Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Display Method:
Ambient noise level of Sichuan region from seismic stations
Xie Jiangtao, Lin Liping, Zhao Min, Chen Liang
 doi: 10.11939/jass.20200148
[Abstract](0) [FullText HTML](0) [PDF 4330KB](0)
Abstract:
Based on the three-component continuous waveform data observed by sixty permanent seismic stations in Sichuan Seismic Network recorded from January 1, 2015 to December 31, 2018, this paper calculated the noise power spectral densities and corresponding probability density functions, and then gave the statistical characteristics of noise power spectral density at different frequencies and the variations of the noise level at different regions and frequencies. The results show that the high-frequency seismic noise of most stations is affected by the human activities, production mode and lifestyle nearby, which has obvious seasonal and diurnal variations. The noise level increases during summer and decreases during winter, with the lowest level during the Spring Festival in the whole year; and the geographical spatial distribution is not obvious. At double-frequency microseism band, the noise level increases during winter and decreases during summer, and has obvious seasonal variation, with an average of 1−5 dB, which has obvious geographical spatial distribution characteristics. The average noise level in eastern Sichuan is the highest, followed by Panxi region, and the lowest in western Sichuan plateau. The microseism peaks have different amplitudes and occur at different frequencies in summer and winter, with the peaks shifted by 1−2 s toward longer periods in the winter. Compared with the double-frequency microseism band, at primary microseism band the noise energy is weaker, the seasonal variation is not obvious, and the difference of noise level in geographical distribution is significantly reduced. While the long-period (>20 s) noise level has no obvious seasonal variation and geographical spatial distribution difference. In addition, installing seismographs in caves and borehole can effectively reduce the influence of noise sources, temperature and pressure on high-frequency and long-period observations, and the noise level is lower than that of shallow installations.
Interference characteristics of new energy power generation on electromagnetic observation environment:Taking the electromagnetic observation of Xuzhuangzi station of Tianjin as an example
Ma Yong, Li Ning, Xu Xuegong, Bi Jinmeng
 doi: 10.11939/jass.20210028
[Abstract](8) [FullText HTML](3) [PDF 1872KB](3)
Abstract:
New energy power generation is an important measure taken by the countries to replace non-renewable energy sources to achieve sustainable utilization of power resources under the sustainable development strategy. Wind power and photovoltaic power generation are clearly defined as the key development areas of renewable energy. These big power generation facilities will have a great impact on the original seismic observation environment. Based on the electromagnetic observation project of the Xuzhuangzi station of Tianjin, aiming at the wind power and photovoltaic power generation facilities in the observation area around the station in recent years we analyzed the temporal and spatial changes of electromagnetic observation data in different observation environments by the field experimental measurement, and summarized the influence characteristics of the construction and operation of wind power generation and photovoltaic power generation on electromagnetic observation environment interference. The results show that the impact of wind turbine is significantly less than that of photovoltaic power generation equipment on the electromagnetic observation environment, which can be regarded as an interference point to regulate the distance between wind turbine and observation facilities, and in order to ensure that the photovoltaic power generation equipment does not affect the electromagnetic observation, it is necessary to treat the two systems as a whole, so as to maintain a large enough safety distance between the two systems. This paper can provide a reference for the protection and evaluation of electromagnetic observation environment by building similar new energy power generation equipment in the seismic field, which is of important practical significance for protecting the observation environment of seismic stations.
Construction and verification of onsite ground motion prediction models for seismic intensity instrument
PENG Chaoyong, ZHENG Yu, XU Zhiqiang, JIANG Xudong, YANG Jiansi
 doi: 10.11939/jass.20210075
[Abstract](199) [FullText HTML](94) [PDF 1707KB](69)
Abstract:
Using the initial P-wave early warning parameters to construct onsite ground motion prediction models, so as to quickly release an alarm message when it reaches the predefined threshold, is a key issue of the onsite earthquake early warning system, which is directly related to the accuracy and timeliness of the early warning information. For MEMS-based seismic intensity instrument (SII) with poor data quality, the obtained displacement record after two integrations has a large deviation, which will lead to more false and missed alarms. Therefore, for waveforms recorded by SII, in this paper, we adopted Butterworth filters of different orders (1−4) to build several onsite ground motion prediction models based on the P-wave 3 seconds data and the whole P-wave window. These models are relationships between Pd and PGV, Pd and peak ground motion acceleration (PGA), velocity amplitude (Pv) and PGV, Pv and PGA, acceleration amplitude (Pa) and PGV, Pa and PGA, respectively. The models is then verified using the collected MEMS-based seismic event records from the Sichuan-Yunnan Demonstration Early Warning Network. The results show that for the SII records, the two optimal onsite ground motion prediction models are the relationship between Pv and PGV and the one between Pa and PGA obtained by the first-order Butterworth filter processing and derived from the whole P-wave window. In specific applications, two or more statistical relationships should be simultaneously adopted to predict onsite ground motion, and observed ground motion values should be used as additional judgment conditions to reduce the probability of false and missed alarms.
