Ultra-low-frequency Emissions Research Articles

Investigating ultra-low frequency emissions and total electron content anomalies as earthquake precursors in Sumatra (2019–2020)

Ultra-low frequency (ULF) emissions and total electron content (TEC) anomalies are potential earthquake precursors for short-term earthquake prediction. This study comprehensively examines ULF emissions and TEC anomalies associated with the Sumatra earthquakes spanning 2019–2020. Precursor ULF emissions are identified through the polarization power ratio calculations involving Z-component and H-component geomagnetic data, meticulously recorded by the Magnetik Acquisition Data System (MAGDAS). Employing the Single Station Function Transfer technique, ULF anomalies directed toward the epicenter of earthquakes are recognized as reliable precursors. Among the 29 earthquakes scrutinized, 25 were heralded by ULF emissions acting as precursors. Among these, 15 instances solely exhibited ULF emissions, unaccompanied by TEC anomalies, while the remaining ten were linked to TEC anomalies. The investigation of TEC anomalies employs advanced autocorrelation techniques rooted in the analysis of correlation coefficient deviations. TEC anomalies are meticulously acquired from the observations of the Sumatran GPS Array (SuGAr) stations. Furthermore, this study illuminates a positive correlation (R = 0.65) between the lead time of ULF emissions and TEC anomalies. Moreover, the frequency of ULF emissions and TEC anomalies also showcases a direct relationship with earthquake magnitude. Heightened frequencies of ULF emissions and TEC anomalies as earthquake precursors correspond to more significant magnitudes in ensuing earthquakes. The outcomes of this research significantly augment the understanding of employing ULF emissions and TEC anomalies as effective earthquake precursors in the context of Sumatra.

Analysis of ULF Emissions in Solar Winds as A Short-Term Earthquake Precursor

The physical destruction and fatalities caused by earthquake events compelled scientists to develop Earthquake predictions. Due to limited seismometer sensitivity, it was impossible to detect earthquake events; therefore, non-seismological is established, which in Ultra Low Frequency (ULF). The study of electromagnetic waves (EM) in the Ultra-Low Frequency (ULF) range is a promising tool for investigating seismomagnetic effects that act as an earthquake precursor. This study analysed the ULF frequency range as a short-term earthquake precursor at a depth < 100 km and an epicentral distance < 100 km (distance from Cebu magnetometer station) measured using a ground magnetometer installed in Cebu (10.36oN, 123.91oE), in the Philippines. This study also intended to determine the emission of magnetic pulsation (Pc4 and Pc5), solar wind parameters and near equatorial geomagnetic storms (SYM/H) in low latitude regions prior to an earthquake event. Findings show that the most evident ULF that acts as a potential earthquake precursor was at a frequency range of Pc5 (1.7 - 6.7 mHz) compared to Pc4 (6.7 - 22 mHz). It also shows that high solar wind changes and geomagnetic storms respond to the emission of ULF magnetic pulsations (Pc4 and Pc5) prior to earthquake events at low latitudes. Thus, it can be concluded that magnetic pulsations are signatures that indicate the probability of short-term earthquakes precursor.

ULF geomagnetic anomaly associated with the Sumatra-Pagai Islands earthquake swarm during 2020

Observations of earthquake precursors via geomagnetic anomalies from ultra-low frequency (ULF) could provide an expectation for short-term earthquake prediction. However, there are still several obstacles in determining this precursor, one of which is the presence of a precursor bias if the earthquake occurs at a close time and location. To cope with this problem, we analysed six earthquakes with a magnitude >5 on Pagai Island, Mentawai, Sumatra, during 2020. These earthquakes have epicentres close to each other and occur within a short time (one month). This study used geomagnetic data recorded by the Magnetic Data Acquisition System (MAGDAS) magnetometer network at Sicincin station (SCN), West Sumatra, and Kepahiyang station (KPY), Bengkulu, with a span of one month before the earthquake. The anomaly of ULF emission was analysed using the power spectral density method at a frequency of 0.012 Hz for the H and Z components of the geomagnetic data. The onset time of the ULF emission anomaly was determined by the standard deviation value (pz + σ, pz – σ) of the SZ/SH power ratio polarization. The disturbance storm time index (Dst) was used to ensure that the anomaly occurred was not caused by geomagnetic storm. Also, the single station transfer function was used to calculate the azimuth, and the empirical formula between the anomaly amplitude and the magnitude was used to calculate the earthquake magnitude as the validation of the anomaly source. Several ULF anomalies as earthquake precursors were observed, especially at the KPY station. The estimated azimuth shows a good accuracy compared to the earthquake's azimuth, with an average value of 97.8%. Furthermore, the earthquake magnitude calculated from the ULF anomaly shows a good agreement with the actual earthquake magnitude, with an average accuracy of 98.13%.

