2021 Vol. 43, No. 5

Display Method:
Characteristics of seismic ambient noise in Sichuan region
Xie Jiangtao, Lin Liping, Zhao Min, Chen Liang
2021, 43(5): 533-550. doi: 10.11939/jass.20200148
Based on the three-component continuous waveform data recorded by sixty permanent seismic stations in Sichuan seismic network from January 1, 2015 to December 31, 2018, this paper calculated the noise power spectral densities and corresponding probability density functions, then gave the statistical characteristics of noise power spectral density at different frequencies, and finally analyzed the characteristics of noise level at different regions and frequencies. The results show that the high-frequency seismic noises of most stations are affected by the nearby human activities, production mode and lifestyle, 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 distribution is not obvious. For double-frequency microseisms, 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 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, the noise energy at primary microseism band 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 no difference in geographical distribution. In addition, installing seismographs in caves and borehole can effectively reduce the influence of noise sources, temperature and pressure on high-frequency band and long-period observations, therefore the noise level is lower than that of shallow installations.
Spatio-temporal characteristics of repeating seismic events in the middle of Tianshan orogenic belt
Tang Lanlan, Zhang Yifeng, Yao Daoping
2021, 43(5): 551-568. doi: 10.11939/jass.20200192
Based on the seismic waveform data recorded in the stations of Xinjiang regional seismic network from 2009 to 2017, the repeating earthquakes in the middle of Tianshan orogenic belt and its periphery in Xinjiang were determined and relocated by using the waveform cross correlation technique and the master event approach. The results show that 11 618 of the 30 181 events are repeating events, which consist of 2395 groups of doublets and clusters, accounting for 38.5% of the total events. According to the statistical results of the distance between doublets before and after repeating events relocation, it is estimated that the system location error in the research area is about 5−10 km. In addition, combined with the latest source classification results in this area, the results show that repeating earthquakes of different source types have different spatial and temporal distribution characteristics. Repeating quarry blasts appear mostly as clusters, 93.6% of them occur during the daytime, and they also exhibit a seasonal pattern with more events in summers and fewer ones in winters. Tectonic earthquakes occurred in various thrust faults in the Tianshan orogenic belt, and occurred randomly at any time, and the monthly frequency of tectonic events is relatively stable during the studied period. Repeating induced earthquake locations indicate that most of them are located near large gas/oil fields and water reservoirs, but some also geographically overlay tectonic earthquakes in some regions. The occurring time characteristics of induced earthquakes are similar to those of tectonic earthquakes, which appears as random distribution within 24 hours.
Structural characteristics of Yanyuan basin deduced from seismic survey and its dynamic implication
Zhang Pingchuan, Yu Changqing, Qu Chen, Qiu Longjun, Li Hengqiang
2021, 43(5): 569-583. doi: 10.11939/jass.20200119
In this paper, an artificial seismic line and a short period natural seismic array covering Yanyuan 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. 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 Yanyuan basin is mainly divided into three layers by seismic characteristics. The shallow layer is the Cenozoic sedimentary stratum with low P wave velocity ranging from 1.0 km/s to 2.1 km/s, and it is characterized by good continuity and strong reflection. The S wave velocity is low in the south and high in the middle of Yanyuan basin, and the trend of the low-velocity anomaly in the south is in agreement with the strike of Yanyuan fault. Cenozoic basin is thick in the south and thin in the north, where the sedimentary and structure are controlled by Yanyuan fault. The middle layer is medium-low P wave velocity ranging from 3.5 km/s to 4.5 km/s, and it is characterized by discontinuous reflection axis and weak reflection, which can be interpreted as Triassic strata. The S-velocity becomes higher gradually and the change becomes smaller in general. The deep layer is high velocity, characterized by chaotic seismic reflection and weak reflection energy, interpreted as Paleozoic strata. Furthermone, there is a detachment surface in the upper crust, which is the interface between sedimentary and basement. A series of faults are developed from the detachment to the surface, and these faults are also an earthquake-prone area in Yanyuan basin. One of the faults is Jinhe-Qinghe fault, which is the boundary fault between Yanyuan basin and Kangdian block.
Anomalous characteristics of geomagnetic vertical strength polarization before the 2017 Milin MS6.9 earthquake in Tibet
Li Xia, Feng Lili, Zhao Yuhong, Liu Lei, Gou Zhidong, Fan Wenjie, He Manqiu, Liao Xiaofeng, Aisa Yisimayili
2021, 43(5): 584-594. doi: 10.11939/jass.20200196
Based on the second sampling data from 65 geomagnetic stations in Chinese mainland, we extracted the ultra-low frequency electromagnetic anomaly signals from the epicenter of the 2017 Milin MS6.9 earthquake in Tibet and its vicinity using geomagnetic vertical strength polarization method, and analyzed their spatio-temporal evolution characteristics. The results show that a large range of geomagnetic vertical strength polarization high value anomalies appeared in the Qinghai-Xizang (Tibetan) Plateau before the MS6.9 earthquake, and the abnormal process lasted for nine days. The high value anomaly began on October 30, 2017. The high value lasted for three days, followed by a short period of decline, a turning point and an increase. Afterwards the high value anomaly appeared again and lasted for four days. In this process, the time-series curves of each high-value station show single peak or double peak. The spatial distribution map shows that high value anomalies occur repeatedly in the western part of Chinese mainland, especially at the junction of Bayankhara and Qiangtang blocks on the Tibetan Plateau. The anomaly appeared, expanded, contracted, disappeared, expanded and disappeared, and the abnormal area reached its maximum value on October 31. Ten days after the end of the abnormal process, the Milin MS6.9 earthquake occurred, with its epicenter 5 km from the anomaly threshold line of October 31. Comprehensive analysis indicates that this anomaly has strong temporal and spatial correlation with the Millin earthquake and can be regarded as reliable seismic electromagnetic precursor.