The anomaly characteristics of water temperature in the Eryuan well before the 2021 Yangbi MS6.4 earthquake in Yunnan,China
Ma Yuchuan, Yan Rui, Hu Xiaojing
 doi: 10.11939/jass.20210142
[Abstract](58) [FullText HTML](21) [PDF 2094KB](31)
Abstract:
Dynamic characteristics of fault hydrogen concentration in Aksu and its earthquake reflection efficiency
Zhong Jun, Wang Bo, Yan Wei, Ma Yuchuan
 doi: 10.11939/jass.20210007
[Abstract](32) [FullText HTML](13) [PDF 2265KB](10)
Abstract:
This paper firstly analyzed the periodic components of the fault hydrogen concentration observation data in Aksu, and then explored the relationship between temperature, atmospheric pressure and the annual periodic components of hydrogen concentration using the linear regression and cross-correlation methods. At the same time, the Molchan error chart method is used to quantitatively verify the earthquake reflection efficiency of fault hydrogen in Aksu. The results show that: ① Fault hydrogen concentration in Aksu has good annual and semi-daily periodic components; ② There is a good positive correlation between the annual dynamic changes of fault hydrogen concentration and temperature, and a general correlation between the annual dynamic changes of hydrogen concentration with atmospheric pressure, indicating that the annual periodic changes of hydrogen concentration are mainly affected by temperature. The results from Molchan diagram method show that the fault hydrogen concentration anomaly in Aksu is more sensitive to moderate-strong earthquakes around the observation point, suggesting good earthquake reflection efficiency.
Anomalous characteristics of geomagnetic vertical strength polarization before the Milin MS6.9 earthquake in 2017
Li Xia, Feng Lili, Zhao Yuhong, Liu Lei, Gou Zhidong, Fan Wenjie, He Manqiu, Liao Xiaofeng, Aisa Yisimayili  
 doi: 10.11939/jass.20200196
[Abstract](244) [FullText HTML](112) [PDF 4945KB](17)
Abstract:
Based on the data of 65 geomagnetic observation stations in mainland China, the magnetic anomaly signals in the seismic source area are extracted by the method of geomagnetic vertical intensity polarization and the anomalous space-time scanning results are analyzed. The analysis shows that there was a large scale synchronous geomagnetic anomaly before the MS6.9 earthquake in Milin, Tibet. Since October 30, 2017, the calculation results showed a high value pattern of single peak or double peak, which lasted for 3 days. It reappeared two days later and lasted for four days, the abnormal area shows an evolutionary process of enlargement-shrinkage- disappearance -enlargement-disappearance. The high value abnormal repetition area is located in western China, mainly at the junction of the Bayan Hala block and the Qiangtang block on the Qinghai-Tibet Plateau. The abnormal process lasted for 9 days, 19 days after which a MS6.9 earthquake occurred in Milin, Tibet, with the epicenter located 5km from the threshold line of October 31, 2017.Comprehensive analysis shows that the abnormal high value of geomagnetic vertical intensity polarization has nothing to do with the external field, but reflects the underground electromagnetic signal, and this method has a certain indication significance for the moderate earthquakes in mainland China.
