/jass.20210104
Abstract:
This paper collected the seismic travel-time data both from temporary stations employed after Yangbi M6.4 earthquake and the seismic networks in Yangbi region and its vicinity from January of 2015 to 28 May, 2021 and performed the high-resolution inversion for three-dimensional velocity structure and accurate hypocentral locations by using double-difference seismic tomography method. The relocation results show that the sequence spread in the NW−SE direction along the Weixi-Qiaohou-Weishan fault. The focal depth in the area is generally in the range of 2−5 km with high dip angle of 80°. The three-dimensional velocity structure shows that the spatial distribution characteristics of the Yangbi M6.4 earthquake sequence are closely related to the velocity structure. The epicenter of the Yangbi M6.4 main shock was located near the P- and S-wave high-to-low-velocity anomaly transitional zones, which are favorable for occurrence of moderate-strong earthquakes, and the aftershocks are mainly distributed in the brittle region with low vP, high vS and low vP/vS. In addition, along the strike of Yangbi earthquake sequence, there are totally different velocity structures on both sides of main earthquake, and significant higher vP and lower vS anomalies are observed in the northwest of the Yangbi M6.4 main shock compared with the southeast part, which may obstruct the northwestward slipping of the seismogenic fault of Yangbi M6.4 earthquake, leading to the striking unilateral source rupture.
This paper collected the seismic travel-time data both from temporary stations employed after Yangbi M6.4 earthquake and the seismic networks in Yangbi region and its vicinity from January of 2015 to 28 May, 2021 and performed the high-resolution inversion for three-dimensional velocity structure and accurate hypocentral locations by using double-difference seismic tomography method. The relocation results show that the sequence spread in the NW−SE direction along the Weixi-Qiaohou-Weishan fault. The focal depth in the area is generally in the range of 2−5 km with high dip angle of 80°. The three-dimensional velocity structure shows that the spatial distribution characteristics of the Yangbi M6.4 earthquake sequence are closely related to the velocity structure. The epicenter of the Yangbi M6.4 main shock was located near the P- and S-wave high-to-low-velocity anomaly transitional zones, which are favorable for occurrence of moderate-strong earthquakes, and the aftershocks are mainly distributed in the brittle region with low vP, high vS and low vP/vS. In addition, along the strike of Yangbi earthquake sequence, there are totally different velocity structures on both sides of main earthquake, and significant higher vP and lower vS anomalies are observed in the northwest of the Yangbi M6.4 main shock compared with the southeast part, which may obstruct the northwestward slipping of the seismogenic fault of Yangbi M6.4 earthquake, leading to the striking unilateral source rupture.
2021, 43(4): 410-426.
doi: 10.11939/jass.20200101
Abstract:
In this paper, 470 broadband seismic records of natural earthquakes with M≥4.0 between May 2014 and May 2019 recorded by 121 fixed stations in and around Yunnan Province are collected. 6 976 high-quality vertical wave data is processed by using the reverse two-station method. The attenuation imaging of Lg wave at 1 Hz with spatial resolution less than 100 km in Yunnan area is inverted. The inversion results show that the Q0 value of Lg wave in Yunnan is in 60 to 300 range, and the whole is in low Q0 background, with significant changes in lateral heterogeneity. The distribution characteristics of low Q0 value in Yunnan area reflect the strong attenuation of Lg wave in Yunnan area. The Q0 value in the west side of Honghe fault is relatively low, in 50 to 160 range, and in the east side is relatively high, in 120 to 200 range. The distribution characteristics are consistent with the distribution of sedimentary layer thickness. Loose sedimentary layer may be the main cause of high attenuation in the east side. The Q0 value of Lg wave in Yunnan area shows a similar differential distribution with the distribution of surface heat flow, which may be related to frequent earthquakes, long-term strong tectonic movement and deep material upwelling with volcanic activity.
