After the 21 September 1999 M7.6 devastating earthquake, a program entitled the integrated Search for Taiwan Earthquake Precursor (iSTEP-1, 2002/4/1-2006/3/31), which consists of a main project and five sub-projects, was conducted to search credible precursors in seismological variations, geomagnetic and gravity fields, ground surface deformations, and ionospheric electron density anomalies, as well as to evaluate the statistical significance of observed precursors in Taiwan. Results reveal that anomalies in P-wave velocity, ground surface deformation, geomagnetic field intensity, ionospheric electron density could appear few years, months, and days before large earthquakes in Taiwan, respectively. An integrated ground-based seismo-electromagnetic observation system, including eight networks of magnetometers, electrode arrays, corona probes, FM tuners, Doppler sounding systems, ionosondes, GPS receivers, and all sky cameras, has been constructed and routinely operating to monitor earthquake precursors in the lithosphere, atmosphere, and ionosphere and to find possible lithosphere-atmosphere-ionosphere coupling in the Taiwan area. Several statistical analyses were developed to validate the observed anomalies to be credible precursors. Due to its worldwide availability, the statistical results showed that the ionospheric total electron content (TEC) derived by ground-based GPS receivers were most likely to be a credible precursor. Succeeding the iSTEP-1, the iSTEP-2 (integrated Study for Taiwan Earthquake Precursors, 2006/8-2012/7) project adding with satellite observations was conducted to have a longer time period for data collection and analysis, as well as to develop physical and statistical models. Although it was not officially funded but supported by basic ionospheric research projects, the integrated ground-based observation still has been operating uninterruptedly. Many new observations possibly related to seismo-lithospheric precursors of the earth's surface magnetic field and the GPS surface deformation, seismo-atmospheric precursors of the infrasound signal, and seismo-ionospheric precursors (SIPs) in the electron density profile, the electron temperature, ion density, and neutral temperature probed by satellites were reported. The TEC in the global ionosphere map (GIM) routinely published (with a 2- or 4-day time delay) allows us to monitor temporal SIPs at a specific location, and to conduct spatial analysis discriminating the observed SIPs from global effects, such as solar flares, magnetic storms, etc. Statistical analyses for detecting both temporal and spatial precursors in the ionospheric TEC are developed. Meanwhile, ionospheric model simulations are also introduced to find causal mechanisms explaining the observed SIPs. The iSTEP-3 (integrated Study for Taiwan Earthquake Precursors, 2012/8-2016/7), which is proposed to focus on the SIP study, consisting of a main project and three sub-projects is formally funded. The main project continues to operate the integrated ground-based observation system, develops physical models, and compares model simulations with observed precursors, while the three sub-projects aim to develop a near real-time GIM with a 4-hour time delay for worldwide SIP monitoring, to monitor lithosphere, atmosphere, and ionosphere precursors, to find the precursor link, and to conduct earthquake hazard assessment with observed precursors, respectively.

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In order to confirm whether electromagnetic (EM) waves are excited by earthquakes or not, we have been observing EM signal in the earth, using an EM sensor system installed into a deep borehole. By shifting down the monitoring frequency from VLF to ELF range in December 2011, we have been able to detect EM signals excited by seismic waves, and have clarified the excitation mechanism of the EM pulses in the earth. Through detailed analysis of the EM signals based on the clarified behaviors of EM waves excited by seismic waves, we have found a method of deriving electrical parameters of the earth's medium.

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We have carried out rock fracture experiments in order to elucidate the mechanism of microwave emission due to rock fracture. We revealed experimentally that microwave is emitted when metals contact each other. If metal to metal contacts happen in the rock fracture machine, it is concerned that the desired microwave emission by rock fracture is contaminated by the emission originated by the metal to metal contact. Therefore, we connected and shortened all parts of the machine by wires in order to keep the homogeneous distribution of potential. As a result, we demonstrated that the emission due to metal to metal contact was almost completely suppressed. Pursuing the rock fracture experiments using the shortened machine, we successfully detected the microwave at 300 MHz in all rock cases.

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Analysis of lightning electromagnetic field using the finite-difference time-domain (FDTD) method have been studied in recent year. However,large-scale analysis on real environment have not been considered,because the FDTD method requires huge computational cost on large-scale analysis. We proposed a three-dimensional moving window FDTD (MW-FDTD) with parallel computation and applied to large-scale analysis of lightning electromagnetic field with a real numerical terrain model. In this paper, we have studied about large-scale analysis of lightning electromagnetic field with real urban models considering many buildings using our MW-FDTD with parallel computation. We have shown effects of observed electromagnetic field on urban model.

