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In order to mitigate landslide risks, monitoring of ground water is essential. Previous indoor experiments suggest that the self-potential approach has an advantage to establish the early warning system in performance to detect related signals and in cost. To evaluate results obtained from the previous indoor experiments, in-situ measurements are required. To achieve this, we selected a test slope at Nishiikawa, Tokushima, Japan and we performed preliminary electrical resistivity exploration (ERT) and core-sampling. The results of ERT show that there are low resistive areas in shallow depth ( < 3 m). The core-sampling results also indicate that there exists the structure which corresponds to slip surface. In addition, anisotropy in magnetic susceptibility was examined and the samples around the estimated the slip surface region provide the oblate ellipsoid characteristics. These results of ERT and core analyses are consistent and make convince the identification of the slip surface. Based on these preliminary results, we set up the in-situ self-potential observation at the slope and started measurements.

? 2016 The Institute of Electrical Engineers of Japan. The ionospheric anomalies possibly associated with large earthquakes have been reported by many researchers. In this paper. Total Electron Content (TEC) and tomography analyses have been applied to investigate the spatial and temporal distributions of ionospheric electron density prior to the 2011 Off the Pacific Coast of Tohoku earthquake (Mw9.0). Results show significant TEC enhancements and an interesting three dimensional structure prior to the main shock. As for temporal TEC changes, the TEC value increases 3-4 days before the earthquake remarkably, when the geomagnetic condition was relatively quiet. In addition, the abnormal TEC enhancement area in space was stalled above Japan during the period. Tomographic results show that three dimensional distribution of electron density decreases around 250 km altitude above the epicenter (peak is located just the east-region of the epicenter) and increases the mostly entire region between 300 and 400 km.

© 2015 Elsevier Ltd. As one of the most promising candidates for short-term earthquake forecasting, the seismo-electromagnetic phenomena have been intensively studied for several decades. Recently, Xu et al. (2013) have reported unusual behaviors of geomagnetic diurnal variations in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To validate this result, further investigations have been applied in this study. Geomagnetic data of 16 years' long term observation have been analyzed using the same method in Xu et al. (2013). Ratios of diurnal variation range between the target station Esashi (ESA) which is about 130 km from the epicenter and the remote reference station Kakioka (KAK) about 300 km distant to the epicenter have been computed. After removing seasonal variations revealed by wavelet transform analysis, the 15-day mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event. This anomaly is unique over a 16-year long background, and further discussions indicate that this anomaly is unlikely caused by strong geomagnetic storms or a statistical fluke. Therefore, this study has provided a strong support to the previous results in Xu et al. (2013).

? 2015 The Institute of Electrical Engineers of Japan. A numerical simulation has been done to evaluate the performance of the ionospheric tomography using the residual minimization training neural network (RMTNN) method. The results indicated that reconstruction with high-precision is possible when the standard deviation of the noise is about 2.5% or less of the average value of observed data (Slant TEC: STEC). Moreover, in the daytime when the value of STEC becomes large, the signal to noise ratio (SNR) increases and reconstruction accuracy becomes high; at night when the SNR falls conversely, it becomes low. Results of detectability tests show that the RMTNN method has a good performance around F-layer height with shape and peak intensity reconstruction. In conclusion, the developed RMTNN ionospheric tomography is effective in reconstructing 3D electron density distribution from realistic STEC data in the daytime, and is able to estimate images around F-layer.

© 2015 Academic Journals Inc. Analysis of ionospheric precursor is not easy because the ionosphere is very dynamic as well as the earthquake phenomena. If the analysis method is the same dynamics with the earthquake phenomena, the estimation of earthquake parameters is possible to be realized. Neural network is an adaptive system that changes its structure to solve the problem during a learning phase. Therefore, the neural network is potentially to estimate the parameter of earthquake based on ionospheric precursor. A preliminary attempt was made to construct the neural network that can estimate the epicenter area. The GIM-TEC star method is useful to determine ionospheric anomalies associated with large earthquakes as ionospheric precursor data. The Kriging method is good to interpolate GIM-TEC star data as input of neural networks to estimate the epicenter area. The conclusion of five models of the ionosphere anomalies due to seismic activity show that the epicenter is at the edge of the less developed anomalies, whereas, for the growing anomalies, the epicenter is always located near the boundary of high and low density of TEC anomalies. The boundary is projection of the boundary of the unstressed and stressed rock area below the earth’s surface.

© 2013 John Wiley & Sons, Ltd. Knowledge of the mechanisms of rain-induced shallow landslides can improve the prediction of their occurrence and mitigate subsequent sediment disasters. Here, we examine an artificial slope's subsurface hydrology and propose a new slope stability analysis that includes seepage force and the down-slope transfer of excess shear forces. We measured pore water pressure and volumetric water content immediately prior to a shallow landslide on an artificial sandy slope of 32°: The direction of the subsurface flow shifted from downward to parallel to the slope in the deepest part of the landslide mass, and this shift coincided with the start of soil displacement. A slope stability analysis that was restricted to individual segments of the landslide mass could not explain the initiation of the landslide; however, inclusion of the transfer of excess shear forces from up-slope to down-slope segments improved drastically the predictability. The improved stability analysis revealed that an unstable zone expanded down-slope with an increase in soil water content, showing that the down-slope soil initially supported the unstable up-slope soil; destabilization of this down-slope soil was the eventual trigger of total slope collapse. Initially, the effect of apparent soil cohesion was the most important factor promoting slope stability, but seepage force became the most important factor promoting slope instability closer to the landslide occurrence. These findings indicate that seepage forces, controlled by changes in direction and magnitude of saturated and unsaturated subsurface flows, may be the main cause of shallow landslides in sandy slopes.