In this paper, 470 broadband seismic records of natural earthquakes with M≥4.0 between May 2014 and May 2019 recorded by 121 fixed stations in and around Yunnan Province are collected. 6 976 high-quality vertical wave data is processed by using the reverse two-station method. The attenuation imaging of Lg wave at 1 Hz with spatial resolution less than 100 km in Yunnan area is inverted. The inversion results show that the Q0 value of Lg wave in Yunnan is in 60 to 300 range, and the whole is in low Q0 background, with significant changes in lateral heterogeneity. The distribution characteristics of low Q0 value in Yunnan area reflect the strong attenuation of Lg wave in Yunnan area. The Q0 value in the west side of Honghe fault is relatively low, in 50 to 160 range, and in the east side is relatively high, in 120 to 200 range. The distribution characteristics are consistent with the distribution of sedimentary layer thickness. Loose sedimentary layer may be the main cause of high attenuation in the east side. The Q0 value of Lg wave in Yunnan area shows a similar differential distribution with the distribution of surface heat flow, which may be related to frequent earthquakes, long-term strong tectonic movement and deep material upwelling with volcanic activity.
2021, 43(4): 427-440.
doi: 10.11939/jass.20210169
Abstract:
The differences of the seismic characteristics and frequency of the Huailai explosions, the Sanhe quarry explosions and the natural earthquakes with low magnitude are discussed. The results show that the two different area explosions have obviously different seismic characteristics and frequency distribution, Huailai explosion has stronger P wave energy than S wave and fast attenuation; The main frequencies of P wave and S wave in Sanhe quarry explosion are lower than that in Huailai explosion, S wave and surface wave are confused, and the low frequency developed obviously at different distances; While for natural earthquakes, the effective frequency band is wider and the frequency components are more complex than explosions. Pg/Sg spectral ratios in small-magnitude earthquakes and explosions were studied and cross-band spectral ratios were explored. Results obtained show that the high frequency （＞5 Hz）Pg/Sg spectral ratio discriminants can completely distinguish explosions from low magnitude earthquakes; The spectral ratios of the cross-band related to low frequency （0−2 Hz） of Sg can effectively identify explosions in these two areas, Pg/Sg discriminants of the crossed frequency band can better reflect the difference characteristics of different types of events than that of the traditional single frequency band.
The differences of the seismic characteristics and frequency of the Huailai explosions, the Sanhe quarry explosions and the natural earthquakes with low magnitude are discussed. The results show that the two different area explosions have obviously different seismic characteristics and frequency distribution, Huailai explosion has stronger P wave energy than S wave and fast attenuation; The main frequencies of P wave and S wave in Sanhe quarry explosion are lower than that in Huailai explosion, S wave and surface wave are confused, and the low frequency developed obviously at different distances; While for natural earthquakes, the effective frequency band is wider and the frequency components are more complex than explosions. Pg/Sg spectral ratios in small-magnitude earthquakes and explosions were studied and cross-band spectral ratios were explored. Results obtained show that the high frequency （＞5 Hz）Pg/Sg spectral ratio discriminants can completely distinguish explosions from low magnitude earthquakes; The spectral ratios of the cross-band related to low frequency （0−2 Hz） of Sg can effectively identify explosions in these two areas, Pg/Sg discriminants of the crossed frequency band can better reflect the difference characteristics of different types of events than that of the traditional single frequency band.
2021, 43(4): 441-452.
doi: 10.11939/jass.20200150
Abstract:
In this paper, we use a Bayesian principle-based gravity leveling method to uniformly process 16 periods of land-based mobile gravity observations from 2009-2017 in North China, and study the dynamic characteristics of the regional gravity field in conjunction with four earthquakes （M≥4.5） that have struck the studied area since 2009. Further, we study the characteristics of the isostatic gravity anomaly in North China, and analyze the relationship between regional gravity field changes and deep density structure based on the Bouguer gravity anomaly, sedimentary sequences gravity anomaly and Moho gravity anomaly in the studied area to characterize the three-dimensional density structure in North China. The results show that, the four earthquakes （M≥4.5） in the studied area in recent years have been located near the contours of the regional differential gravity field and the zero-value of the cumulative gravity field, the abnormally shifting gradient zones and the bends of the gradient zones, the unbalanced zones in the deep structure where the theoretical equilibrium thickness differs from theactual crustal thickness, and the high and low densities of the three-dimensional density structure inside the transition zone.