OBSERVATION OF ULF ELECTROMAGNETIC EMISSIONS BEFORE THE M 7.8 NEW ZEALAND EARTHQUAKE OF NOVEMBER 13, 2016

We analyzed the ground geomagnetic data obtained from a 3-component fluxgate magnetometer at the Eyrewell Geomagnetic Observatory (New Zealand) (43.474 °S, 172.393 °E) from October 1 to December 31, 2016. The study aimed to investigate electromagnetic precursors associated with the M 7.8 New Zealand earthquake of November 13, 2016. This earthquake occurred 54 km northeast of Amberley (New Zealand). Its epicenter was located 158 km from the Eyrewell Observatory. We used three methods focused on the polarization ratio, fractal dimension and principal component analysis to identify anomalies in the geomagnetic data. The time series showed an enhanced polarization ratio at two times, October 20 and October 30, 2016, i.e. before the occurrence of the New Zealand earthquake, and a value ~1 or more during these instances. Since the global geomagnetic indices Kp and Dst were normal in these cases, the enhanced polarization ratio may be related to the preparation phase of the New Zealand earthquake. To further classify them, we applied the principal component analysis to the magnetic data on component H. The first three principal components showed more than 90 % of the variance of the original ultra-low frequency (ULF) magnetic field time series. The first principal component was found to be well correlated with the storm index (Dst) recorded during this period. Again, the second principal component was dominated by daily variations, which were the periodic component of the recorded ULF magnetic field. The temporal variation of the third principal component was analyzed to verify a possible correlation between the ULF emissions and the occurrence of the earthquake. The fractal dimension of components D and Z of the magnetic data decreased initially and sharply increased three days before the New Zealand earthquake.

Optimum frequency identification of anomalous geomagnetic signal related to earthquake precursor in Sumatra Island

Many scientific studies has detected ULF (ultra-low frequency) emission before an earthquake. An earlier study of Loma Prieta 7.1 M earthquake in 1989 shows ULF anomaly ranged between 0.01-10 Hz before an earthquake. A similar study of Kushiro earthquake 6.4 Mw shows that the ULF anomaly can be detected at a frequency range of 0.022-0.1 Hz. Furthermore, a study conducted in Sumatra Island from 2007-2012 shows the ULF anomaly at 0.01-0.06 Hz. Newer study on the 2009 Padang earthquake and Mentawai 2010 earthquake shows ULF emission between 0.012-0.022 Hz. Based on the aforementioned researches, a study is conducted on frequency range of 0.01-0.09 Hz to identify the optimum ULF frequency. Study conducted in this case uses magnetic diurnal variation data from Sumatra region from 2017 to 2018, especially earthquakes recorded at Gunung Sitoli (GSI), and Sicincin (SCI) stations. Previous studies above provide an overview of ULF ranges associated with earthquakes. Based on all above, an analysis is conducted to find the precursors using data of past earthquake events by utilizing Fast Fourier Transform to transform time-domain data to frequency domain, bandpass filters to eliminate noise, power ratio of Z/H to determine the time window, and Single Station Transfer Function to determine the location window. Lastly, moving average is used to assist determination of time window. Magnetic anomalies between 0.01-0.03 Hz, with amplitude exceeding monthly average, were observed before the earthquakes. Analysis in this frequency range shows occurence of earthquake 5-6 days after geomagnetic anomalies were detected. This study shows that location window prediction is possible with 100% accuracy. Therefore, 0.01-0.03 is currently the optimum frequency for precursor analysis.

