Electromagnetic anomalous variations of the geomagnetic field, lightning activity, ionospheric F2-peak plasma frequency, GPS total electron content (TEC), etc. have been observed around the epicenter few days before the 21 September (local time) 1999 M7.7 Chi-Chi earthquake. The TEC over the epicenter anomalously and significantly decreases in the afternoon period on day 1, 3, and 4 before the Chi-Chi earthquake, which generally agrees with TEC decrease anomalies day 1-5 and day 10-15 appearing prior to M≥5.0 earthquakes in Taiwan during the 6-year period of 1994/1/1-1999/9/20.  Temporal and spatial analyses of the global ionospheric map (GIM) shows that TEC anomalously and significantly decrease specifically over the epicenter day 3-4 and day 10-15 before the Chi-Chi earthquake.  To find possible physical mechanisms causing the TEC decrease anomalies before the Chi-Chi earthquake, the equatorial ionization anomaly of TEC along the Taiwan longitude during September 1999 and plasma quantities of the ion density, ion temperature, and ion velocity measured by DMSP (Defense Meteorological Satellite Program) satellites are examined. It is found that westward seismo-electric fields around the epicenter area day 3-4 and day 10-15 before the Chi-Chi earthquake are essential.

How to cite: Liu, J.-Y., Wen, Y.-C., Chang, F.-Y., Lin, C.-Y., and Chen, Y.-I.: 25th anniversary study: the anomalous electromagnetic signals appear before the 21 September 1999 M7.7 Chi-Chi earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4932, https://doi.org/10.5194/egusphere-egu25-4932, 2025.

Earthquake-triggered geomagnetic variations typically exhibit time delays, with disturbances detected first at near-source stations and later at more distant locations. On April 2, 2024, a destructive M 7.4 earthquake struck the eastern Taiwan region, China, triggering an intriguing set of geomagnetic disturbances. Leveraging a high-density, three-component geomagnetic observation network with a 1 Hz sampling rate, we analyzed data from eight stations ranging from 84 to 320 km from the epicenter. Our findings reveal a striking pattern of simultaneous geomagnetic disturbances in both the X and Z components at stations within 114 km of the epicenter, with no similar disturbances observed further away. These disturbances persisted for 350 to 413 seconds, with peak amplitudes of 0.45 nT and 0.3 nT in the X and Z components, respectively. Notably, the Z-component disturbances exhibited opposite phases across the affected stations, suggesting the presence of electric currents near the epicenter. The time delay between the earthquake and the geomagnetic disturbances aligns with the expected propagation of acoustic waves from the earthquake's epicenter to the ionosphere. Using the Biot-Savart law, we estimated the location and intensity of the electric currents responsible for these disturbances. Our calculations place the currents approximately 17 km north of the epicenter, at an altitude of ~80 km, with an intensity of ~120 A and an azimuth of 301.5°. Furthermore, high-frequency Doppler sounders confirmed that acoustic waves propagated to the lower ionosphere, generating electric currents that led to the observed simultaneous geomagnetic disturbances, and continued upward to perturb the higher ionosphere. This study reveals a novel class of earthquake-triggered geomagnetic variations, offering new insights into the interaction between seismic events and the ionosphere.

How to cite: Mao, Z.: Novel Geomagnetic Disturbances Triggered by the M 7.4 Earthquake in Taiwan region: Evidence of Electric Currents, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4615, https://doi.org/10.5194/egusphere-egu25-4615, 2025.

