Abstract: Based on the field investigation of the Luding MS6.8 earthquake occurred on September 5, 2022, combined with the remote sensing interpretation, the development characteristics and controlling factors of coseismic geohazards triggered by the Luding earthquake are summarized, and the evolution trend of coseismic geohazards is studied and judged. According to the different phases of prevention and mitigation of coseismic geohazards, corresponding suggestions are given in order to benefit the prevention and control of geohazards. The results show that the coseismic geohazards of the Luding earthquake are mainly small-medium-sized collapses and landslides, which are concentratedly distributed in three areas: Moxi town and Hailuogou valley, Wandong village of Detuo town and both banks of Daduhe river in Detuo town. Mainshock and aftershocks, Xianshuihe active fault, topography, special rock and sediment mass are the main controlling factors for spatial distribution of coseismic landslides triggered by the Luding earthquake. The geohazards after Luding earthquake will be extremely active in the next 10 years. It is necessary to pay close attention to the high-locality and steep slopes on both banks of the Moxihe river and its tributaries, the Daduhe river, the high-locality terrace along the Daduhe river, the edge area around the Moxi platform, and the debris flows in Moxi town and Detuo town where coseismic landslides are densely developed. Therefore, according to the management logic of emergency prevention of coseismic geohazards, it is suggested that local governments should follow the three phases of detailed investigation, restoration and reconstruction, and long-term planning to carry out geohazard prevention and mitigation work.
Zhang J J，Chen L，Li Y L，Liu M S，Shi S W，Yi J S，Zhang W，Zhang S L，Sun J H，Yang D，Xian J L，Cai J J. 2023. Development characteristics and controlling factors of co-seismic geo-hazards triggered by the Luding MS6.8 earthquake on September 5，2022. Acta Seismologica Sinica，45（2）：167−178. doi: 10.11939/jass.20220215.
Abstract: At 12:52 on September 5, 2022, a MS6.8 earthquake occurred in Luding County, Garze Prefecture, Sichuan Province. The earthquake caused severe damage and heavy casualties in Luding County and its surrounding areas. In order to reproduce the ground motion influence field of the earthquake and analyze the spatial distribution characteristics of near-field ground motion, the deterministic asperity source model is combined with the random source model to obtain the kinematic hybrid source model. Then, the hybrid source model is implemented into the SPECFEM 3D, and the whole-process broadband （0.1−5 Hz） ground motion simulation based on the spectral element method and kinematic hybrid source model is realized. The results from the simulation of Luding earthquake are as follows. Firstly, the simulation results are compared with the time history records of six stations, the corresponding response spectra and the NGA-West2 ground motion attenuation curves to test the applicability of the method. Secondly, the three-component velocity wavefield snapshots of the earthquake is given to demonstrate the directional effect and local site effect of the near field when the seismic wave propagates. Finally, the peak acceleration （PGA） and peak velocity （PGV） maps of the ground motion in the range of 100 km×100 km centered on the Luding area are given, and the spatial distribution characteristics of the ground motion in the near field region for the Luding earthquake are analyzed. Based on the simulation results, the seismic intensity distribution map is given. The results show that the epicenter PGA and PGV is close to 600 cm/s2 and 50 cm/s, respectively, and the seismic intensity reaches Ⅸ degree. Due to the influence of mountain-canyon topography in Luding area on the ground motion, the peak of ground motion is significantly amplified at the top of the mountain and the bottom of the canyon, with the amplification of PGA and PGV of 1.9 times and 1.5 times, respectively. The amplification of PGA and PGV at the bottom of the canyon is 1.7 times and 1.4 times. Therefore, attention should be paid to the phenomenon of earthquake amplification and possible secondary geological disasters in mountain-canyon region.
Zhao J X，Ba Z N，Kuo C Y，Liu B J. 2023. Broadband ground motion simulations applied to the Luding MS6.8 earthquake on September 5，2022 based on spectral element method. Acta Seismologica Sinica，45（2）：179−195. doi: 10.11939/jass.20220190.
Li Z，Liu R F，Wang Z B，Hu Y S，Kong H D. 2023. Determination of focal mechanism and seismic energy of Luding MS6.8 earthquake on September 5，2022. Acta Seismologica Sinica，45（2）：196−202. doi: 10.11939/jass.20220212.