Daily resolution data retrieved from the 1243 ground-based Global Positioning System (GPS) stations in Japan are utilized to expose surface displacements before the destructive M9 Tohoku-Oki earthquake (March 11, 2011). Variations in the residual GPS data, in which effects of the long-term plate movements, short-term noise and frequency-dependent variations have been removed through a band-pass filter via the Hilbert-Huang transform, are compared with parameters of the focal mechanism associated with the Tohoku-Oki earthquake for validation. Analytical results show that the southward movements, which were deduced from the residual displacements and agree with the strike of the rupture fault, became evident on the 65th day before the Tohoku-Oki earthquake. This observation suggests that the shear stress played an important role in the seismic incubation period. The westward movements, which are consistent with the angle of the maximum horizontal compressive stress, covered entire Japan and formed an impeded area (142°E, 42°N) about 75. km away from the epicenter on the 47th day prior to the earthquake. The horizontal displacements integrated with the vertical movements from the residual GPS data are very useful to construct comprehensive images in diagnosing the surface deformation from destructive earthquakes along the subduction zone. © 2013 Elsevier Ltd.

© Author(s) 2014. CC Attribution 3.0 License. Indonesia is one of the most seismically active regions in the world and mitigation of seismic hazard is important. It is reported that Ultra low frequency (ULF) geomagnetic anomalies are one of the most convincing phenomena preceding large earthquakes (EQs). In this paper we have analyzed geomagnetic data at Pelabuhan Ratu (PLR) (7.01° S, 106.56° E), Sukabumi, West Java, Indonesia, from 1 September 2008 to 31 October 2010. There are twelve moderate-large EQs (M ≥ 5) within 160 km from the station during the analyzed period. The largest one is the M = 7.5 EQ (depth=57 km, epicentral distance=135 km, 2 September 2009) based on EQ catalog of Indonesian Meteorological, Climatological and Geophysical Agency (BMKG). To investigate the ULF geomagnetic anomalous variations preceding all the EQs, spectral density ratio at the frequency range of 0.01±0.003 Hz based on wavelet transform (WT) and detrended fluctuation analysis (DFA) have been carried out. The spectral density ratio results show the enhancements a few weeks before the largest EQ. The enhancement persists about one week and reaches a maximum on 16 August 2009. At the same time, the result of the DFA presents the decrease of α value. For other EQs, there are no clear increases of the spectral density ratio with simultaneous decrease of α value. When these phenomena occur, the value of Dst index shows that there are no peculiar global geomagnetic activities at the low latitude region. The above results are suggestive of the relation between the detected anomalies and the largest EQ.

This paper describes a field test to verify a newly discovered phenomenon of microwave emission due to rock fracture in a volcano. The field test was carried out on Miyake Island, 150. km south of Tokyo. The main objective of the test was to investigate the applicability of the phenomenon to the study of geophysics, volcanology, and seismology by extending observations of this phenomenological occurrence from the laboratory to the natural field.We installed measuring systems for 300. MHz, 2. GHz, and 18. GHz-bands on the mountain top and mountain foot in order to discriminate local events from regional and global events. The systems include deliberate data subsystems that store slowly sampled data in the long term, and fast sampled data when triggered. We successfully obtained data from January to February 2008. During this period, characteristic microwave pulses were intermittently detected at 300. MHz. Two photographs taken before and after this period revealed that a considerably large-scale collapse occurred on the crater cliff. Moreover, seismograms obtained by nearby observatories strongly suggest that the crater subsidence occurred simultaneously with microwave signals on the same day during the observation period.For confirmation of the microwave emission caused by rock fracture, these microwave signals must be clearly discriminated from noise, interferences, and other disturbances. We carefully discriminated the microwave data taken at the mountaintop and foot, checked the lightning strike data around the island, and consequently concluded that these microwave signals could not be attributed to lightning. Artificial interferences were discriminated by the nature of their waveforms. Thus, we inferred that the signals detected at 300. MHz were due to rock fractures during cliff collapses. This result may provide a useful new tool for geoscientists and for the mitigation of natural hazards. © 2013 Elsevier B.V.

Earthquake-related anomalous electromagnetic phenomena have been reported in various frequency ranges over the past few decades. Investigation of the anomalous propagation of VHF transmitter waves is a promising approach to short-term prediction and crustal activity monitoring. Anomalous propagation is believed to be generated by disturbances of the atmosphere above the epicenter or along the propagation path prior to large earthquakes. Consequently, over-the-horizon propagation has been received. A recent study shows that the appearance of anomalies was signifi cantly enhanced within 5 days of earthquakes with M ≥4.8. However, there is no information on the scattering location, that is, on the direction of wave arrival. Therefore, a simple interferometer system for VHF radio waves to identify the space-time position of earthquake-related atmospheric disturbances has been developed and installed at Chiba University. This paper describes the newly developed interferometer system and presents the results of fundamental tests to evaluate the performance of the new interferometer at Chiba. Data on over-the-horizon propagation of VHF radio waves obtained from 1-year continuous measurement at Chiba are described. These are possible radio duct propagations and possible earthquake-related anomalous propagations. © 2013 Wiley Periodicals, Inc.

