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Zhang, Yiqun
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Zhang, Yiqun
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- PublicationOpen AccessAnalysis of Swarm Satellite Magnetic Field Data for the 2015 Mw 7.8 Nepal Earthquake Based on Nonnegative Tensor Decomposition(2022-08-04)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; A nonnegative tensor decomposition (NTD) approach has been developed to analyze the ionospheric magnetic field data of the Swarm Alpha and Charlie satellites for the Mw7.8 2015 Nepal earthquake. All available satellite data were analyzed regardless of geomagnetic activity. We used the amplitude time–frequency spectra of the two-satellite data to build third-order tensors and decomposed them into three components. One of these components seems to be more affected by seismicity. In particular, the cumulative number of anomalous tracks of this component displays accelerated growth that conforms to a sigmoid fit from 60 to 40 days before the mainshock. Subsequently, until ten days before the earthquake, it shows a weak accelerating trend that obeys a power-law behavior and then resumes linear growth after the mainshock. Moreover, the cumulative anomaly was indicated not to be caused by geomagnetic activity, solar activity, or other nonseismic factors. An investigation of the foreshocks around the epicenter reveals that the cumulative Benioff strain also exhibited two accelerated growths before the mainshock, which is consistent with the cumulative result of ionospheric anomalies. In the first acceleration stage, seismicity appeared in the region surrounding the epicenter, and most of the ionospheric anomalies were offset away from the epicenter. During the second acceleration stage, some foreshocks occurred closer to or on the mainshock fault, and ionospheric anomalies also appeared near two faults around the epicenter. Furthermore, the correspondence between the ionospheric anomalies and the anomalies in different geolayers can be explained by the lithosphere–atmosphere–ionosphere coupling model.136 89 - PublicationOpen AccessCSES-01 Electron Density Background Characterisation and Preliminary Investigation of Possible Ne Increase before Global Seismicity(2023)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ; ; ; ; ; ;In this paper, we provide a characterisation of the ionosphere from April 2018 to September 2022 for 48 investigated months. We used the data of the China Seismo Electromagnetic Satellite (CSES-01), which is a sun-synchronous satellite with five days of revisit time and fixed local time of about 2 a.m. and 2 p.m. The unique orbit of CSES-01 permitted us to produce a monthly background of the ionosphere for night- and daytime with median values acquired during geomagnetic quiet time in equatorial and mid-latitude regions (i.e., between 50° S and 50° N of geographical latitude). We compared the obtained CSES-01 monthly median values with the solar activity in terms of sunspot numbers, and we found a high correlation of 0.89 for nighttime and 0.85 for daytime between the mean sunspot number and the maximum of the characterised CSES-01 Ne map values. In addition, we extracted all the anomalous positive increases in CSES-01 electron density and compared them with the Worldwide M5.5+ shallow earthquakes. We tested two different definitions of anomaly based on median and interquartile range or (mild) outliers. We tried two relationships between anomalies inside Dobrovolsky’s area before the earthquake and the magnitude of the same seismic events: one which considers distance in space and time and a second which only uses the anticipation time of the anomaly before the earthquake. Using both anomaly definitions, we searched the best coefficients for these two laws for mid-latitude and equational regions. We found that the best coefficients are independent of the anomaly definition, but better accuracy (greater than 80%) is obtained for the outlier definition. Finally, using receiving operating characteristic (ROC) curves, we show that CSES-01 increases seem statistically correlated to the incoming seismic activity.51 14 - PublicationOpen AccessExploration of the 2021 Mw 7.3 Maduo Earthquake by Fusing the Electron Density and Magnetic Field Data of Swarm Satellites(2024)
; ; ; ; ; ; ; ; ; ; ;; ;; ; ; ;Earthquake is a complex and multivariate problem. Using a single parameter to extract anomalies is difficult to completely and truly reflect the preparation activity before earthquakes. In this paper, we develop a fusion anomaly extraction method based on principal component analysis (PCA) and non-negative tensor decomposition (NTD). It extracts anomalies by combining features of different parameters, which can obtain earthquake-related signals from dataset and reveal some weak anomalies hidden in individual parameters. By PCA-NTD, we fused the electron density and magnetic field data from the Swarm satellites to explore the possible precursors of 2021 M7.3 Maduo earthquake and compared the results with those of the single-parameter analyses. The cumulative value of fusion anomalies indicates two acceleration stages before the mainshock: from -51 to -24 days, following a sigmoid trend, and from -21 to the earthquake occurrence, following a power-law behavior. The second acceleration is more pronounced than the first one, and its critical point occurs near the date of the Maduo earthquake. Spatially, these anomalies are located around important fault zones (Altun fault, Jiali fault, and Red River fault) and the epicenter region, which likely reflect the northward stress and eastward stress experienced in the seismogenic area before the mainshock. As the earthquake approaches, the anomalies become more concentrated and closer to the impending epicenter. Furthermore, the ionospheric anomalies correspond well with the anomalous phenomena of lithospheric activity and atmospheric thermal radiation, which support a multi-channel lithosphere-atmosphere-ionosphere coupling (LAIC).53 16 - PublicationOpen AccessWorldwide Statistical Correlation of Eight Years of Swarm Satellite Data with M5.5+ Earthquakes: New Hints about the Preseismic Phenomena from Space(2022)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ;; ; ; ;; ; ; ; ; ; ; ; ; ; ; ;Nowadays, the possibility that medium-large earthquakes could produce some electromagnetic ionospheric disturbances during their preparatory phase is controversial in the scientific community. Some previous works using satellite data from DEMETER, Swarm and, recently, CSES provided several pieces of evidence supporting the existence of such precursory phenomena in terms of single case studies and statical analyses. In this work, we applied a Worldwide Statistical Correlation approach to M5.5+ shallow earthquakes using the first 8 years of Swarm (i.e., from November 2013 to November 2021) magnetic field and electron density signals in order to improve the significance of previous statistical studies and provide some new results on how earthquake features could influence ionospheric electromagnetic disturbances. We implemented new methodologies based on the hypothesis that the anticipation time of anomalies of larger earthquakes is usually longer than that of anomalies of smaller magnitude. We also considered the signal’s frequency to introduce a new identification criterion for the anomalies. We find that taking into account the frequency can improve the statistical significance (up to 25% for magnetic data and up to 100% for electron density). Furthermore, we noted that the frequency of the Swarm magnetic field signal of possible precursor anomalies seems to slightly increase as the earthquake is approaching. Finally, we checked a possible relationship between the frequency of the detected anomalies and earthquake features. The earthquake focal mechanism seems to have a low or null influence on the frequency of the detected anomalies, while the epicenter location appears to play an important role. In fact, land earthquakes are more likely to be preceded by slower (lower frequency) magnetic field signals, whereas sea seismic events show a higher probability of being preceded by faster (higher frequency) magnetic field signals.184 60