    Earthquake-induced geohydrological changes have been monitored and investigated in the last fifty years. However, most of the previous studies focused on the effects of a single earthquake event on different observations or multiple-independent events on many different data sources which arising uncertainties from different mechanisms and site effects. The quantitative analysis of earthquake-induced geohydrological changes and their effects on soil liquefaction remains a challenge. In order to complete the soil liquefaction potential map in Taiwan and improve the accuracy of the analysis and evaluation, the Central Geological Survey of the Ministry of Economic Affairs conducts a six-year plan from 2018 to 2023. The geological and geohydrological data thoroughly collected by previous projects serve as a solid foundation for this 4-year project to systematically probe the coseismic geohydrological changes and their effects on soil liquefaction.
     In the four year of this project, four primary tasks had been finished including (1) database establishment of the long-term groundwater level observations in the study areas, (2) three-dimensional hydrogeological structures construction of the study areas,, (3) case study of induced groundwater level changes and liquification in three catastrophic earthquakes (4) methodology development and establishment of the coseismic geohydrological changes and their effects on soil liquefaction potential. 
     The establishment and management of the long-term groundwater level observations in the study areas were done in this year. The statistical program was merged into processing procedures for data analysis. Also, external observational data produced by the Water Resources Agency and Central Weather Bureau was integrated. The three-dimensional hydrogeological models were established based on the models constructed in T-PROGS (Transition Probability Geostatistical Software). With limited drilling data, the three-dimensional hydrogeological models could be applied to estimate and build an underground database for those areas with no data. In addition, the geological zoning after geological research and judgment could serve as a reasonable geological basis for subsequent interpolation of soil liquefaction.
    The hourly and secondly data of groundwater level variations and the hourly river discharge variations triggered by the 1999 Chi-Chi earthquake, 2016 Meinong earthquake and 2018 Hulien earthquake are checked and analyzed to investigate the responses under different hydrogeological conditions in west and south regions of epicenter. The increased groundwater levels are shown to consistent with the horizontal peak ground velocity (PGV), which imply that the increased groundwater levels might result from the buildup pore water pressure induced by shear strain, like the liquefaction mechanism.
    Uncertainties associated with groundwater level measurement or incorrect representation of regional groundwater level could easily lead to erroneous assessments of liquefaction potential in regional areas. These uncertainties result from spatial and temporal groundwater level variability and/or measurement error. Natural variability also makes it difficult to correctly identify the groundwater depth. The groundwater level fluctuates in response to recharge and discharge. In this project, an alternative methodology was adopted in order to overcome these inherent uncertainties.

How to cite: Lai, W.-C., Wang, S.-J., and Lin, Y.-Y.: The study of coseismic geohydrological changes and its effects on soil liquefaction potential in Taiwan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3450, https://doi.org/10.5194/egusphere-egu25-3450, 2025.

In this research, we explore the EM response generated by earthquake fault-slip attributable to the piezomagnetic effect. To achieve this, we integrate the elastodynamic equations with Maxwell's equations. We put forward a semi-analytical method for simulating seismo-electromagnetic fields within a horizontally-stratified model. In this model, the coupled equations are resolved in the frequency-wavenumber domain. Subsequently, the seismo-electromagnetic responses in the time-space domain are obtained through the Hankel transform and the inverse Fourier transform. We carry out numerical simulations to examine the characteristics of the EM signals triggered by a fault - slip source. The results indicate that the piezomagnetic effect can generate both magnetic and electric fields. For an Mw 6.0 earthquake, at a receiver 85 km from the epicenter, the coseismic electric field can reach approximately ~0.1 μV/m, and the coseismic magnetic field can reach about 0.1 nT. This demonstrates that the EM fields resulting from the piezomagnetic effect are detectable by current EM equipment. Furthermore, we apply this method to simulate the observed coseismic EM data from an actual earthquake. The predicted magnetic fields show a high degree of consistency with the data, validating the effectiveness of our method.

How to cite: Gao, Y. and Zhao, J.: Modeling of electromagnetic fields generated by an earthquake due to piezomagnetic effect, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6273, https://doi.org/10.5194/egusphere-egu25-6273, 2025.

Over the past 10 years, increasingly intensive studies of ionospheric plasma structures have been carried out using data from the Low-Frequency Array for radio astronomy (LOFAR) radio telescope system. Recently more and more attempts are taken to combine LOFAR ionospheric studies with other monitoring techniques such as GNSS observations. LOFAR detects scattering of high-frequency (HF) (MHz) electromagnetic waves (EMW) on the above-mentioned plasma structures. Astrophysical sources of such EMWs may be, for example, radio galaxies, supernovae remnants and pulsars. The plasma structures under investigations are excited due to impacts on the ionosphere “from below”, namely powerful natural hazards, including typhoons, volcanoes and earthquakes, and “from above”, in particular strong magnetic storms, accompanied by corresponding disturbances of auroral currents and due to flows of charged particles; as well as because of solar flares, eclipses and the terminator, as well as various plasma instabilities and nonlinearities, etc. Now we are focusing on identifying quasi-periodic and quasi-wave ionospheric disturbances in the low frequency range, including traveling ionospheric disturbances (TIDs). Results will be presented (1) based on developed models of linear and nonlinear TIDs in the presence of corresponding linear and nonlinear atmospheric gravity waves (AGWs); (2) appropriate modulation of the ionospheric plasma; (3) examples of scattering of high-frequency (HF) (MHz) electromagnetic waves (EMWs) on plasma structures, including the characteristics of the Doppler shift on moving plasma structures, taking into account birefringence in ionospheric plasma, as well as results regarding the qualitative influence on the scattering characteristics of EMWs such factors as height and horizontal dimensions of the ionospheric plasma scatterer. Algorithms are being developed to take into account the influence of plasma resonances and photochemical interactions on the characteristics of emerging ultra-low frequency (ULF) plasma structures. A database has been created that includes dynamic spectra of plasma disturbances with reference to the times of sunrise and sunset over a number of months in 2024. Observations of plasma structures were carried out at various LOFAR stations. Slant TEC maps are being developed on the basis of GNSS data. Spectral processing (using in particular Fast Fourier Transform, wavelets and transformation from dynamic spectra to Doppler shifter spectra) of the LOFAR and GNSS data is carried out with the aim of identifying quasi-periodic and quasi-wave ultra-low frequency (ULF) plasma structures with subsequent comparison “Theory-Experiment”.