Geoelectrical fluctuations are the end product of several geophysical phenomena. In particular, geoelectrical signals measured in seismically active areas can be attributed to stress and strain changes associated with earthquakes. In order to study the geoelectrical potential difference changes associated with the Earth's crustal activities, it is important to distinguish them from other noises because they are generally weak. The complexity of this problem has called for development of advanced statistical methods suitable for investigation of the heterogeneous nature of these fluctuations. In this paper we analyzed the time dynamics of short-term variability of the geoelectrical field measured at Kiyosumi station located in the southern part of Boso peninsula, Japan. We tried to apply principal component analysis (PCA) and singular spectral analysis (SSA) to detect and reduce precipitation effects which are the most intense noise in geoelectrical potential difference data. The results show that an application of PCA to two orthogonal electric data can detect the precipitation and SSA has a potential to remove the precipitation effects. © 2012 Wiley Periodicals, Inc.

By using multiple reference stations, we have developed a method to get reliable ULF global geomagnetic variations. This background is extremely useful for detecting local anomalous behaviors. In this paper we report on variable tools developed to identify the anomalies in two frequency ranges: daily variations and variations of 10- to 1000-second span. For estimating background daily variations, the periodical model has been applied for data observed at three reference stations and a study station. Comparison between the first principal component of the periodical data from the reference stations and the periodical data derived from the target station generally provides high correlation. For data with 100-second periods after wavelet filtering, the nighttime energy variations have been investigated among three reference stations and a study station. Similar principal component analysis as the diurnal variation has been performed and results also show high correlation between the variation at the target and the global background. These tendencies suggest that the two proposed methods are effective in automatically identifying the anomalous patterns. Examining the original data, we can obtain details of waveforms and distinguish whether the anomalies are related to underground activities or simply to some artificial noises. © 2012 Wiley Periodicals, Inc.

The Parkinson vectors derived from 3-component geomagnetic data via the magnetic transfer function are discussed with respect to epicentre locations and hypocentre depths of 16 earthquakes (M &ge; 5.5) in Taiwan during a period of 2002-2005. To find out whether electric conductivity changes would happen particularly in the seismoactive depth ranges, i.e. in the vicinity of the earthquake foci, the frequency dependent penetration depth of the electromagnetic waves (skin effect) is taken into account. The background distributions involving the general conductivity structure and the coast effect at 20 particular depths are constructed using the Parkinson vectors during the entire study period. The background distributions are subtracted from the time-varying monitor distributions, which are computed using the Parkinson vectors within the 15-day moving window, to remove responses of the coast effect and underlying conductivity structure. Anomalous depth sections are identified by deviating distributions and agree with the hypocentre depths of 15 thrust and/or strike-slip earthquakes with only one exception of a normal fault event. © 2013 Author(s).

In order to clarify the ultra low frequency (ULF) seismo-magnetic phenomena, a sensitive geomagnetic network has been installed in Kanto, Japan. In this study, we have analyzed geomagnetic data observed during the past decade in Izu and Boso Peninsulas. Energy of ULF geomagnetic signals at the frequency around 0. 01 Hz has been investigated by wavelet transform analysis. To identify anomalous changes in ionospheric disturbances, the station Memabutsu has been chosen as a reference station. Case studies of magnitude 6 class earthquakes have demonstrated that there are unusual geomagnetic energy enhancements in the vertical component before the main shocks. Statistical studies by superposed epoch analysis have indicated that, before a sizeable earthquake, there are clearly higher probabilities of ULF anomalies than after the earthquake: statistical results of daily counts were found significant at about 3-4 weeks before, 1 week before, few days before, and 1 day after the event for Seikoshi station in Izu and around 2 weeks before, few days before, and 1 day after the event for Kiyosumi station in Boso, respectively. © 2012 Springer Science+Business Media Dordrecht.

There have been many reports on ultra-low-frequency (ULF) electromagnetic phenomena associated with earthquakes in a very wide frequency range. In this study, unusual behaviors of geomagnetic diurnal variations prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw9.0) have been reported. Ratios of diurnal variation range between the target station Esashi (ESA) which is about 135. km from the epicenter and the remote reference station Kakioka (KAK) about 302. km distant to the epicenter have been computed. The results of 10-day running mean of the ratios showed a clear anomaly exceeding the statistical threshold in the vertical component about 2. months before the mega event. This anomaly is unique over a 3-year background, and the further stochastic test indicates that this anomaly is unlikely a random anomaly, which is highly suggestive of correlation with the mega event. The original records of geomagnetic fields of the ESA station also exhibit continuous anomalous behaviors for about 10. days in the vertical component approximate 2. months prior to the Mw9.0 earthquake. © 2013 The Authors.