Considering the potential of IAR emissions for ionospheric sounding

Knowledge of the ionospheric state allows us to adjust the forecasts of radio wave propagation, specify the environment models, and follow the changes of space weather. At present, probing of the ionosphere is produced by radio sounding with ground ionosondes, as well as by raying signals from satellites. We want to draw attention to the possibility of the diagnosis of the ionospheric parameters by detecting ultra-low frequency (ULF) electromagnetic emission generated in the so-called ionospheric Alfvén resonator (IAR). To do this, we present observations of the IAR emission made simultaneously for the first time at three stations using identical induction magnetometers. The stations are within one-hour difference of local time, two of them are mid-latitudinal; the third one is situated in the auroral zone. We compare frequency and frequency difference between adjacent harmonics of the observed multi-band emission with ionospheric parameters measured at the stations using ionosondes and GPS-observations. Diurnal variations of the ionospheric and ULF emission characteristics are also compared. The results show that there is quite a stable correlation between the resonant frequencies of the resonator bands and the critical frequency of the F2 layer of the ionosphere, namely, the frequency of the IAR emission varies inversely as the critical frequency of the ionosphere. This is due to the fact that the frequency of oscillation captured in the resonator is primarily determined by the Alfvén velocity (which depends on the plasma density) in the ionospheric F2 layer. The correlation is high; it varies at different stations, but is observed distinctly along the whole meridian. However, coefficients of a regression equation that connects the ionosphere critical frequency with DSB frequency vary significantly from day to day at all stations. The reason for such a big spread of the regression parameters is not clear and needs further investigation before we are able to develop a method for evaluating the ionosphere critical frequency using the IAR emission observations. Such a method may prove to be useful as an additional alternative to the basic method for probing the ionosphere using digisondes. This is especially important for auroral regions, where the presence of strong absorption, shielding by a lower layer, stratification, the presence of echoes, etc. make the sounding difficult.

A Study on the Correlation of Milk Yield of Cows with Seismicity and ULF Magnetic Field Variations

A statistical study on the basis of one-year data of 2014 has been performed in order to find whether abnormal animal behavior is related with seismic activity and also whether the ULF (Ultra Low Frequency) electromagnetic radiation might be a possible sensory mechanism of abnormal animal behavior. Abnormal animal behavior has been studied with the use of digitally recorded milk yield of cows at Ibaraki Prefecture Livestock Station, and the ULF magnetic field changes have been studied with the data at a magnetic observatory of Kakioka. As the result of correlation analyses, clear responses are observed for both the milk yield of cows and ULF magnetic field changes (both ULF radiation (ULF emissions from the lithosphere) and ULF depression (as an indicator of lower ionospheric perturbations)) for most powerful and not distant earthquakes (EQs) with magnitude > 6, that is, the milk yield of cows is found to exhibit a conspicuous depletion about 17 - 18 days before an EQ, though the correlation coefficient is not so big. Another important objective in this paper is to identify that ULF radiation is the main agent of abnormal behavior so that we have compared the temporal evolutions of milk yield of cows, ULF radiation and ULF depression for three major EQs. As a result, it is found that ULF radiation happens, at least, during the periods of abnormal depletion of milk yield of cows.

The 2011 eruption of Aso volcano, Japan, and its signature on the geomagnetic field measurements

High-resolution (1-s) geomagnetic data from the MAGDAS Network, Kyushu University, Japan, were examined to detect any local anomalous geomagnetic behavior that is possibly linked with the eruption of the Aso volcano, Kyushu Island, Japan, during 2011. Geomagnetic data recorded in the vicinity of the Aso volcano at the Kuju (KUJ) station, Japan, were analyzed and compared with those recorded at the Darwin (DAW) station in Australia, near the conjugate point of the KUJ station. The KUJ station is located about 20 km northeast of the Aso volcano. The amplitude range of daily variations was calculated and examined for the three geomagnetic components (H-, D-, and Z-components) along with the amplitude of ultra–low-frequency emissions in the Pc3-4 ranges (Pc3; 10–45 s and Pc4; 45–150 s). Data possessing and analysis reveal some local anomalous geomagnetic variations at the KUJ station during the 2011 volcanic activities of the Aso volcano. On the other hand, no anomalous changes were observed at the DAW station during that time. Therefore, the observed anomalous geomagnetic variations are considered to be of a local volcanic origin.

Anomalous ULF Emissions and Their Possible Association with the Strong Earthquakes in Sumatra, Indonesia, during 2007-2012

Eleven strong Sumatran earthquakes, with their epicenter less than 550 km away from the Kototabang (KTB) geomagnetic station (2007-2012), were studied to examine the occurrence of anomalous ultra-low frequency emissions (ULF-EM). Anomalous ULF signals, possibly associated with the earthquake's precursors, were determined by the Welch ratio SZ/SH at 0.06 Hz at the KTB station. These ULF anomalies were then compared with geomagnetic data observed from two reference stations in Darwin and Davao, to prevent misinterpretation of global geomagnetic disturbances as precursors. This study aims to analyze the relationship between earthquake magnitude and hypocenter radius, and seismic index against lead time during ULF-EM anomalies. We used the polarization ratio Welch method in terms of power spectrum density to evaluate the geomagnetic data by overlapping windows and applying fast Fourier transform (FFT). The results showed anomalous variations in onset and lead time, determined using the standard deviation controlling the SZ/SH power pattern. Our positive correlation between lead time of ULF emission and earthquake magnitude as well as between lead time and seismic index. It shows a negative correlation between hypocenter distances to KTB station against lead time.