Abstract: Due to high sensitivity to S-wave velocity, Rayleigh-wave dispersion curves of the fundamental and higher modes are usually used to invert near-surface S-wave velocities in engineering geophysical exploration. For the model containing a low-velocity layer, the dispersion curves of the fundamental and higher modes show two typical characteristics. One typical characteristic is that the crossover would be observed between different modes, and the fundamental mode shows obvious indication of low-velocity characteristics in interested frequency ranges. For the other kind of model with low-velocity layers, the dispersion curves have no visual crossing phenomenon in the frequency range of interest, and the low-velocity characteristics may not be observed in the measured dispersion curves. For the latter model containing a low-velocity layer, which is often encountered in practice, investigations on the inversion of multi-mode Rayleigh waves are conducted in this paper based on seismic reflection data. The studies show that if the observed fundamental-mode dispersion curve does not include the frequency band sensitive to the depth of the low-velocity layer, the inversion based on the fundamental-mode alone may not be able to recover the low-velocity characteristics of the model. But the low-velocity layer can be reconstructed accurately by inversion considering both the fundamental and higher mode Rayleigh waves even the observed fundamental mode dispersion curve has no obvious indication of low-velocity characteristics.
Feng X Z，Lu L Y，Wang S T，Qin T W. 2023. Constraint on low-velocity layer using higher mode Rayleigh waves in the shallow structure research. Acta Seismologica Sinica，45（2）：203−222. doi: 10.11939/jass.20210189.
Abstract: After the impoundment of the Three Gorges Reservoir in 2003, shallow earthquakes occurred frequently, which had a great impact on local productions and lives. The study of shallow velocity structure is of great significance for shallow seismic disaster assessment and disaster prevention and reduction in Zigui area. In this paper, based on the vertical component continuous waveform records of 24 mobile seismic stations in Zigui area of Hubei Province from June to July of 2020, the empirical Green’s functions between the stations are obtained by cross-correlation of the ambient seismic noise, the Rayleigh wave group velocity dispersion curves on the period 0.6−5 s are extracted, and the inversion for the three-dimensional S-wave velocity model is obtained within 6 km near the surface of the region. The results show that the S-wave velocity of Zigui basin and its south adjacent area is significantly lower than that of the eastern fault area, which is consistent with the structural evolution and sedimentary characteristics of different structural blocks in the studied area; an earthquake with MS4.2 occurred on the vertical high-velocity and low-velocity junction area of the study area in 2014. The S-wave velocity decreases obviously near the Yangtze River region in the fault zone, indicating that the Yangtze River water penetrates into the fault area. Therefore, the frequent occurrence of earthquakes in Zigui area is related to the water load and water infiltration of the Three Gorges reservoir.
Hu J T，Xie J，Wei Z G，Jin C. 2023. Shallow velocity structure and seismogenic environment in the Zigui section of the Three Gorges Reservoir region of China. Acta Seismologica Sinica，45（2）：223−233. doi: 10.11939/jass.20210194.
Abstract: Seismic phase picking is the first key step of mine microseisms detection, and its accuracy often directly affects the quality of subsequent event processing, so we proposed a method for P-arrival picking of mine microseisms which is based on deep learning method. Firstly the CNNDet model is constructed for events detection and P-arrival pre-picking, and then the CGANet model was constructed to accurately pick up the P-arrival time for the detected events by introducing the self-attention mechanism and the gated recurrent unit. Comparison with STA/LTA, DPick and PpkNet shows that the precision and the recall ratio of seismic event detection by our method are more than 98% for the test sets, and the mean error and the standard deviation of P-arrival are 0.014 s and 0.051 s, respectively. Our method is superior to the above three methods in terms of precision, the recall ratio and the standard deviation. In addition, the experimental tests on samples with different SNRs prove that our method can still maintain high precision on the condition of low SNR. In the source location, our method also shows more excellent performance. The P-arrival picking method proposed in this paper which is based on gated recurrent unit and self-attention mechanism provides a new idea for microseisms monitoring and accurate identification of rock burst and other disasters.
Jiao M R，Dong F J，Luo H，Yu J K，Ma L. 2023. P-arrival picking method of mine microseisms by fusing of GRU and self-attention mechanism. Acta Seismologica Sinica，45（2）：234−245. doi: 10.11939/jass.20220034.