How to cite: Rapoport, Y., Krankowski, A., Błaszkiewicz, L., Grimalsky, V., Fron, A., Kotulak, K., Flisek, P., Petrishchevskii, S., and Grytsai, A.: Modeling and experimental investigations for Modern Radio-Diagnostics of the impacts on ionosphere “from above and from below” using LOFAR and GNSS Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5623, https://doi.org/10.5194/egusphere-egu25-5623, 2025.

How to cite: Zhang, J., Hu, H., and Gao, Y.: Global Analytical Simulation of Acoustic-Gravity Wave Propagation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7929, https://doi.org/10.5194/egusphere-egu25-7929, 2025.

In the measurement of water current velocities, sound waves are commonly used to detect the Doppler effect caused by small scattering particles. The widely known Acoustic Doppler Current Profilers (ADCPs) operate based on this principle (Gould et al., 2001). Although ADCPs are widely used for measuring water current velocity, single-point current meters are considered a better choice for precise and continuous measurements of near-seafloor water current velocity.

In this study, a single-point current meter (Aquadopp-6000m Current Meter) was mounted on the Yardbird-BB OBS (Lin et al., 2024) to form a Seafloor Current Meter (SCM). The SCM was used to measure and record water current disturbances above the OBS seismic sensor. The data collected by the SCM can be used not only for analyzing the overall OBS orientation, the time of contact with the seafloor, the moment the seismic sensor detached from the A-frame and settled onto the seafloor, and the sound speed, temperature, and pressure profiles during the instrument's descent, but also for broader applications.

By increasing the sampling rate to 1 sps, the continuous observation data can be analyzed alongside OBS data to study the relationship between background seismic noise and seafloor current velocity, as well as changes in seafloor current velocity before and after seismic events. Moreover, atmospheric pressure changes—occurring even thousands of kilometers away before a typhoon forms—can affect seawater pressure, seafloor current velocity, and subtle variations in seafloor temperature, which in turn influence ocean sound speed.

This study analyzes and discusses SCM data collected in the northeastern offshore waters of Taiwan, at the western end of the Okinawa Trough..

Reference:

Gould, J., B. Sloyan, and M. Visbeck. (2013). In Situ ocean observations: a brief history, present status and future directions. In, G. Siedler, S. Griffies, J. Gould, and J. Church. (eds.) Ocean Circulation and Climate: A 21st Century Perspective. 2nd Ed. (HASH(0xa0a5e98), Oxford, GB. Academic Press, pp. 59-82. https://eprints.soton.ac.uk/358924/

Lin, C.R., Y.C. Liao, C.C. Wang, B.Y. Kuo, H.H. Chen, J.P. Jang, P.C. Chen, H.K. Chang, F.S. Lin and K.H. Chang. (2024). Development and evaluations of the broadband ocean bottom seismometer (Yardbird-BB OBS) in Taiwan. Terr Atmos Ocean Sci. 35, 4. Doi: https://doi.org/10.1007/s44195-024-00062-w

How to cite: Lin, C.-R.: Data Collection and Applications of Seafloor Water current Velocity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2355, https://doi.org/10.5194/egusphere-egu25-2355, 2025.

Seismic velocity is particularly sensitive to clay degradation, measuring relative seismic velocity (dv/v) with seismometers has emerged as a powerful geophysical technique for monitoring ground surface activities. Previous studies have considered the seismic velocity variations over large-scale area under seasonal weather while neglecting the impact of short-term rainfall on clay landslides. We aim to elucidate the quantitative variations of dv/v on shallow clayed slope measured by short-distance sites under rainfall influence. In this study, we deployed five seismometers and one rain gauge on the Juemo Village landslide (Sichuan Province, China) from June 21, 2022, to September 12, 2022. The cross-correlation method was employed to calculate the variations in the travel time of the same seismic phase between two seismic sites. The results show a clear drop in velocity under rainfall conditions, with a relative variation of approximately 15%. The observations could enhance the recognize of slope failure related to rainfall and provide a possible indicator of landslide early warning.