Are Pre-Seismic Ulf Electromagnetic Emissions Considered as a Reliable Diagnostics for Earthquake Prediction?

The ultra low frequency (ULF) electromagnetic (EM) emissions, a short-term or operative parameter under observation for diagnosing seismic regimes have been examined for their reliability with efforts aimed to consider them as a reliable diagnostics. It has now been realized that there exists serious problems regarding these seismogenic ULF emissions and so there is a need to develop different methods of analysis which not only detect the pre-seismic ULF-EM emissions but also associate it for predicting the location, magnitude and time of occurrence of imminent earthquakes.

Foreshocks of strong earthquakes

The specific enhancement of ultra-low-frequency (ULF) electromagnetic oscillations a few hours prior to the strong earthquakes, which was previously mentioned in the literature, motivated us to search for the distinctive features of the mechanical (foreshock) activity of the Earth’s crust in the epicentral zones of the future earthquakes. Activation of the foreshocks three hours before the main shock is revealed, which is roughly similar to the enhancement of the specific electromagnetic ULF emission. It is hypothesized that the round-the-world seismic echo signals from the earthquakes, which form the peak of energy release 2 h 50 min before the main events, act as the triggers of the main shocks due to the cumulative action of the surface waves converging to the epicenter. It is established that the frequency of the fluctuations in the foreshock activity decreases at the final stages of the preparation of the main shocks, which probably testifies to the so-called mode softening at the approach of the failure point according to the catastrophe theory.

Analysis of space- and ground-based parameters prior to an earthquake on 12 December 2009

The present study includes observations of anomalies at ionospheric and ground levels using data from global positioning system (GPS)-based total electron content (TEC), ionospheric electron density by Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER), and ground-based ultra-low-frequency (ULF) emission prior to an earthquake that occurred on 12 December 2009. The analysis of the data from these three different techniques show relations with preseismic activities. The GPS-based TEC showed enhancement three days prior to the day of the earthquake. On converting the satellite azimuth into latitude and longitude, a large number of passes giving anomalous TEC values were observed covering a zone 17–23° N and 70–74° E (i.e. around the epicentre). Electron density and temperature as measured by DEMETER also show enhancement of the tracks around the epicentre one day prior to the earthquake. For ground-based ULF emission, we used different analysis techniques for the detection of abnormal changes in ULF emission before the earthquake. We observed changes in different parameters such as polarization ratio and fractal dimension. Clear enhancement is observed for polarization parameters (Z/X, Z/Y) and fractal analysis (for the Z component). The results provide useful information in acquiring data prior to earthquakes.

A study of correlation between seismicity and mental health: Crete, 2008–2010

We compared the number of M > 2 earthquakes in an area, including the island of Crete, Greece, to the number of admissions to the Psychiatric Inpatient Unit of the University of Crete (IPU/UoC), during the period 2008–2010. We found that when earthquakes with M 6.4 occurred in the region examined, the number of monthly admissions to the Acute Care Unit of the IPU/UoC, NAA, reached lowest values, whereas this number increased when the monthly number NE of small (M < 3) earthquakes increased. We also found a positive correlation between the total number of monthly admissions NA and NE (r = 0.601/P = 0.001). When a daily resolution analysis was performed for the month with the highest value of NE, we found that an abrupt increase in the number of small earthquakes was followed by an increase in the number of admissions after ∼2 days (during that month, from a total of 71 patients, 38/10 people were diagnosed with schizophrenia/bipolar disorder). We hypothesize that seismic activity might be a significant contributing factor influencing the frequency of admissions of psychotic disorders in Crete in the period 2008–2010 and that the beneficial/adverse effects are related to the anomalous electric field/extra low frequency–ultra low frequency emissions.

The ultra-low-frequency magnetic disturbances associated with earthquakes

The ultra-low-frequency (ULF) electromagnetic emission is recently recognized as one of the most promising candidates for short-term earthquake (EQ) prediction. This paper reviews previous convincing evidence on the presence of ULF emissions before three major EQs. Then, we present further statistical study on the ULF occurrence, our networks of ULF monitoring in different spatial scales in Japan and finally we present several signal processings to identify the seismogenic emissions by showing latest results for recent large EQs.