Reactivated landslides have often posed a significant hazard to human lives and properties in many regions of Indonesia. In order to mitigate the hazard, a better understanding of mechanism leading to the reactivation of landslides is necessary. For this purpose, a longterm monitoring and numerical analysis of an active landslide in a volcanic soil slope in West Java, Indonesia was conducted. Monitoring instruments consisted of jet-fill tensiometers, inclinometers, open stand-pipes, and a tipping-bucket rain-gage. The records of pore-water pressure responses of the soil slope show that a transient perched water table could develop within the shallow soil profile, and the groundwater table could rise significantly in the middle portion of the slope during heavy rainstorms. Meanwhile, the inclinometer records show the existence of a multiple sliding zone at a maximum depth of about 17 m. Based on the monitoring data, a couple analysis of seepage and slope stability was performed to evaluate the effect of rainwater infiltration on long-term stability of the slope. The numerical results indicated that the total rainfall of at least 550 mm, with intensity of more than 80 mm/day was the triggering hydrological condition for reactivation of the landslide. © Springer-Verlag Berlin Heidelberg 2013.

An ionospheric anomaly prior to the 2007 Southern Sumatra earthquake (M8.5) was observed by GPS receivers around the Sumatra islands. In this paper, to investigate the three-dimensional structure of electron density in the ionosphere, a tomographic approach (Residual Minimization Training Neural Network; RMTNN) has been used. Results of the tomographic approach are consistent with those of the total electron content (TEC) approach. We found that a significant decrease of electron density takes place at altitudes of 250 to 400 km, especially at an altitude of 330 km. But the altitude at which the maximum electron density has been observed remains unchanged. The obtained structure has a region of decreased density of Integrated Electron Content (IEC) in the southwest area (at altitudes of 400 to 550 km) and in the northern area (at altitudes of 250 to 400 km). The global tendency of the decreased electron density expands to the east with higher altitudes and it is concentrated in the southern hemisphere over the epicenter. These results show that the high capability of the RMTNN method can be used for estimation of the ionospheric electron density distribution possibly associated with earthquake. © 2012 Wiley Periodicals, Inc.

On 1 September 2004 a vulcanian eruption, the strongest since 1983, occurred at Mt. Asama, one of the most active volcanoes in Japan. An ash plume that rose 2km above the crater was emitted, together with incandescent blocks ejected several kilometers away from the summit crater. Subsequently, after a short quiescence period of about 2weeks, a series of minor eruptions took place from 14 September. They were considered unusual for Mt. Asama for the presence of juvenile material dominant among fallout. Small and moderate eruptions continued even in the following months to end in December 2004. On the basis of GPS observations, a dike intrusion occurred under the western flank of Mt. Asama about 6weeks before 1 September eruption, triggering the magma ascension towards the surface. This phenomenon produced many signs of unrest including a thermal activity observed on the ground from the end of July 2004. The retrospective analysis presented in this paper, performed applying the RST VOLC algorithm to Moderate Resolution Imaging Spectroradiometer (MODIS) data, shows the occurrence of pre-eruptive hot spots at Mt. Asama during 1-31 August and 11-12 September 2004. These results extend observations of a previous MODIS-based independent study and show the occurrence of a distinct phase of thermal unrest, of short-time duration, a few days before the 14 September minor eruption. Moreover, by the temporal trend of radiative power from infrared data, time of occurrence of dome extrusion is also estimated, in good agreement with independent radar observations. These outcomes seem to be compatible with different mechanisms of upward propagation of magma speculated by previous authors, confirming the potential of infrared satellite observations in identifying thermal precursory signals of volcanic eruptions, provided that proper detection algorithms are used. © 2012 Elsevier B.V.

This study uses a network that is comprised of 10 total intensity magnetometers to detect azimuthal propagation of seismo-magnetic emission waves during 26 earthquakes that occurred between July 2007 and December 2008 in Taiwan. The propagation azimuth and phase velocity of the seismo-magnetic waves are calculated using frequency wavenumber analysis at the ultra low frequency of 0.05 Hz every 30 min. We superimpose the derived azimuths within a moving window of 30 days as the monitored distributions, and the entire dataset as the background distribution. We also find the propagation azimuths of the seismomagnetic anomalies of each earthquake by subtracting the background from the monitored distributions. The results show that frequency wavenumber analysis can be applied to evaluate azimuthal propagation of seismo-magnetic emission waves using a scalar of geomagnetic total intensity fields. The success detection rate of seismo-magnetic anomalies increases from 62% of the 26 earthquakes to 77% using the surface magnetic anomalous reference tip (SMART) to substitute the epicenters. Meanwhile, the odds proportions between the azimuths of the seismomagnetic emission waves towards and away from SMART reveal the associated anomalous propagation. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

One of the strongest earthquakes, with magnitude M 8.9, occurred at the sea bottom near to the east coast of Japan on March 11, 2011. This study is devoted to the investigation of anomaly disturbances in the main magnetic field of the Earth and in ultra-low frequency magnetic variations (F <10 Hz) observed before this earthquake. Secular variations of the main geomagnetic field were investigated using three-component 1-h data from three magnetic observatories over the 11-year period of January 1, 2000, to January 31, 2011. The Esashi and Mizusawa magnetic stations are situated northwest of the earthquake epicenter, at distances of around 170 km to 200 km, and the Kakioka observatory is situated southwest of the earthquake epicenter, at a distance of about 300 km. During this period, there were four local anomalies in the secular variations. The last anomaly was the biggest, which began around 3 years prior to the earthquake moment. All of the anomalies can be most distinctly recognized, in the form of differences in the corresponding magnetic components at these remote magnetic stations. For investigations of the ultra-low frequency magnetic field disturbances, three-component 1-s data at two magnetic stations (Kakioka and Uchiura) were used. The Uchiura station is situated 119 km south of Kakioka, at a distance of about 420 km from the earthquake epicenter. Data from the time interval of February 18, 2011 to March 10, 2011 (only at night-time: 01:00 to 04:00 local time) were investigated in a wide frequency range. In the frequency range of 0.033 Hz to 0.01 Hz, there was the clearest anomaly, seen as a decrease in the correlation coefficients of the corresponding magnetic components at these two stations, from February 22, 2011. Differences in the Z components showed an increase, and became positive after this date. This might suggest that the ultra-low frequency lithospheric source appeared north of the Kakioka station. Outside this specified frequency range, the anomalies were not well defined. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