In this paper, we use a Bayesian principle-based gravity leveling method to uniformly process 16 periods of land-based mobile gravity observations from 2009-2017 in North China, and study the dynamic characteristics of the regional gravity field in conjunction with four earthquakes （M≥4.5） that have struck the studied area since 2009. Further, we study the characteristics of the isostatic gravity anomaly in North China, and analyze the relationship between regional gravity field changes and deep density structure based on the Bouguer gravity anomaly, sedimentary sequences gravity anomaly and Moho gravity anomaly in the studied area to characterize the three-dimensional density structure in North China. The results show that, the four earthquakes （M≥4.5） in the studied area in recent years have been located near the contours of the regional differential gravity field and the zero-value of the cumulative gravity field, the abnormally shifting gradient zones and the bends of the gradient zones, the unbalanced zones in the deep structure where the theoretical equilibrium thickness differs from theactual crustal thickness, and the high and low densities of the three-dimensional density structure inside the transition zone.
2021, 43(4): 453-462.
doi: 10.11939/jass.20210115
Abstract:
This paper analyzes and studies the magnetic anomaly characteristics of each component of the lithospheric magnetic field in the view of pre-earthquake changes of the lithospheric magnetic field near the epicenter of the Madoi MS7.4 earthquake using geomagnetic field vector data of the mainland of China in 2020 and 2019. The results show that before the Madoi MS7.4 earthquake, the lithospheric magnetic field components near the epicenter all changed to varying degrees. The Madoi MS7.4 earthquake epicenter is located at the weakly magnetic anomaly region, and near the zero-variation line of all the components. The Madoi MS7.4 earthquake epicenter is located between the high gradient belt and the low gradient belt of all the components. This paper confirms that the lithospheric magnetic anomaly will be changed before the earthquake, and summarizes characteristics of geomagnetic components magnetic anomaly, and provides a case for the future study of the seismo-magnetism, especially the earthquakes that magnitude larger than 7.0.
This paper analyzes and studies the magnetic anomaly characteristics of each component of the lithospheric magnetic field in the view of pre-earthquake changes of the lithospheric magnetic field near the epicenter of the Madoi MS7.4 earthquake using geomagnetic field vector data of the mainland of China in 2020 and 2019. The results show that before the Madoi MS7.4 earthquake, the lithospheric magnetic field components near the epicenter all changed to varying degrees. The Madoi MS7.4 earthquake epicenter is located at the weakly magnetic anomaly region, and near the zero-variation line of all the components. The Madoi MS7.4 earthquake epicenter is located between the high gradient belt and the low gradient belt of all the components. This paper confirms that the lithospheric magnetic anomaly will be changed before the earthquake, and summarizes characteristics of geomagnetic components magnetic anomaly, and provides a case for the future study of the seismo-magnetism, especially the earthquakes that magnitude larger than 7.0.
2021, 43(4): 463-473.
doi: 10.11939/jass.20200128
Abstract:
With the increase of seismic observation data, the application of automatic processing technology in earthquake event classification, a basic work of seismic monitoring, is becoming more and more important. In this paper, 417 explosion events and 519 natural earthquake events are selected from the rich natural and non-natural seismic observation data of the Inner Mongolia Regional Seismological Network as the original data for the study. After preprocessing, such as interception and filtering, the original data is transformed into log amplitude spectrum in time-frequency domain by short-time Fourier transform, and convolution neural network with three convolution layers is used as classifier to distinguish earthquakes from explosion events. Five folds cross validation results show that the average accuracy of the algorithm used in this paper is 97.33%, and the accuracy of the test set is 98.03%. Our model has applied more original information in the classification of natural earthquake and explosion events, therefore can get a higher accuracy and better stability.
With the increase of seismic observation data, the application of automatic processing technology in earthquake event classification, a basic work of seismic monitoring, is becoming more and more important. In this paper, 417 explosion events and 519 natural earthquake events are selected from the rich natural and non-natural seismic observation data of the Inner Mongolia Regional Seismological Network as the original data for the study. After preprocessing, such as interception and filtering, the original data is transformed into log amplitude spectrum in time-frequency domain by short-time Fourier transform, and convolution neural network with three convolution layers is used as classifier to distinguish earthquakes from explosion events. Five folds cross validation results show that the average accuracy of the algorithm used in this paper is 97.33%, and the accuracy of the test set is 98.03%. Our model has applied more original information in the classification of natural earthquake and explosion events, therefore can get a higher accuracy and better stability.