Abstract: Based on spatial autocorrelation （SPAC） method, the dispersion curves of Rayleigh wave have been extracted from the microtremor recorded by 21 stations belong to the dense short-period array deployed in Guangdong-Hong Kong-Macao Greater Bay area, and then the inversion for shallow S-wave velocity structures within a depth of 1 km beneath the Panyu district, Guangzhou city, have been performed. The results show that the velocity beneath the array is obviously low within 0.25 km depth, ranging from 1.17 km/s to 1.59 km/s, while the velocity increases steadily to 2.88 km/s between 0.25 km to 1 km depth. This indicates that the stability and the reliability of the method, which also implies that the SPAC is an effective, economical and environmental method for detecting the shallow fine structures in densely populated urban areas, and it will play an increasingly important role in the exploration of shallow structure in urban areas in the future.
Pan Q，Shen X Z. 2023. Spatial autocorrelation method based on dense short-period seismic array and its application in the Guangdong-Hong Kong-Macao Greater Bay. Acta Seismologica Sinica，45（2）：246−257. doi: 10.11939/jass.20220003.
Abstract: In seismic observation, seismic data generally contain ambient noise, which reducesthe efficiency of seismic analysis. Traditional denoising methods usually need a priori knowledge of noise, and some effective data will be lost when filtering. To solve this problem, this paper proposes a seismic data denoising method based on complete ensemble empirical mode decomposition with adaptive noise （CEEMDAN） and Hurst exponent. Firstly, the signal is decomposed into a series intrinsic mode functions （IMF） by CEEMDAN method. Secondly, the Hurst exponent is used to identify the filtered IMF component. Finally, the IMF component of seismic data is reconstructed to realize data denoising. Compared with the denoising effect of traditional methods, the filtering ability of this method for low SNR waveforms is improved by 32%, and the filtering ability for high SNR waveforms is 6 times higher. At the same time, as shown in the denoising results of geomagnetic data, this method can completely filter subway noise from geomagnetic signal waveform.
Mao S R，Shi S P，Yu Z J，Su M Y，Li S，He J X，Fu H，Zhang Q. 2023. A seismic data denoising method based on complete ensemble empirical mode decomposition with adaptive noise and Hurst exponent. Acta Seismologica Sinica，45（2）：258−270. doi: 10.11939/jass.20210165.
Abstract: A MS6.0 earthquake with a focal depth of 16 km occurred with the epicenter of （28.34°N, 104.90°E） in Changning county, Yibin city, Sichuan Province on June 17, 2019. There were three strong aftershocks above MS5.0 within five days after the Changning earthquake and another MS5.6 earthquake occurred in Gongxian County on July 4. This paper relocated 1948 ML≥2.0 earthquakes in Changning-Xingwen area during 2019 to 2020 by the double difference method based on the seismic phase observation report provided by China Earthquake Networks Center. The distribution of 1 823 earthquakes above ML2.0 after relocation shows that the aftershock sequence in the northern area of Changning spreads along the NW−SE direction anticline. Moreover, the earthquakes in southern area is more scattered. We calculated source parameters such as seismic moment and stress drop based on 208 earthquakes above ML3.0 in Changning-Xingwen area, Sichuan from 17 June 2019 to 31 December 2020, and analyzed the evolution characteristics of seismic stress drop in the region before and after the Changning MS6.0 earthquake on June 17, 2019. The results show that the stress drop of ML3.0−3.9 earthquakes significantly increased in Changning-Xingwen area before the moderate strong aftershocks, and most of the earthquakes above ML4.0 occurred in the high stress drop area of ML3.0−3.9 earthquakes in the study area.
He X J，Zhang X D，Meng L Y，Wang Y. 2023. Spatio-temporal relationship between source parameters such as stress drop and significant earthquakes in Changning-Xingwen area. Acta Seismologica Sinica，45（2）：271−284. doi: 10.11939/jass.20210196.
Abstract: The geoelectrical resistivity observation is one of the most important methods in the study of earthquake precursor, and the characteristics of the stray current during subway operation in the geoelectrical resistivity observation are useful for analyzing the geoelectric observation data and the anomaly variation before earthquakes. Based on the study on the generation mechanism, the quantitatively results of the influence of stray current on geoelectrical resistivity observation were given in this paper. The result shows that the effect distance can reach dozens or even a hundred kilometers. Through monitoring the stray current in some geoelectrical resistivity observation stations around cities, such as Tongzhou station in Beijing, Qingguang, Baodi and Tanggu stations in Tianjin, Jiangning station in Jiangsu and Xinchengzi station in Liaoning, the amplitude and frequency range of the stray current signal as well as its spatial distribution characteristics were analyzed. It shows that the amplitude of the signal ranges from several to tens millivolt, and the period range is mainly from 50 s to 200 s. Due to the influence of stray current, the signal-to-noise ratio is decreased by 10 to 30 dB, and the accuracy of the observation data is more than 20 times worse than the specified requirement. According to the characteristics of the stray current propagation, several methods which can be used to suppress the signal were proposed in this paper, and it will provide a foundation for the identification of anomaly information from the observation data and anti-interference technology study.