Structural characteristics of Yanyuan basin deduced from geophysical data and it dynamic implication
Zhang Pingchuan, Yu Changqing, Qu Chen, Qiu Longjun, Li Hengqiang
 doi: 10.11939/jass.20200119
[Abstract](293) [FullText HTML](143) [PDF 3414KB](40)
Abstract:
In this paper, an artificial seismic line and a short period natural seismic array covering the basin are set up. The seismic energy attribute is extracted from the artificial seismic data and the seismic velocity structure is obtained by seismic tomography inversion. Meanwhile the S-wave velocity distribution at different depths is obtained by inverting the empirical Green’s functions retrieved from cross-correlation of the ambient noise data recorded by the short period seismic array. The results show that the Yanyuan basin is mainly divided into three layers by seismic reflection characteristics. The shallow layer is the Cenozoic sedimentary stratum with low velocity ranging from 1000 m/s to 2100 m/s, and it is characterized by good continuity, strong reflection. The Cenozoic basin is thick in the south and thin in the north. The sedimentary and structure are controlled by Yanyuan fault. The middle layer is medium-low velocity ranging from 3500 m/s to 4500 m/s, it is characterized by that the reflection axis is discontinuous and the reflection is weak, which can be interpreted as Triassic strata. The deep layer is high velocity, characterized by chaotic seismic reflection and weak reflection energy, interpreted as Paleozoic strata. It is found that there is a detachment surface in the upper crust, which is the interface between sedimentary and basement, and a series of faults are developed from the detachment to the surface. One of the faults is Jinhe-Qinghe fault, which is the boundary fault between Yanyuan basin and Kangdian block. In addition, these faults are also an earthquake-prone area in Yanyuan basin.
Display Method:
Three-dimensional seismic velocity structure beneath the M6.4 Yangbi,Yunnan earthquake region
Du Guangbao, Wu Qingju, Zhang Xuemei
2021, 43(4): 397-409.   doi: 10.11939/jass.20210104
[Abstract](367) [FullText HTML](122) [PDF 44515KB](103)
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.
Study on Lg wave attenuation imaging in Yunnan
Liu Sen, Bian Yinju, Wang Tingting, Lu Zhinan
2021, 43(4): 410-426.   doi: 10.11939/jass.20200101
[Abstract](133) [FullText HTML](44) [PDF 4428KB](16)
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.
Research on seismic characteristics and identification of artificial explosion in different areas and natural earthquake
Wang Tingting, Bian Yinju, Yang Qianli, Ren Mengyi
2021, 43(4): 427-440.   doi: 10.11939/jass.20210169
[Abstract](257) [FullText HTML](132) [PDF 3538KB](41)
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.
Gravity field characteristics and seismogenic model in North China
Xu Weimin, Shi Lei, Chen Shi, Lu Hongyan
2021, 43(4): 441-452.   doi: 10.11939/jass.20200150
[Abstract](94) [FullText HTML](38) [PDF 9244KB](8)
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.
Characteristic analysis of the lithospheric magnetic anomaly before the Madoi MS7.4 earthquake on 22th May 2021
Dong Chao, Chen Bin, Yuan Jiehao, Wang Zhendong, Wang Can
2021, 43(4): 453-462.   doi: 10.11939/jass.20210115
[Abstract](230) [FullText HTML](104) [PDF 2006KB](37)
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.
Classification of seismic events based on short-time Fourier transform and convolutional neural network
Zhang Fan, Yang Xiaozhong, Wu Lifei, Han Xiaoming, Wang Shubo
2021, 43(4): 463-473.   doi: 10.11939/jass.20200128
[Abstract](133) [FullText HTML](42) [PDF 7570KB](20)
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.
Random noise suppression of seismic data based on deep convolution neural network
Chen Tian, Yi Yuanyuan
2021, 43(4): 474-482.   doi: 10.11939/jass.20200135
[Abstract](126) [FullText HTML](57) [PDF 2557KB](12)
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.
Empirical relationship of stochastic uncertainty of source parameters in relative local area
Li Zongchao, Gao Mengtan, Sun Jize, Chen Xueliang, Zhang Bo
2021, 43(4): 483-497.   doi: 10.11939/jass.20200153
[Abstract](202) [FullText HTML](70) [PDF 7203KB](38)
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.
Regularity research on the seismic response of characteristic parameters for ground motion peak to soil slope
Du Lurong, Zhang Jiangwei, Chi Mingjie, Chen Su
2021, 43(4): 498-507.   doi: 10.11939/jass.20200149
[Abstract](157) [FullText HTML](50) [PDF 2379KB](16)
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.
Research on application of focal mechanism and site conditions in judgment of Hebei earthquake influence field
Sun Lina, Qi Yuyan, Chen Ting, Wang Xiaoshan
2021, 43(4): 508-520.   doi: 10.11939/jass.20200133
[Abstract](127) [FullText HTML](66) [PDF 2673KB](15)
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.
Comparative analysis and transformation relations between China and the US site classification systems in building seismic code provisions
Zhou Jian, Li Xiaojun, Li Yaqi, Kang Chuanchuan
2021, 43(4): 521-532.   doi: 10.11939/jass.20200164
[Abstract](62) [FullText HTML](16) [PDF 2314KB](14)
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.