2021, 43(4): 474-482.
doi: 10.11939/jass.20200135
Abstract:
Random noise suppression of seismic data is essential in seismic data processing. Since the seismic data recorded by the geophone is usually noisy, this kind of noisy data can be regarded as a manifestation of low signal-to-noise ratio. Low SNR data will affect subsequent processing of seismic data, such as migration and imaging. In this paper, we aim to improve the imaging quality of seismic data and propose an intelligent noise reduction framework for convolutional neural network to adaptively learn seismic signals from noisy seismic data. In order to speed up network training and avoid gradient disappearance during training, we add residual learning and batch normalization methods to the network, and use ReLU activation function and Adam optimization algorithm to optimize the network. In addition, the two datasets, Marmousi and F3, are used to train and test the network. A fully trained network can not only retain weak features in learning, but also remove random noise. First, fully train the network, extract random noise from it, and retain the learned seismic data features, and then estimate the waveform features in the test set by reconstructing the seismic data. The processing results of synthetic records and field data show the potential of deep convolutional neural network in random noise suppression tasks, and experimental verification shows that the deep convolutional neural network framework has a good denoising effect.
Random noise suppression of seismic data is essential in seismic data processing. Since the seismic data recorded by the geophone is usually noisy, this kind of noisy data can be regarded as a manifestation of low signal-to-noise ratio. Low SNR data will affect subsequent processing of seismic data, such as migration and imaging. In this paper, we aim to improve the imaging quality of seismic data and propose an intelligent noise reduction framework for convolutional neural network to adaptively learn seismic signals from noisy seismic data. In order to speed up network training and avoid gradient disappearance during training, we add residual learning and batch normalization methods to the network, and use ReLU activation function and Adam optimization algorithm to optimize the network. In addition, the two datasets, Marmousi and F3, are used to train and test the network. A fully trained network can not only retain weak features in learning, but also remove random noise. First, fully train the network, extract random noise from it, and retain the learned seismic data features, and then estimate the waveform features in the test set by reconstructing the seismic data. The processing results of synthetic records and field data show the potential of deep convolutional neural network in random noise suppression tasks, and experimental verification shows that the deep convolutional neural network framework has a good denoising effect.
2021, 43(4): 483-497.
doi: 10.11939/jass.20200153
Abstract:
In the numerical simulation of strong ground motion of future destructive earthquakes, the accuracy of source parameters selection has a great impact on the results of ground motion prediction. There are many uncertain factors in determining source parameters, including both random and cognitive uncertainties. Based on a large number of seismic events and literature researches, this paper focuses on statistical analysis of source parameters with random uncertainty characteristics by using statistical methods. Through regression analysis, a mathematical model is established to explain the randomness and uncertainty of source parameters in the form of empirical formula. In order to study the scaling relation characteristics of source parameters in local regions, we get more empirical relations which are more suitable for local seismic densely regions, especially those of the local regions including the Chinese mainland. This paper more than 1 700 seismic events with MW≥5.5 are selected from the global CMT catalogue. The empirical relationship of source parameters in earthquake intensive areas is studied by using statistical methods, including focal depth, magnitude, seismic moment, rupture area, etc. The number of seismic samples of asperity in a relatively large local range is increased, so as to obtain more suitable experience for local areas to calculate source parameters from the perspective of statistics relationship. The statistical results show that there are differences between the empirical relationship of source parameters obtained from local earthquake cases and those obtained from unlimited regional cases, especially when it comes to fault rupture area and asperity related parameters. The empirical relationship of source parameters obtained from local earthquake cases is more representative. When using the empirical formula obtained in this paper to calculate the focal parameters required for the strong ground motion of future destructive earthquakes, the ground motion prediction results will better reflect the real ground motion characteristics of the target area. It could improve the reliability of the ground motion prediction results.