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
2021, 43(5): 595-604. doi: 10.11939/jass.20210028
New energy power generation is an important measure taken by the countries to replace non-renewable energy sources so as 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 fields 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 system on the electromagnetic observation environment, which can be regarded as an interference point to regulate the distance between wind turbine and observation facilities. In order to ensure that the photovoltaic power system does not affect the electromagnetic observation, it is necessary to treat each system as a whole and maintain a sufficiently large separation distance between them. 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.
Design and implementation of ionospheric multi-parameter anomaly monitoring system in Sichuan-Yunnan region
Liu Jiang, Xu Rui, Chen Cong, Rao Taiguo, Li Xingquan
2021, 43(5): 605-614. doi: 10.11939/jass.20210038
This paper introduced the design ideas and function implementation of ionospheric multi-parameter anomaly monitoring system in Sichuan-Yunnan region, and then applied the system to monitoring test of the MS6.0 earthquake on June 17, 2019 in Changning, Sichuan. The results show that the system realizes the real-time monitoring of abnormal changes such as global and China regional VTEC, site VTEC and foF2, which is helpful to carry out the demonstrative seismic-ionospheric anomaly monitoring. The graphic and data results can provide evidences for the identification of seismic-ionospheric anomalous disturbance, and ionospheric precursor information for earthquake monitoring and prediction in Sichuan-Yunnan region.
Dynamic characteristics of fault hydrogen concentration in Aksu and its earthquake reflecting efficiency
Zhong Jun, Wang Bo, Yan Wei, Ma Yuchuan
2021, 43(5): 615-627. doi: 10.11939/jass.20210007
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, respectively. At the same time, the Molchan error diagram method was used to quantitatively verify the earthquake reflecting efficiency of fault hydrogen in Aksu. The results show that: ① Fault hydrogen concentration in Aksu has clear annual and semi-daily periodic components; ② There is a significant 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 and atmospheric pressure, indicating that the annual periodic changes of hydrogen concentration are mainly affected by temperature. The results from Molchan error diagram method show that the fault hydrogen concentration anomaly in Aksu is more sensitive to moderate-strong earthquakes around the observation point, suggesting better earthquake reflecting efficiency.
Empirical prediction models of time-averaged shear wave velocity vS20 and vS30 in Sichuan and Yunnan areas
Jia Lin, Xie Junju, Li Xiaojun, Wen Zengping, Chen Wenbin, Zhou Jian
2021, 43(5): 628-642. doi: 10.11939/jass.20200193
The time-averaged shear wave velocity of overburden soil is an important parameter for site classification and reflecting site effects on ground motion, which is widely used in earthquake ground motion prediction models. Using the lithology and wave velocity profile data of 973 boreholes in Sichuan and Yunnan, we study the regional prediction model of the average shear wave velocity. Based on the bottom constant velocity (BCV) model, log-linear model and Markov independent model, the empirical prediction models of vS20 and vS30 in this region were established. The results show that, the BCV method has the largest prediction error. When the depth of the shear wave velocity is less than 10 m, this method will significantly underestimate the average wave velocity of the actual site. Based on the log-linear model of Boore method, we establish an empirical prediction model. By comparison, we find that the average wave speed prediction results in Sichuan and Yunnan are close to those in Beijing and California, and significantly lower than those in Japan. Through the comparative analysis of prediction error of three different extrapolation methods, we find that the prediction results based on Markov independence model have the smallest error at different depths, and it is preferred to use this method to set up regional prediction model.
Construction and verification of onsite ground motion prediction models for seismic intensity instrument
Peng Chaoyong, Zheng Yu, Xu Zhiqiang, Jiang Xudong, Yang Jiansi
2021, 43(5): 643-655. doi: 10.11939/jass.20210075
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 micro-electro-mechanical-systems-based seismic intensity instrument 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 seismic intensity instrument, in this paper, we adopted Butterworth filters of different orders (1−4) to build up several onsite ground motion prediction models based on the P-wave 3 seconds data and the whole P-wave window. These models are the relationships between displacement amplitude PD and peak ground velocity PGV, PD and peak ground acceleration PGA, velocity amplitude PV and PGV, PV and PGA, acceleration amplitude PA and PGV, PA and PGA, respectively. The models are then verified using the collected micro-electro-mechanical-systems-based seismic event records from the Sichuan-Yunnan Demonstration Early Warning Network. The results show that for the seismic intensity instrument 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 development in seismic application research of VLF/LF radio waves
Zhang Xuemin
2021, 43(5): 656-673. doi: 10.11939/jass.20210143
Radio waves from very low frequency and low frequency (VLF/LF) transmitters, as an active signal for communication and navigation purpose, show great advantages in ionospheric monitoring and application research with their long-distance propagation feature in the waveguide between surface and lower ionosphere. This paper summarized the developments in detecting technology of VLF/LF radio waves, their data analysis methods, case study and statistical research, coupling processes and channels in lithosphere-atmosphere-ionosphere. On the basis of the significant achievements of this technology in earthquake application, and the researches in the origin of big disturbances in VLF/LF observations, the future development plan has been considered to build up a stereo-monitoring system in China by combining our long-history ground-based network and new satellite platform in electromagnetic field, to fully use of this technology in earthquake research.
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
2021, 43(5): 674-677. doi: 10.11939/jass.20210142