Despite early optimism, pre-earthquake anomalous phenomena can be determined by using enhanced amplitude at the ultra-low-frequency range from geomagnetic data via the Fourier transform. In reality, accuracy of the enhanced amplitude in relation to earthquakes (deduced from time-varied geomagnetic data) would be damaged by magnetic storms and/or other unwanted influences resulting from solar activity and/or variations in the ionosphere, respectively. We substitute values of the cross correlation between amplitudes, summarized from the earthquake-related (0.1-0.01 Hz) and the comparable (0.01-0.001 Hz) frequency bands, for isolated amplitude enhancements as indexes of determination associated with seismo-magnetic anomalies to mitigate disturbance caused by magnetic storms. A station located about 300 km away from the others is also taken into account to further examine whether changes of the cross correlation values are caused by seismo-magnetic anomalies limited within local regions or not. Analytical results show that the values suddenly decrease near epicenters a few days before and after 67% (= 6/9) of earthquakes (M > = 5) in Taiwan between September 2010 and March 2011. Seismo-magnetic signals determined by using the values of cross correlation methods partially improve results yielded from the Fourier transform alone and provide advantageous information of earthquake locations. © 2012 Author(s). CC Attribution 3.0 License.

The Morlet wavelet transform is commonly utilized to understand amplitude distributions in the frequency domain. To compare amplitude distributions at two different stations, the wavelet coherence as a normalized numerical index is employed. The wavelet coherence at two stations away from an epicenter often approaches 1 and their relative phase presents synchronous variations suggesting that geomagnetic fields are dominated by changes in the ionosphere and/or magnetosphere. When a station located near an epicenter is added to the analytical process, the small wavelet coherence at the period of approximately 0.5 day appears and disappears suddenly 40 days before and few days after earthquakes, respectively. Analytical results are in agreement with ratio changes of daily variation ranges at two stations during earthquakes reported in previous studies and shed light on location estimation of epicenters for forthcoming events using geomagnetic intensity fields.

Earthquake-related anomalous electromagnetic phenomena have been reported in various frequency ranges in a few decades. Investigation on the anomalous propagation of VHF transmitter waves is one of promising approaches on the short-term prediction and crustal activity monitoring. The anomalous propagation is considered to be generated by disturbances of the atmosphere above the epicenter or along the propagation path prior to large earthquakes. Consequently, over-horizontal propagation has been received. A recent study shows that the appearance of anomalies was significantly enhanced within 5 days before earthquakes with M &ge; 4.8. However, there is no information on the scattered place, that is, the direction of wave arrival. Therefore, a simple interferometer system for VHF radio wave to identify the position between space-time of earthquake-related atmospheric disturbances has been developed and installed at Chiba University. In this paper, we will show you the developed interferometer system and results of fundamental tests to evaluate the performance of developed and installed interferometer at Chiba. In addition, facts on invisible propagation of VHF radio wave obtained from 1-year continuous measurement at Chiba are described in this paper. Those are possible radio duct propagations and possible earthquake-related anomalous propagations.

By using multiple reference stations, we have developed a method to get reliable ULF global geomagnetic variations. This background is extremely useful for detecting local anomalous behaviors. This paper demonstrates developed variable tools to identify the anomalies in two frequency ranges; daily variation and 10-1000 seconds. For estimating background daily variations, the periodical model has been applied for data observed at 3 reference stations and a study station. Comparison between the first principal component of the periodical data from the reference stations and the periodical data derived from the target station provides high correlation in general. For data with 100 sec periods after wavelet filtering, the nighttime energy variations have been investigated among 3 reference stations and a study station. The similar principal component analysis as the diurnal variation has been performed and results also show high correlation between the variation at the target and the global background. These tendency suggest that the developed two methods are effective to identify the anomalies in automatic. Examining the original data, we can know details of waveform and distinguish whether the anomalies is related to underground activities or just some artificial noises.

The ionospheric anomaly prior to the 2007 SouthernSumatra earthquake (M8.5) was observed by GPS receivers around Sumatra islands. In this paper, to investigate the three-dimensional structure of electron density in Ionosphere, a tomographic approach (Residual Minimization Training Neural Network; RMTNN) has been performed. The results of the tomographic approach are consistent with those of total electron content (TEC) approaches. We found that the significant decreases take place in the heights of 250-400 km, especially at 330 km height. But the height which gives the maximum electron density is not changed. The obtained structure is that the decreased region exists in the southwest side of Integrated Electron Content (IEC) (400-550 km altitudes) and in the northern side of IEC (250-400 km altitudes). Global tendency of the decreases area is expanded to the east with an altitude and it is concentrated in the southern hemisphere of over the epicenter. These resultsshow that the high capability of RMTNN method for the estimation of the ionospheric electron density distribution possibly associated with earthquake.