Wang L W，Zhang Y，Zhang X G，Hu Z. 2023. Characteristics of subway stray current in geoelectrical resistivity observation. Acta Seismologica Sinica，45（2）：285−301. doi: 10.11939/jass.20210197.
Abstract: This study adopts the geomagnetic measurements of the Chinese mainland in 1960.0, 1970.0, 1980.0, 1990.0, 2000.0, and the Taylor polynomial model to build up a medium-scale crustal magnetic anomaly model with low spatial resolution, associated with the fourth-generation geomagnetic comprehensive model （CM4）. The figures of the anomaly field are drawn. The whole trends of the field are studied by putting all measurements into 2000.0. The differences in anomalies among different epochs have also been analyzed by the newest three-dimensional surface spline （3DSS） model derived from all available ground and aeromagnetic data. Results show that the intensity of the northward component X decreases with time, and its distribution shrinks to the southwest; the positive area of eastward component Y gradually moves to the west, and the negative area gradually occupies most of Chinese mainland; the distribution of vertical component Z is stable and is mainly negative in the study area, which implies that other factors do not likely influence the radial component. For the overall distribution of magnetic anomalies, components X, Z, total intensity F and horizontal component H are almost negative, which differs from CM4. Regarding the data consistency, the distribution of F has some similar features to the 3DSS model, which can illustrate more middle and small wavelength structures of the anomaly field. Large differences among the three models mainly come from the different measurement numbers, locations, and modeling theories.
Yang Y J，Feng Y. 2023. Differences of geomagnetic anomalous field over Chinese mainland and its adjacent areas during 1960−2000 based on Taylor polynomial model. Acta Seismologica Sinica，45（2）：302−314. doi: 10.11939/jass.20220042.
Abstract: The magnetic field data of Swarm satellite are influenced by geomagnetic activity and local time. In this paper, the influence of geomagnetic activity was firstly removed by variational mode decomposition, and then we built a time-varying background field to eliminate the influence of local time on the data. Based on the established time-varying background field, we used the Swarm satellite magnetic field data to conduct a pre-earthquake anomaly analysis of the 2020 Jamaica earthquake. We calculated the amount of energy of the magnetic data inside the earthquake affected area, then compared with the threshold value of the time-varying background field. If the difference between the amount of energy of a track inside the earthquake affected area and the corresponding background value is over the threshold value, the track was considered to be pre-earthquake anomalies. The results show that the cumulative number of anomalous tracks in function of the time shows accelerated growth from 50 days to 43 days before the earthquake. In addition, we also built day-side and night-side background fields to compare with the time-varying background field. By analyzing the differences between the anomalous orbits extracted based on the two methods, we find that the high-value background fields will be pulled down by the low-value background fields, which leads to some nonanomalous orbits being wrongly identified as anomalous orbits. For the same reason, the low-value background fields will be pulled up by the high-value background fields, which leads to some abnormal orbits cannot be recognized. It is because that the day-side and night-side background fields were built by the data of multiple local times magnetic field data, which cannot identify the great variations of magnetic field at different local time. However, the time resolution of time-varying background field is high, so it can highlight the great variations of magnetic field at different local time. As a result, the establishment of time-varying background field is crucial for the accurate detection of anomalous orbits. Further, we analyzed the cumulative number of anomalies in lithosphere, atmosphere and ionosphere, and explained the time correlation of these anomalies, which proved that these anomalies may be related to the seismogeny of the Jamaica earthquake.
Zhu K G，He X D，Fan M X，Marchetti D，Wang T，Wen J M，Zhang Y Q. 2023. Time-varying background field and anomaly analysis of Swarm satellite magnetic field data before the 2020 Jamaica MW7.7 earthquake. Acta Seismologica Sinica，45（2）：315−327. doi: 10.11939/jass.20210172.
