Ichiro Tomizawa

Results of statistical analysis of low-frequency seismogenic EM emissions as precursors to earthquakes and volcanic eruptions

Abstract Since 1980, we have detected impulsive noise bursts of seismogenic emissions at 82 kHz, 1525 Hz and 36 Hz with a multipoint detection network around the Tokyo region and the Izu peninsula. In earlier times, this system has recorded EM signals prior to several earthquake events. And it has also recorded volcanic eruptions on 15 and 21 November 1986 at Mt. Mihara on Ohshima Island, on 12 July 1989 in Itoh Bay in the Izu peninsula region, and since June 1991 at Mt. Unzen on the Shimabara peninsula. To obtain a clear explanation of the source mechanism of these emissions, we are progressing with statistical analyses of these emission characteristics using the data of 29 events over the last 5 years.

Observation of low-frequency electromagnetic emissions as precursors to the volcanic eruption at Mt. Mihara during November, 1986

Abstract At the time of the major volcanic eruptions on November 15 and 21, 1986, at Mt. Mihara, Izu-Ohshima Island, in the Tokyo territory of Japan, one detector of a multi-point (multi-static) direction finding network for detection of electromagnetic emissions was operating. Since October 20, impulsive noise bursts at 82 kHz were recorded, and the occurrence of these anomalous burst-like noise observations increased after November 3. At 10–16 JST (Japanese Standard Time, UT + 9 hr) on November 14, several strong noise bursts were observed, and the first major eruption occurred at the main summit crater of this volcano at 16 JST on November 15. At 10–12 JST on November 21, noise bursts were observed, and 4 hr later, 19 new craters erupted suddenly on the side of the mountain, both inside and outside the caldera. These data are the first observations of electromagnetic emissions as precursors to volcanic eruptions correlated in time with the intrusion of dykes in the mountain body.

Direction finding of precursory radio emissions associated with earthquakes: A proposal

Abstract The direction finding of precursory radio emissions associated with earthquakes is proposed, based on the simultaneous measurement of the waveforms of multiple electric and magnetic field components at ELF (1.525 kHz ± 5 Hz) and ULF (0–10 Hz) at a few stations. This measurement enables us to determine the wave mode of the observed signal (for example, its direction of arrival) by measuring such parameters as the wave impedance, wave polarization and wave coherence. From these observations we hope to be able to obtain the first convincing evidence on the precursory emissions of earthquakes.

Characteristics of TEC variations observed at Chofu for geomagnetic storms

Since the middle of 1996, we have made a routine observation of TEC using GPS in Chofu (35.65°N, 139.54°E), Tokyo. We examine in this paper the relationship between the perturbation components of TEC and geomagnetic field variations during 11 storm events for 1997–1998 to clarify the mid-latitude characteristics of TEC variations at the time of magnetic storms. The perturbation components of TEC were derived every 20 seconds by subtracting quiet-time TEC values which are estimated as the average of both 3 days just before and after the storm period. The magnetic field data from Memambetsu Magnetic Observatory (45.92°N, 144.20°E) are used for the identification of the storm-time variations. Our results show that (1) the amplitude level of the TEC variation tends to increase during the first 24 hr of storm and then decrease below its usual-day level with recovery in one or two days later for the typical magnetic storm, and (2) during a negative-value phase of the TEC variation, which follows the initial positive hump structure, the perturbation amplitude of TEC shows a remarkable reduction in summer compared to in winter.

Coseismic ionospheric disturbances at multiple altitudes associated with the foreshock of Tohoku earthquake observed by HF Doppler sounding

Many studies have reported that ionospheric disturbances occur after big earthquakes. One of the causes of these coseismic ionospheric disturbances (CIDs) is the infrasound wave excited by Rayleigh wave propagated on the ground from the epicenter. The infrasound wave propagates upward and produces CIDs. In this study, using HF Doppler sounding system (HFD), CIDs at the different altitudes associated with the foreshock of Tohoku Earthquake were examined.

Altitude and Spatial Angle Dependences of Natural and Secondary Cosmic Ray Radiations on Terrestrial Electronic Devices

For the purpose of studying the radiation effects on electronic devices on the ground, observations of total-time radiation counts of natural and secondary cosmic rays up to an energy of 12 MeV were carried out at two locations with different altitudes: Chofu (53 m) and Sugadaira (1313 m) in Japan, using a multichannel analyzer (MCA) with a NaI detector. Sugadaira recorded more counts by 15% for the natural rays and by 20% for the cosmic rays than Chofu. This small increasing rate in cosmic rays compared with a 3-times increase in the soft error rate (SER) that is predicted at the altitude at Sugadaira suggested that the SER might originate from the higher energy neutrons above 12 MeV, which account for only 3% (0.3 countsh-1cm-2) of the effect below 12 MeV in the neutrons reaching the ground. Moreover, the spatial angle dependences of radiation were investigated by changing the angle between the NaI detector tube and the ground plane, and it was found that in the center of the direction parallel to the ground plane, the counts indicated a maximum immediately under the ground plane for the natural rays, and a maximum at the zenith for the cosmic rays.