Geoelectrical fluctuations are the end product of several geophysical phenomena. In particular geoelectrical signals measured in seismically active areas can be attributed to stress and strain changes, associated with earthquakes. In order to study the geoelectrical potential difference changes associated with the earth's crustal activities, it is important to discriminate from other noises because they are generally weak. The complexity of this problem has suggested the development of advanced statistical methods to investigate the heterogeneous nature of these fluctuations. In this paper we analyzed the time dynamics of short-term variability of geoelectrical field measured at Kiyosumi station, located in southern part of Boso peninsula, Japan. We tried to apply principal component analysis (PCA) and singular spectral analysis (SSA) to detect and reduce precipitation effects which are the most intense noise in geoelectrical potential difference data. The results show an application of PCA to two orthogonal electric data can detect the precipitation and SSA has a potential to remove the precipitation effects.

In this paper, we examine pre-earthquake ionospheric anomalies in time series and perform a statistical test by using total electron content (TEC) derived from global ionosphere maps (GIM) around the Japan area for the first time. The normalized GIM-TEC (GIM-TEC*), which is computed based on 15days backward running mean of GIM-TEC, have been investigated for minimizing possible confounding effects of consecutive earthquakes and identify the abnormal signals. Meanwhile, to reduce the effect of strong geomagnetic activities such as geomagnetic storms, the criterion for removing the GIM-TEC data have been adapted; that is when Dst index exceeds -60nT. Temporal variations of GIM-TEC* for large and destructive earthquakes in Japan have been studied; which are the 2004 mid-Niigata Prefecture Earthquake (M6.8), its aftershock (M6.1), the 2007 offshore mid-Niigata Earthquake (M6.8), and the 2008 Iwate-Miyagi Nairiku Earthquake (M7.2). Although there are some positive and negative TEC anomalies before and after the four earthquakes, there is a tendency that positive TEC anomalies appear 1-5days before all the above earthquakes even during the quiet geomagnetic condition. Superposed epoch analyses have been performed for the statistical analysis of TEC anomalies associated with M≥6.0 earthquakes during the 12-year period of May 1998-May 2010. The statistical result indicates the significance of the positive TEC anomalies 1-5days before earthquakes within 1000km from the epicenter around Japan. © 2010 Elsevier Ltd.

To mitigate volcanic hazards, a new algorithm for monitoring volcanic activity using satellite remote sensing in mid-infrared range (MODIS) has been developed. Nighttime data of Band 20 is used for brightness temperature imaging, and data at Bands 31 and 32 are analyzed for attempting to remove the cloud effects. It is found that a certain threshold is effective for cloud detection in the tropical region. Moreover, the differential brightness temperature (S and Smax) between the focused and the reference points is introduced to eliminate the background effects. In addition, the singularity of the differential bright temperature in time (δ and δmax) has been proposed. The developed algorithm has been applied to the practical data associated with the Mt. Merapi volcano in Java, Indonesia and the Mt. Asama volcano in Honshu, Japan. In the case of Mt. Merapi volcano, the results suggest the effectiveness of the developed method. For the case of Mt. Asama, the proposed method cannot remove the cloud influences. © 2011 Elsevier Ltd.

There are many reports on earthquake-related electromagnetic phenomena. Anomalous ULF geomagnetic field changes associated with earthquake is one of the most convincing and promising phenomena due to deeper skin depth. Since ULF signals associated with large earthquakes are weak, effective signal discrimination methods should be required. Several methods for the signal discrimination have been developed so far: which are spectrum density ratio analysis, geomagnetic transfer function analysis, fractal analysis, principal component analysis, direction finding analysis, and so on. In this study, we investigate ULF geomagnetic changes possibly associated with the 2008 Iwate-Miyagi Nairiku earthquake based on spectral density ratio analysis. Geomagnetic data observed at Esashi, where the epicentral distance is about 47 km and Kakioka, the distance is about 317 km, and as a reference station have been analyzed. Wavelet transform have been performed for the spectral density analysis instead of the conventional FFT method. Before the earthquake, the variation of spectral density ratio, Sz/. Sx and Sz/. Sy, at the nearest station of Esashi exhibits an apparent increase from the trend. On the contrary, there are no corresponding significant changes at a remote station of Kakioka. After investigating the singularity of the increase using normalized spectrum density ratio, the enhancement is the most significant in intensity and duration for the all analyzed period. The level of peak is beyond the 3. σ and its duration is 3 days. The lead time is about 3-4 weeks before the earthquake. At the periods from 3 to 105 s, similar anomalous changes occurred. These facts suggest the anomalous change is a possible candidate of earthquake-related ULF magnetic change. © 2010 Elsevier Ltd.

There have been many reports on ULF electromagnetic phenomena associated with the 2000 Izu Islands earthquake swarm. In this study, seismo-magnetic anomalies are presented by examining energy variations of signatures at the periods around 100 s. Geomagnetic data observed at three stations in Izu Peninsula from 1 January 2000 to 30 April 2001 have been analyzed and wavelet transform has been performed. In order to indentify anomalous changes from ionosphere disturbances, Kakioka station has been chosen as a reference station, and the similar data analysis has been performed. The results suggest that the unusual energy enhancement of the Z component, which only appears in Izu Peninsula from late June until early November 2000, might possibly be one of electromagnetic phenomena associated with the 2000 Izu Islands earthquake swarm. © 2011 Author(s).