In the numerical simulation of strong ground motion of future destructive earthquakes, the accuracy of source parameters selection has a great impact on the results of ground motion prediction. There are many uncertain factors in determining source parameters, including both random and cognitive uncertainties. Based on a large number of seismic events and literature researches, this paper focuses on statistical analysis of source parameters with random uncertainty characteristics by using statistical methods. Through regression analysis, a mathematical model is established to explain the randomness and uncertainty of source parameters in the form of empirical formula. In order to study the scaling relation characteristics of source parameters in local regions, we get more empirical relations which are more suitable for local seismic densely regions, especially those of the local regions including the Chinese mainland. This paper more than 1 700 seismic events with MW≥5.5 are selected from the global CMT catalogue. The empirical relationship of source parameters in earthquake intensive areas is studied by using statistical methods, including focal depth, magnitude, seismic moment, rupture area, etc. The number of seismic samples of asperity in a relatively large local range is increased, so as to obtain more suitable experience for local areas to calculate source parameters from the perspective of statistics relationship. The statistical results show that there are differences between the empirical relationship of source parameters obtained from local earthquake cases and those obtained from unlimited regional cases, especially when it comes to fault rupture area and asperity related parameters. The empirical relationship of source parameters obtained from local earthquake cases is more representative. When using the empirical formula obtained in this paper to calculate the focal parameters required for the strong ground motion of future destructive earthquakes, the ground motion prediction results will better reflect the real ground motion characteristics of the target area. It could improve the reliability of the ground motion prediction results.
2021, 43(4): 498-507.
doi: 10.11939/jass.20200149
Abstract:
We randomly selects 100 ground motion records with different ground motion peak characteristic parameters, and obtains the peak ground acceleration （PGA）, peak ground velocity （PGV） and peak ground displacement （PGD） of each ground motion through baseline correction and integration. Based on finite element numerical simulation, the seismic response law of soil slopes is studied by comparing the characteristics of different peak characteristic parameters of the same observation point with the characteristics of the same peak characteristic parameters of different observation points, and the analysis of the characteristics of seismic peak characteristic parameters and the seismic response of soil slope correlation. The calculation results show that PGA, PGV and PGD have a good positive correlation with the seismic response of the soil slope, and the average correlation coefficients are 0.868, 0.981, 0.926, respectively. The correlation of PGV is better than that of PGA and PGD, while the correlation of PGD is slightly better than PGA. Therefore, it is recommended to use PGV as a parameter index in the evaluation of soil slope stability.
We randomly selects 100 ground motion records with different ground motion peak characteristic parameters, and obtains the peak ground acceleration （PGA）, peak ground velocity （PGV） and peak ground displacement （PGD） of each ground motion through baseline correction and integration. Based on finite element numerical simulation, the seismic response law of soil slopes is studied by comparing the characteristics of different peak characteristic parameters of the same observation point with the characteristics of the same peak characteristic parameters of different observation points, and the analysis of the characteristics of seismic peak characteristic parameters and the seismic response of soil slope correlation. The calculation results show that PGA, PGV and PGD have a good positive correlation with the seismic response of the soil slope, and the average correlation coefficients are 0.868, 0.981, 0.926, respectively. The correlation of PGV is better than that of PGA and PGD, while the correlation of PGD is slightly better than PGA. Therefore, it is recommended to use PGV as a parameter index in the evaluation of soil slope stability.
2021, 43(4): 508-520.
doi: 10.11939/jass.20200133
Abstract:
The determination of seismic influence field in post-earthquake work, a quickly gived reasonable map of seismic intensity distribution, was an important basis for emergency rescue after the earthquake and was important for the government to understand the disaster situation, deploy work and estimate the disaster loss. In this paper, the isoseismal maps of moderate strong earthquakes in Hebei Province were collected and sorted out, it was compared with the theoretical isotherm map generated by the regional earthquake intensity attenuation relationship based on the focal mechanism solution. The results show that, with the increase of magnitude, the similarity was higher between the theoretical isoseismal map and the actual isoseismal map in the high intensity area （≥Ⅶ）, the theoretical isoseismal map was calculated by the attenuation relationship of seismic intensity with focal mechanism solution. In addition, according to the spatial variation of aftershock frequency within 24 hours after the earthquake, the theoretical isoseismal line in the polar region is modified, which is more consistent with the actual isoseismal line. That is to say, the theoretical calculated intensity is closer to the actual investigation intensity value. Finally, the grid of Hebei area was divided, and the bedrock PGA of the earthquake case on each grid center point was calculated according to the attenuation relationship of ground motion. Then, the site category attributes were extracted, considered site amplification factor, and the conversion of bedrock PGA to surface PGA was completed. The surface PGA was converted into intensity and compared with the actual earthquake isoseismal map. The results show that the similarity between the seismic influence field calculated by considering the site amplification effect and the actual isoseismal line was very high in the high intensity area, and the similarity is higher than the intensity attenuation relation method based on the focal mechanism solution.