How to cite: Liu, Y. and wang, F.: The Variation Characteristics of Delay Times of Seismic Signals in Shallow Slopes Under Rainfall Conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5426, https://doi.org/10.5194/egusphere-egu25-5426, 2025.

Disturbances occurring near the Earth's surface, such as earthquakes, tsunamis, volcanic eruptions, typhoons, and hurricanes, can trigger severe perturbations or fluctuations in the ionosphere. These perturbations or fluctuations typically have periods of tens of minutes and propagate outward from their origin at speeds of several hundred to thousands of meters per second. Most previous studies have utilized Total Electron Content (TEC) observations from dense ground-based Global Navigation Satellite System (GNSS) receiver networks to monitor the horizontal propagation of these perturbations or fluctuations. On the other hand, these disturbances or fluctuations also propagate vertically upward, penetrating the atmosphere and disturbing the ionosphere. Several studies have employed multiple ground-based instruments, such as seismometers, infrasound systems, magnetometers, high-frequency Doppler sounding systems, ground-based GNSS receivers, and ionosondes, to detect and investigate the vertical propagation of these perturbations. These studies have demonstrated the importance and complexity of disturbances originating from the lithosphere. Due to the scarcity of instruments primarily installed on land, observing and studying vertical disturbances remain challenging. Therefore, radio occultation (RO) techniques on Low Earth Orbit (LEO) satellites, which globally detect atmospheric and ionospheric structures, provide valuable insights for monitoring and studying phenomena in regions lacking ground-based instruments, such as oceans, deserts, and polar areas. This presentation introduces the use of RO techniques from the FORMOSAT-3/COSMIC (F3/C) and FORMOSAT-7/COSMIC-2 (F7/C2) missions to monitor ionospheric vertical disturbances caused by events such as the magnitude 9.0 Tohoku earthquake on 11 March 2011, the magnitude 7.8 Nepal earthquake on 25 April 2015, and the underwater volcanic eruption near Tonga on 15 January 2020, etc. The results show that intense disturbances originating from the lithosphere should be regarded as significant drivers that alter ionospheric structures and dynamics. Studying vertical disturbances benefits us understand the propagation of fluctuations and dynamic changes across various geospheres.

How to cite: Sun, Y.-Y.: Ionospheric Vertical Disturbances Induced by Lithospheric Activities: Insights from Radio Occultation Technique, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6483, https://doi.org/10.5194/egusphere-egu25-6483, 2025.

On September 16, 2021, an Ms 6.0 earthquake occurred in Luxian, Luzhou City, Sichuan Province, China, following the Ms 6.0 Changning earthquake on June 17, 2019, another significant seismic event in the Sichuan Basin. The epicenter of the Luxian earthquake was situated within the NE-trending Huaying Mountain Folded Fault Zone, and the maximum seismic intensity reached Level VII, resulting in 3 fatalities and 159 injuries. Some studies propose that this earthquake may be associated with the development of nearby shale gas extraction platforms, exhibiting characteristics of both induced and natural seismicity, thereby holding considerable research significance. This study identifies multiple pre-seismic anomalous signals preceding the Luxian Ms 6.0 earthquake, including total electron content (TEC), geomagnetic variations, velocity structure changes, b-value fluctuations, and fault stress anomalies. Employing machine learning techniques, Log-Periodic Power Law (LPPL) models, and other analytical methods, we conduct a comprehensive assessment of the indicative capacity and predictive efficacy of these pre-seismic anomalies. The findings reveal that certain anomalous signals exhibit predictive relevance concerning the key parameters of the mainshock. Furthermore, the seismic mechanisms of medium- to strong-magnitude induced earthquakes appear to be similar to those of natural earthquakes, potentially demonstrating more pronounced and sustained pre-seismic anomalies. The coupled analysis of these signals offers valuable insights for improving the prediction of future high-magnitude seismic events.

How to cite: Wang, X. and Hu, J.: Coupling Diverse Pre-Seismic Anomaly Features for Inferring Critical Information on the Ms6.0 Earthquake in Luxian, Sichuan, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14339, https://doi.org/10.5194/egusphere-egu25-14339, 2025.