Electromagnetic phenomena associated with crustal activities have been reported in a wide frequency range (DC-HF). In particular, ULF electromagnetic phenomena are the most promising among them because of the deeper skin depth. However, ULF geoelctromagnetic data are a superposition of signals of different origins. They originated from interactions between the geomagnetic field and the solar wind, leak current by a DC-driven train (train noise), precipitation, and so on. In general, the intensity of electromagnetic signals associated with crustal activity is smaller than the above variations. Therefore, in order to detect a smaller signal, signal discrimination such as noise reduction or identification of noises is very important. In this paper, the singular spectrum analysis (SSA) has been performed to detect the DC-driven train noise in geoelectric potential difference data. The aim of this paper is to develop an effective algorithm for the DC-driven train noise detection. © 2011 Author(s).

A tomographic approach is used to investigate the fine structure of electron density in the ionosphere. In the present paper, the Residual Minimization Training Neural Network (RMTNN) method is selected as the ionospheric tomography with which to investigate the detailed structure that may be associated with earthquakes. The 2007 Southern Sumatra earthquake (M = 8.5) was selected because significant decreases in the Total Electron Content (TEC) have been confirmed by GPS and global ionosphere map (GIM) analyses. The results of the RMTNN approach are consistent with those of TEC approaches. With respect to the analyzed earthquake, we observed significant decreases at heights of 250-400 km, especially at 330 km. However, the height that yields the maximum electron density does not change. In the obtained structures, the regions of decrease are located on the southwest and southeast sides of the Integrated Electron Content (IEC) (altitudes in the range of 400-550 km) and on the southern side of the IEC (altitudes in the range of 250-400 km). The global tendency is that the decreased region expands to the east with increasing altitude and concentrates in the Southern hemisphere over the epicenter. These results indicate that the RMTNN method is applicable to the estimation of ionospheric electron density. © 2011 Author(s).

Three-dimensional ionospheric tomography is effective for investigations of the dynamics of ionospheric phenomena. However, it is an ill-posed problem in the context of sparse data, and accurate electron density reconstruction is difficult. The Residual Minimization Training Neural Network (RMTNN) tomographic approach, a multilayer neural network trained by minimizing an objective function, allows reconstruction of sparse data. In this study, we validate the reconstruction performance of RMTNN using numerical simulations based on both sufficiently sampled and sparse data. First, we use a simple plasma-bubble model representing the disturbed ionosphere and evaluate the reconstruction performance based on 40 GPS receivers in Japan. We subsequently apply our approach to a sparse data set obtained from 24 receivers in Indonesia. The reconstructed images from the disturbed and sparse data are consistent with the model data, except below 200 km altitude. To improve this performance and limit any discrepancies, we used information on the electron density in the lower ionosphere. The results suggest the restricted RMTNN-tomography-assisted approach is very promising for investigations of ionospheric electron density distributions, including studies of irregular structures in different regions. In particular, RMTNN constrained by low-Earth-orbit satellite data is effective in improving the reconstruction accuracy. Copyright © 2011 by the American Geophysical Union.

We report seismo-ionospheric precursors of anomalous decreases in the total electron content (TEC) appearing day 5 prior to an M9.3 earthquake, the largest one in the last five decades, which occurred in Sumatra-Andaman, Indonesia on 26 December 2004. Sequences of global ionosphere maps of the TEC derived from worldwide ground-based receivers of the global positioning system (GPS) are used to statistically study the temporal and spatial precursors of the earthquake. It was found that the temporal precursor of the GPS TEC around the epicenter was significantly reduced during the afternoon period on d 5 before the earthquake. The spatial precursors prominently, persistently, and simultaneously appear around the epicenter and its conjugate areas of the SumatraAndaman earthquake.

Many researchers studied the relationships between appearances of geomagnetic anomalies and their distances to earthquake epicenters or faults. Yet, occasionally some magnetometer stations located nearby earthquake epicenters and/or faults do not observe geomagnetic anomalies. In this paper, a new hybrid system which simultaneously takes the hypocenter and fault plane solution into account is constructed to examine 38 earthquakes interpreted to be associated with geomagnetic anomalies during the period 1988-2001 in Taiwan. The Surface Magnetic Anomaly Reference Tip (SMART) of the new system is used instead of the epicenter or the fault to investigate statistically the distance relationship between the anomalies and the earthquake parameters. Results show that the anomalies gather along the fault and in the belt zone to the SMART. Possible mechanisms causing the anomalies in the two zones are proposed and discussed. Characteristics of the anomaly might shed some light on locations of faults before earthquake occurrences. © 2010 Elsevier B.V.

In this paper, the total electron content (TEC) of Global Ionosphere Maps (GIMs) is normalized and employed to study the seismo-ionospheric anomalies at the time of the 12 May 2008 M 8.0 Wenchuan Earthquake. The space weather conditions are taken into account. It is found that remarkable reductions appear locally around the epicentre and their conjugate points during the daytime of 29 April and 6-10 May 2008. A global study and a strict criterion are applied to detect anomalies. Results show that the anomalies on 29 April and 6 and 7 May 2008, which are respectively days 13, 6 and 5 before, are possibly related to the earthquake. The conjugate signature implies the seismo-generated electric field is essential. © 2010 Taylor & Francis.