The determination of seismic influence field in post-earthquake work, a quickly gived reasonable map of seismic intensity distribution, was an important basis for emergency rescue after the earthquake and was important for the government to understand the disaster situation, deploy work and estimate the disaster loss. In this paper, the isoseismal maps of moderate strong earthquakes in Hebei Province were collected and sorted out, it was compared with the theoretical isotherm map generated by the regional earthquake intensity attenuation relationship based on the focal mechanism solution. The results show that, with the increase of magnitude, the similarity was higher between the theoretical isoseismal map and the actual isoseismal map in the high intensity area （≥Ⅶ）, the theoretical isoseismal map was calculated by the attenuation relationship of seismic intensity with focal mechanism solution. In addition, according to the spatial variation of aftershock frequency within 24 hours after the earthquake, the theoretical isoseismal line in the polar region is modified, which is more consistent with the actual isoseismal line. That is to say, the theoretical calculated intensity is closer to the actual investigation intensity value. Finally, the grid of Hebei area was divided, and the bedrock PGA of the earthquake case on each grid center point was calculated according to the attenuation relationship of ground motion. Then, the site category attributes were extracted, considered site amplification factor, and the conversion of bedrock PGA to surface PGA was completed. The surface PGA was converted into intensity and compared with the actual earthquake isoseismal map. The results show that the similarity between the seismic influence field calculated by considering the site amplification effect and the actual isoseismal line was very high in the high intensity area, and the similarity is higher than the intensity attenuation relation method based on the focal mechanism solution.
2021, 43(4): 521-532.
doi: 10.11939/jass.20200164
Abstract:
In this study, based on 6 824 borehole profiles, we subdivide the site classes in GB 50011-2010 Code for Seismic Design of Buildings （Chinese code） into more homogeneous sub-classes by different values of the equivalent shear wave velocity （vSe） and site overlaying layers （D）, and quantitatively analysis the effect of each parameters in the site classification schedule in the code. We build the relation between these sub-classes of the China code and classes of the US seismic design code National Earthquake Hazards Reduction Program Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, carry out comparative analysis on two classification schedules, and build the probabilistic transformation relations for interconverting China site classes and the US site classes. The results show that: It is not appropriate to take the average shear wave velocity to a depth of 20 m （vS20） as the proxy for vSe in site classification of China code; for China site class Ⅱ and Ⅲ, different sub-classes have significantly different corresponding relations with the US site classes; the D effectively distinguishes the sites those velocity structures are similar at shallow layers while different at deeper layers; the main part of China site class Ⅱ and Ⅲ are both corresponding to the US site class D, the China site class Ⅱ leans to the US site class C, while the China site class Ⅲ leans to the US site class E; China site class Ⅳ is corresponding to the US site class E; most of the US site class C and D are both corresponding to China site class Ⅱ.It implies that the range of China site class Ⅱ is relatively vast.
In this study, based on 6 824 borehole profiles, we subdivide the site classes in GB 50011-2010 Code for Seismic Design of Buildings （Chinese code） into more homogeneous sub-classes by different values of the equivalent shear wave velocity （vSe） and site overlaying layers （D）, and quantitatively analysis the effect of each parameters in the site classification schedule in the code. We build the relation between these sub-classes of the China code and classes of the US seismic design code National Earthquake Hazards Reduction Program Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, carry out comparative analysis on two classification schedules, and build the probabilistic transformation relations for interconverting China site classes and the US site classes. The results show that: It is not appropriate to take the average shear wave velocity to a depth of 20 m （vS20） as the proxy for vSe in site classification of China code; for China site class Ⅱ and Ⅲ, different sub-classes have significantly different corresponding relations with the US site classes; the D effectively distinguishes the sites those velocity structures are similar at shallow layers while different at deeper layers; the main part of China site class Ⅱ and Ⅲ are both corresponding to the US site class D, the China site class Ⅱ leans to the US site class C, while the China site class Ⅲ leans to the US site class E; China site class Ⅳ is corresponding to the US site class E; most of the US site class C and D are both corresponding to China site class Ⅱ.It implies that the range of China site class Ⅱ is relatively vast.
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