Based on the two Sheveluch eruption events on April 10, 2023 and August 17, 2024, the comprehensive phenomenon of the two volcanic eruption events is described from the analysis of the seismic activity sequence of the lithosphere to the distribution of atmospheric materials and heat. The seismic distribution in Sheveluch volcanic area is mainly shallow-source (0-70 km) and small-earthquake (ML 3.5-4.0). In term of the horizontal evolution of SO2, the SO₂ eruption amount and the duration on April 10, 2023 is larger and longer than on August 17, 2024. In the time series, SO₂ and UV aerosol index obviously respond to the volcano eruption activity. In the vertical dimention, the vertical wind field data show that the SO₂ eruption height is about 100~125hpa, and the boundary layer height between the troposphere and the stratosphere in the Sheveluch volcano region is about 50hpa. The noticeable variation of the temperature profile of the two volcanic eruptions was below 10hpa. Comparing the ozone profile with the temperature profile, the ozone depletion may result in a decrease in stratospheric temperature. The horizontal and vertical migration processes of atmospheric materials during volcanic eruption are described, which is of great significance for the study of multi-layer coupling mechanism.

How to cite: Liu, Q., Gui, L., Ma, X., Xu, J., and Shen, X.: Atmospheric physicochemical multi-parameter horizonal and vertical mitigation response of two recent Sheveluch volcano eruptions in Kamchatka, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7648, https://doi.org/10.5194/egusphere-egu25-7648, 2025.

A FORMOSAT-5 (FS-5) satellite was launched on 25 August 2017 CST into a 98.28° inclination sun-synchronous circular orbit at 720 km altitude along the 1030/2230 local time sectors.  Advanced Ionospheric Probe (AIP), a piggyback science payload developed by National Central University for the FORMOSAT-5 satellite, has measured in-situ ionospheric plasma concentrations at a 1,024 Hz sampling rate over a wide range of spatial scales for more than 7 years.  Dramatical ionospheric plasma density and velocity modulation caused by natural hazards like 2022 Hunga Tonga–Hunga Haʻapai eruption and 2024 Mother Day geomagnetic storm had been observed clearly by FS-5/AIP and will be presented in the talk.

How to cite: Chao, C.-K. and Huang, W.-R.: Ionospheric Plasma Response to Natural Hazards Observed by Advanced Ionospheric Probe Onboard FORMOSAT-5 Satellite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3188, https://doi.org/10.5194/egusphere-egu25-3188, 2025.

An M7.4 magnitude earthquake struck Hualien, Taiwan, on April 3, 2024, at 07:58:12 local time. We collected data from ground-based Global Navigation Satellite System (GNSS) receivers near the epicenter and retrieved coseismic displacements using precise point positioning (PPP) and ionospheric total electron content (TEC) from dual-frequency phase observations received from geostationary Earth orbit (GEO) satellites. The TEC data reveal not only coseismic ionospheric disturbances occurring ~10 minutes after the earthquake, but also disturbances occurring ~1 minute post-event, which are time-synchronized with the coseismic displacements. This type of TEC disturbance is consistently observed across three stations. We projected the displacement in two directions: the line-of-sight (LOS) between the satellite and receiver, and the normal plane to the LOS. The trends in TEC are consistent with the displacements projected onto the LOS normal plane, with a magnitude relationship of approximately 0.05 TECU increase in TEC per 10 cm increase in displacement. In contrast, displacements projected along the LOS direction do not show the same trends in TEC. Therefore, we confirm that LOS movement caused by seismic shaking of GNSS receivers affects the calculation of GNSS-based GEO-TEC.

How to cite: Rao, H.: Seismic shaking recorded simultaneously on TEC and PPP: a case study of the Hualian earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4721, https://doi.org/10.5194/egusphere-egu25-4721, 2025.

In this study, we analyzed ionosonde observations in Eastern Asia to investigate the responses of the sporadic E (Es) layer to severe atmospheric disturbances caused by the Tonga volcanic eruptions on 15 January 2022. The most notable feature observed was the disappearance of the Es layer after approximately 10:00 UT, attributed to the vertical drift induced by the eruptions. The Es layer reappeared intermittently after 13:00 UT, coinciding with the arrival of the tropospheric Lamb wave. To understand the mechanism behind this intermittence, we also analyzed horizontal wind data in the mesosphere and lower thermosphere regions, recorded by meteor radars. Wind disturbances with periods of approximately 20 hours contributed to the nighttime formation of the Es layer on January 15.

How to cite: Wang, J. and Sun, Y.-Y.: Sporadic E responds to the 2022 Tonga volcano eruptions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20923, https://doi.org/10.5194/egusphere-egu25-20923, 2025.