Despite its extreme importance and years of efforts, practical short-term earthquake prediction still remains to be achieved in future. However, earthquake-related electromagnetic phenomena are recently considered as a promising candidate for short-term earthquake prediction. There have been accumulated a lot of evidences of precursory signatures in a wide frequency range. In this paper, the recent studies on seismo-electromagnetics such as pre-seismic phenomena such as ULF geomagnetic anomalous changes and ionospheric disturbances associated the 2004 Sumatra-Andaman earthquake are presented.

Scientists analyzed temporal variations of local data recorded in Taiwan and reported anomalous reductions in the ionospheric total electron content (TEC) on days 4 and 3 before (17 and 18 September) the Chi-Chi earthquake (Mw 7.6, ML 7.3), and days 3 and 1 prior to (19 and 21 October) the Chia-Yi earthquake (ML 6.4) in Taiwan. In this paper, we propose a spatial analysis which compares the data recorded inside and outside of the earthquake area around Taiwan to discriminate whether the anomalies are local (earthquake related) or global (non-earthquake related) phenomena. Results suggest that the anomalies appearing day 3 before the Chi-Chi earthquake, and days 3 and 1 before the Chia-Yi earthquake are local (earthquake related) phenomena and the anomaly appearing on 17 September 1999 (day 4 before) might be the result of both global phenomena (i.e., a geomagnetic storm) and the local effect of the Chi-Chi earthquake.

Despite its extreme importance and years of efforts, practical short-term earthquake prediction still remains to be achieved in future. However, earthquake-related electromagnetic phenomena are recently considered as a promising candidate for short-term earthquake prediction. There have been accumulated a lot of evidences of precursory signatures in a wide frequency range. This paper deals with various electromagnetic phenomena associated with the 2004 Sumatra-Andaman earthquake (M9) in order to show the state of the art of seismo-electromagnetics for prediction research. Both post- and/or co-seismic and pre-seismic phenomena such as ULF geomagnetic and ionospheric disturbances are presented.

The global ionospheric map (GIM) is used to observe variations in the total electron content (TEC) of the global positioning system (GPS) associated with 35 M ≥ 6.0 earthquakes that occurred in China during the 10-year period of 1 May 1998 to 30 April 2008. The statistical result indicates that the GPS TEC above the epicenter often pronouncedly decreases on day 3-5 before 17 M ≥ 6.3 earthquakes. The GPS TEC of the GIM and electron density profiles probed by six microsatellites of FORMOSAT3/ COSMIC (F3/C) are further employed to simultaneously observe seismoionospheric anomalies during an Mw7.9 earthquake near Wenchuan, China, on 12 May 2008. It is found that GPS TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6-4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions, respectively. The F3/C results further show that the ionospheric F2 peak electron density, N nF2, and height, hmF2, significantly decreases approximately 40% and descends about 50-80 km, respectively, when the GPS TEC anomalously reduces. Copyright 2009 by the American Geophysical Union.

Significant geoelectric and magnetic field anomalous changes have been observed prior to and during the Mw7.9 of June 4, 2000 Bengkulu earthquake in the southern Sumatran plate boundary, Indonesia. The mainshock occurred in the intra-oceanic slab and triggered the aftershocks that mainly occurred in the subduction interface of Sumatra Island at a distance of about 220 km from our monitoring sites. We obtained a set of electric and magnetic data for a couple of hours before, during and after the mainshock. Changes of electric field of about 5-10 mV/100 m and magnetic field of about 40 pT were observed at all monitoring sites at about 90 min before the mainshock. These anomalous signals occurred at the same time as the global geomagnetic storm that was clearly observed at Biak, Alice Springs and Kakadu Geomagnetic Observatories. A monitoring instrument located at the so-called sensitive site recorded a remarkable change of geoelectric potential of up to 140 mV/100 m at about 40 s after the origin time of the mainshock. These co-seismic anomalous signals occurred coincidently with the arrival of seismic P-wave. We interpret these signal changes in terms of the electrokinetic effect associated with seismic wave propagation. Crown Copyright © 2008.

The Space Environment Research Center of Kyushu University has obtained geomagnetic data in the Circum-pan Pacific Magnetometer Network (CPMN) region for over 10 years, and has recently deployed a new real-time Magnetic Data Acquisition System (MAGDAS) in the CPMN region and an FM-CW radar network along the 210° magnetic meridian (MM) for space weather research and applications. This project intends to get the MAGDAS network fully operational and provide data for studies on space and lithosphere weather. In connection with this project, we propose a new ultra-low frequency (ULF) wave analysis method to study ULF anomalies associated with large earthquakes using magnetic data. From a case study of the 1999/05/12 Kushiro earthquake with magnitude M = 6.4, we found a peculiar increase of H-component power ratio AR/AM of Pc 3 magnetic pulsations a few weeks before the earthquake, where AR is the power obtained at Rikubetsu station (r = 61 km) near the epicenter and AM is the power obtained at a remote reference station, Moshiri (r = 205 km). It is also found that the H-component power ratio AD/AY of Pc 3 increased three times just a few weeks before the earthquake and after one week decreased to the normal level, where AD is one-day power at Rikubetsu station and AY is the one-year-average power. © 2008 Elsevier Ltd. All rights reserved.