Ultra low frequency pulsation generation under the action of modulated microwave power on ionosphere

A theory of ultra low frequency emission generation by modulation of microwave power in ionosphere heating experiment is presented. The source currents and emitted power of Alfven and magnetoacoustic waves are calculated. Coefficient of efficiency of MHD generation as a function of power frequency modulation is determined.

Magnetic anomalies possibly linked to local low seismicity

Abstract. During the last twenty years a time-synchronized network of magnetometers has operated in Central Italy along the Apennine chain to monitor the magnetic field anomalies eventually related to the tectonic activity. At present time the network consists of five stations. In the past only few anomalies in the local geomagnetic field, possibly associated to earthquakes, has been observed, not least because the network area has shown a low-moderate seismic activity with the epicentres of the few events with Ml≥5 located away from the network station. During 2007 two Ml≈4 earthquakes occurred in proximity of two stations of the network. Here we report the magnetic anomalies in the geomagnetic field that could be related with these tectonic events. To better investigate these two events a study of ULF (ultra-low-frequency) emissions has been carried out on the geomagnetic field components H, D, and Z measured in L'Aquila Observatory during the period from January 2006 to December 2008. We want to stress that this paper refers to the period before the 2009 L'Aquila seismic sequence which main shock (Ml=5.8) of 6 April heavily damaged the medieval centre of the city and surroundings. At present time the analysis of the 2009 data is in progress.

Fractal characteristics of the ground-observed ULF emissions in relation to geomagnetic and seismic activities

Results of fractal analysis of ultra-low-frequency (ULF) emissions registered at a low-latitude observatory, Guam (geomagnetic coordinates Φ m=9°N, Λ m=225°), and at a high-latitude drifting station, North Pole-30 ( Φ m=75°N, Λ m=172°), are presented. The first set of data covers a long period of observations (20 months) including the strong (Ms=8) Guam earthquake of 8 August 1993. The second set of data covers a short period of observations (21 days) in April 1989 during the preparation phase of the big magnetic storm of 25 April 1989. Definite peculiarities in the behavior of ULF emission scaling (fractal) characteristics have been found, which are discussed on the basis of the self-organized criticality concept. The principal common peculiarity for magnetosphere–ionosphere and lithosphere systems is detection of flicker noise ( β∼1, D 0∼2) in a certain frequency range on the preparation phase of strong magnetospheric and seismic events.

Monitoring of ULF (Ultra-Low-Frequency) Geomagnetic Variations Associated with Earthquakes

ULF (ultra-low-frequency) electromagnetic emission is recently recognized as one of the most promising candidates for short-term earthquake prediction. This paper reviews previous convincing evidence on the presence of ULF emissions before a few large earthquakes. Then, we present our network of ULF monitoring in the Tokyo area by describing our ULF magnetic sensors and we finally present a few, latest results on seismogenic electromagnetic emissions for recent large earthquakes with the use of sophisticated signal processings.

The observation of ultra‐low frequency emissions at Nakatsugawa, Japan, in possible association with the Sumatra, Indonesia, earthquake

The observation of direct current (DC)/ultra‐low frequency (ULF) electromagnetic emissions at Nakatsugawa, Gifu, Japan, has indicated that ULF emissions in the frequency below 0.1 Hz took place about 3–10 days before the disastrous Sumatra, Indonesia, earthquake. The most important support to this is based on the goniometric direction finding for those ULF emissions during the period of 17–23 December, which indicates their azimuth angle being very close to the epicentre of the earthquake. So, these ULF emissions between 17–23 December are likely to be associated with the earthquake.

Possible Origin of Ultralow-Frequency Whistlers

The sonographic analysis of records of ultralow-frequency emissions recorded by the induction magnetometer at the Mirnyi observatory (Antarctica) in 1981–1985 has revealed the presence of a special class of signals in the frequency band 0.25–5 Hz having a characteristic dispersion reminiscent of the dynamic spectra of LHR-whistlers observed in the VLF band. The ULF whistlers are observed, as a rule, at morning and evening hours of local time at moderate values of the Kp-index (0 < Kp < 4). The analysis of the frequency–time behavior of observed signals shows that the canalized propagation of short broadband pulses as magnetosonic waves in the layered medium, for example, in the magnetospheric current sheet can serve as a possible cause of the dispersion. Such sporadic phenomena on the magnetopause as microreconnections or FTE-events may be sources of these pulses.