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Our research group has been measuring Extremely Low Frequency (ELF) magnetic fields across Japan. The ELF measurements are mixtures of signals associated with various natural or artificial phenomena. When focus on specific factor, the signals related to other factors distort analysis result. In order to get specific information accurately, we should estimate desired signals or eliminate undesired signals. We found that Image Space Reconstruction Algorithm (ISRA), one of the Nonnegative Matrix Factorization (NMF) algorithm, works better than independent component analysis to estimate the ELF background signal. However, ISRA sometimes failed to estimate the weight vector for the background signal. We considered that ISRA has weakness for outliers and sparse signals because ISRA is based on minimizing L2 (Frobenius) norm between input matrix and projected matrix from estimated matrices. In order to improve robustness, we developed new methods based on minimizing quasi-L1 norm (QL1-NMF). In the experiment using generated signals and ELF observed signals which had outliers, the proposed method estimate background signal more accurately than ISRA and other L1 norm based algorithms.

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Recently, it was shown that the shape of lightning-generated induction magnetic field waveforms in the ELF frequency range is well comparable to that of lightning current waveforms. Therefore, charge amounts of any lightning discharges occurring within the area where the induction magnetic fields are measured can be easily estimated from ELF waveforms by quantitatively evaluating the relation between ELF waveforms and the current waveforms. In this study, we analyzed lightning current waveforms measured by a Rogowski coil installed at Mt. Ogkami and ELF waveforms measured at Onagawa observatory. Based on the quantitative comparison between the ELF waveforms and the current waveforms, empirical equations that enable us to directly convert from the magnetic field intensities into the current intensities and charge amount were obtained. Furthermore, using ELF waveform data obtained at Kuju station in Kyushu and the Japan Lightning Detection Network (JLDN) lightning data, peak current values and charge amounts for the lightning discharges occurring when severe downbursts occurred in the Kanto Plain were estimated by applying the empirical equations. Then, we newly found a clear feature showing that the time variation of charge amounts drastically changed just before the downburst onset.

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In order to evaluate the relationship between the coseismic ionospheric perturbations and the ground perturbations, we compare the coseismic disturbances determined by a numerical simulation with those observed by HF Doppler. HF Doppler observation determines the ionospheric vertical drift speed of the reflection point. The numerical simulation calculates the temporal evolution of the neutral acoustic wave, In this numerical simulation, the ground perturbations observed by seismometers closest to the reflection points are used as sources of the perturbations. The speed estimated from HF Doppler is about an order larger than that calculated from numerical simulation. The difference is because the speed of the ionospheric vertical drift is not same as the neutral atmospheric wave. In order to fix this difference, the ionospheric neutral drift speed is modified considering the effects of the inclination of the magnetic field and the compression of ionospheric plasma. As a result, the both speed are comparable each other.

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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.

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We performed the frequency analysis of time-series data of TEC, and examined the variations of TEC associated with earthquakes. Using Fast Fourier Transform processing, the spectral densities of coseismic TEC variations were calculated. The coseismic variations of TEC were detected in 16 events out of 26 events where the earthquakes larger than M6.4 occurred around Japan since 2000. In most events, the variations at the frequencies of 4.17, 5.21 mHz were observed. The spectral densities reached their maximums about 11 minutes after the earthquake. Since this delay corresponds to the propagation time of acoustic wave from the ground to the ionosphere, it is confirmed that the coseismic variations are due to the acoustic wave generated by the ground/sea surface perturbations. The enhancement of the signal density increases with the magnitude of earthquake and the height of tsunami. Using the enhancement of the TEC variation, the magnitude of wave source can be estimated.

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In order to examine the direction of arrival of VHF radio waves dependent on the natural phenomena, the interferometers have been installed at Katsuyama, Chiba Prefecture and Numata, Gunma Prefecture. In this study, the radio wave at the frequency of 77.1 MHz transmitted from Tokyo Tower (assigned to FM broadcasting of the Open University of Japan) is observed. The interferometers consist of four Yagi antennas and estimate the directions of arrivals of the radio wave. The observational results show that the directions of the wave are affected by the disturbances in the ionosphere and the atmosphere. In this paper, the system of the interferometers is described and the examples of the observation results are presented.

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The Shonai area railroad weather project has investigated fine-scale structure of wind gust dynamics and kinetics such as tornadoes, downbursts, and gust fronts. In this project, in order to improve our understanding of near-surface tornadic features, we developed a linear array of wind and pressure sensors for high resolution in situ measurements in winter tornado cores.  The wind and pressure sensors were deployed along a 1.2-km-long linear array that is located parallel to and about 100 m from the shoreline. Wind data are obtained using 12 two- dimensional fast-response ultrasonic anemometers placed at a height of 5 m at intervals of 100 m. Pressure data are obtained using 25 barometers placed at a height of 50 cm at intervals of 50 m. The pressure ports are designed and loaded to the barometers to reduce the dynamic pressure associated with wind and turbulence. Sampling intervals for both wind and pressure measurements are 100 msec and the sampling are synchronized by the external trigger from data logging computer. In this study, the system overview and the measurement technique will be described, as well as an example of actual winter tornado observations of the system and X-band Doppler radar simultaneously.