Abstract: In response to the shortcomings of the two-step difference method for extracting seismic microwave anomalies proposed in 2011, an improved two-step difference method which incorporates hierarchical clustering and wavelet analysis is proposed to overcome the problems resulted from ignoring the brightness temperature differences caused by ground cover types and the lack of robustness of the brightness temperature background field in the original method, thus making the anomaly results more reasonable in terms of spatial pattern and anomaly amplitude. The method is applied to the microwave anomaly extraction of the MS7.4 earthquake in Maduo County, Qinghai Province on May 22, 2021. The results show that a NE-trending brightness temperature increase strip with a length of about 900 km and an amplitude of 12 K appeared three months before the Maduo earthquake, which has high similarity in spatial distribution and evolution characteristics with the anomaly characteristics of the MS7.1 Yushu earthquake on April 14, 2010. Furthermore, the appearing time and magnitude of the anomalies are related to the magnitude of the earthquakes. The anomalies of MS7.4 Maduo earthquake appeared earlier than MS7.1 Yushu earthquake with larger scale, and the temperatureincrease of microwave brightness due to both earthquakes reached more than 8 K. Preliminary mechanism analysis suggests that the pre-earthquake microwave anomaly stripes are consistent with the spatial distribution of NEward tensional fractures and extensional fracture zones in the region, and the appearance of the anomalies may be related to the geothermal activity of the fracture zones and the emissivity changes of surface rocks before the earthquakes.
Liu S J，Ji M Y，Song L M，Wei L H. 2023. Microwave anomaly of Maduo MS7.4 earthquake derived by improved two-step difference method. Acta Seismologica Sinica，45（2）：328−340. doi: 10.11939/jass.20210193.
Abstract: In this paper, the potential focal area division scheme of the Fifth Generation Seismic Parameter Zoning Map of China combined with the Newmark displacement model is adopted. And according to the engineering geological lithology and topographic elevation data of Longxian, the amplification effect of topography on ground motion and the uncertainty of parameters of Newmark model are also considered. Above all, the instability probability of landslide in Longxian county, Shaanxi Province is given on the condition that the earthquake incidence rate is 10% in 50 years. According to the results, potential earthquake-induced landslide in the study area can be divided into four grades: the very low risk area, the low risk areas, the medium risk areas and the high risk areas. And the high risk areas are mainly concentrated in the mudstone, siltstone and loess covered areas with a slope of more than 40° in Longxian. Among them, Qianhe river and some areas on both sides of Tongguanhe river have higher seismic landslide risk. The results can provide a reference for seismic landslide risk management and land planning in this area.
Fu G C，Pan H，Chen J，Zheng L F. 2023. Probability hazard analysis of potential earthquake-induced landslide：A case study of Longxian County， Shaanxi Province. Acta Seismologica Sinica，45（2）：341−355. doi: 10.11939/jass.20210147.
Abstract: The traditional ground-motion models （GMMs） do not account for pulse effects and may therefore fail to estimate seismic hazards and risk at near-fault sites, where pulse-like ground motions are expected. Thus, the applicability of the NGA-West2 GMMs to the near-fault velocity pulse-like ground motions need to be tested. The near-fault strong ground motions are quantitatively identified from recent earthquakes since 2013 by using wavelet method and taking the uncertainty of pulse orientation into consideration so as to form a new pulse database. Based on the new pulse database, long-period pulses are extracted from the original pulse records by using wavelet method. Based on a quantitative analysis of the epsilon parameter, we quantitatively test the applicability of the NGA-West2 ground-motion model to the near-fault velocity pulse-like ground motions. The results show that the four NGA-West2 models are more suitable for describing the residual recordings in the studied period, but underestimate the original pulse-like ground motions especially around the pulse period. We noted that, among the four NGA-West2 models, the applicability of the CB2018 to the residual ground motions is the best. This study provides an excellent opportunity to quantitatively evaluate the NGA-West2 GMMs and to update these models in the near future, and also provides a basis for incorporating pulse effects into near-fault probabilistic seismic hazard analysis and seismic design.
Zhao X F，Wen Z P，Xie J J，Xie Q C. 2023. Applicability of the Next Generation Attenuation-West2 ground-motion models to the components of near-fault velocity pulse-like ground motions. Acta Seismologica Sinica，45（2）：356−372. doi: 10.11939/jass.20210176.
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