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Study of the electric self-potential (SP) and the magnetic field was conducted at the explosion seismic experiment in Ooyama, Toyama Prefecture. The explosion was induced by 450 kg gun powder placed at a depth of 32.2-75 m. The SP was measured by four electrodes arranged in a line crossing the explosion point, where the distance of each electrode from the explosion ranged from 10 to 100 m. Induced SP changes consisted of two portions, namely, fluctuations with shorter periods (∼10 ms) and fluctuations with longer periods (∼50 ms). Regarding the latter, the amplitude was in proportion to the span length of electrode couples and the fluctuations arrived faster at the electrode couples nearer to the explosion. Further, the increment of ground level in SP was observed in each electrode couple after the explosion, which was concordant with the distribution of apparent resistivity underground studied by the survey. The magnetic field measured by the fluxgate magnetometer at the site 35 m north from the explosion also showed change of a few nT after the explosion. This may have been caused by change of magnetic properties underground.

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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.

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We consider an problem identifying electron density profile in the lower ionosphere by observing electric fields at the Earth's surface in very-low-frequency (VLF) band. The identification problem is one of nonlinear inverse problems, and we solve the problem numerically by using the genetic algorithm in this study. The focus of this study is on appropriate parameters to solve the identification problem by using the VLF (22.2 kHz) wave propagating in the Earth-ionosphere waveguide. Numerical results show that the range of the electron density which can be identified is from 10⁷-10⁸ m^-3, appropriate discretization of the solution space along the altitude is 10 km, and a sufficient number of observation points are necessary to mitigate the effect of noise.

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In order to verify the seismo-electric phenomena before large earthquakes (EQs), stress-stimulated charges (SSCs) in the lithosphere were assumed and the frequency characteristic was evaluated for the electromagnetic field caused by the SSCs, considering a charge generation coefficient and resistivity in crust. Time variation of SSCs was dependent on the stress-changing rate and the degree of dielectric relaxation. Possible electromagnetic spectra due to apparent charges were obtained by the linear system analysis. Because of the low resistivity in ordinary crust, SSCs after a short stress-changing event were quickly recombined and would not contribute much to EQ precursor phenomena. On the other hand, preseismic seismo-electric phenomena such as GPSTEC anomalies are known to last as long as 40 minutes. In order to achieve such long lasting electromagnetic phenomena before an EQ, the resistivity of the interested area should be enough high or the stress changing in crust should last continuously enough long. We can also assume continual stress-changing events at multi-spots around the fault zone which lasting several tens of minutes before large EQs.

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The VLF/LF propagation is extensively used to investigate the ionospheric perturbation in possible association with earthquakes. In this study, we describe about prediction and verification of the South Tokushima Earthquake occurred on 6 February 2015. We pay attention to the propagation of the Miyazaki (JJI) transmitter station and the Anan receiving station.

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In this study, the damping constants of Co/Pt multilayers and hexagonal-closed-packed (hcp) CoPt alloy films are investigated by using a vector network analyzer ferromagnetic resonance spectroscopy (VNA-FMR) and time resolved magneto-optical Kerr effect (TR-MOKE). For Co/Pt multilayer films, very clear linear relationship between FMR frequency and absorption peak width is found. From this relationship, the damping constants of Co/Pt multilayers are evaluated to be about 0.02～0.07 with very small extrinsic contributions. On the other hand, for CoPt alloy films, the absorption peak width becomes wider and exhibits very little FMR frequency dependence. The damping constants of CoPt alloy films evaluated by TR-MOKE, however, are as small as about 0.03～0.06, being almost the same with that Co/Pt multilayers. These discrepancies found in CoPt alloy films can be explained by assuming the large distribution of magnetic anisotropy. Thus the damping constant of CoPt alloy films are some what larger than that of Co/Pt multilayers when these results are compared at same Pt composition.

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This paper deals with dielectric properties of hydrocarbon-based thermosetting resin to evaluate the applicability to electrical insulating materials. Higher thermal stability is needed for various devices as well as electrical insulating materials. Moreover, higher withstand voltage, lower dielectric constant and lower moisture absorption are required for the electrical insulating materials. One of candidate polymer materials satisfying such demands is hydrocarbon-based thermosetting resin that is poly-dicyclopentadiene (P-DCP) and poly-tricyclopentadiene (P-TCP). We measured the dielectric properties such as dielectric constant, dielectric loss, volume resistivity, and breakdown strength, and other properties such as moisture absorption, viscosity, mechanical strength, glass transition temperature, coefficient of linear expansion of those specimens. It was found from the results in particular that P-DCP and P-TCP have the lower dielectric constant and the lower moisture absorption as compared with epoxy resin, while the volume resistivity and the breakdown strength of P-DCP and P-TCP are not different to those of epoxy resin. As a result, it is elucidated that two kinds of hydrocarbon-based thermosetting resin have a potential for application to an electrical insulating material.

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