- freely available
Sensors 2007, 7(7), 1108-1122; doi:10.3390/s7071108
2. Magnetic field sensors and observation system
3. Analysis method of ULF magnetic field variations
3.1. The ratio of vertical to horizontal components of the magnetic field (polarization analysis)
3.2. Principal component analysis
3.3. Direction finding (magnetic field gradient method)
3.4. Direction finding (Goniometric method)
4. Characteristics of seismogenic ULF emissions and generation mechanism
- There is no doubt that ULF emissions take place as a precursor to a relatively large earthquake. The sensitive distance (R) is 70-80 km for magnitude = 6.0, and ∼ 100 km for magnitude = 7.0. The empirical threshold of detection in Fig. 2 is given by 0.025 R ≦ M -4.5.
- The ULF emissions for large earthquakes (with magnitude greater than 6.0), seem to exhibit a typical temporal evolution. First of all, we have a first peak one month to a few weeks before the earthquake, followed by a quiet period and a significant increase in amplitude a few days before the earthquake.
- The amplitude of those seismogenic ULF emissions is found to range from 0.1 nT to a few nT. However, their frequency spectra are not well understand; that is, what is the predominant frequency? Recent studies indicate the importance of the frequency of 10 mHz (period of 100s).
- The observation of ULF emission is a local measurement. So that, only when our observing station happens to be very close to the earthquake epicenter, we can detect seismogenic emissions. Otherwise it is impossible to detect any seismogenic emission, and this is the reason why we do not have abundant data set as is summarized in Fig. 2. The case of Niigata earthquake, does not follow the above-mentioned threshold, so that we have to think of the generation and subsequent propagation for this case.
5. Future direction on a network of magnetic field observation (three components)
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|Station name||Code||Geographic coordinates||Type of magnetometer||Sampling rate||Remark|
|Seikoshi||SKS||34.90°N, 138.82°E||Torsion||50Hz||Western Izu|
|Mochikoshi||MCK||34.88°N, 138.86°E||Torsion||50Hz||Western Izu|
|Kamo||KMO||34.86°N, 138.83°E||Torsion||50Hz||Western Izu|
|Jaishi||JIS||34.70°N, 138.79°E||Torsion||50Hz||Western Izu|
|Unobe||UNB||35.21°N, 140.20°E||Torsion||50Hz||Southern Boso till May, 2001|
|Fudago||FDG||35.19°N, 140.14°E||Torsion||50Hz||Southern Boso from May, 2001|
|Uchiura||UCU||35.16°N, 140.10°E||Torsion||50Hz||Southern Boso|
|Kiyosumi||KYS||35.16°N, 140.15°E||Torsion||50Hz||Southern Boso|
|Iyogatake||IYG||35.10°N, 139.92°E||Torsion||50Hz||Southern Boso|
|Matsushiro||MTS||36.54°N, 138.21°E||Induction||85Hz||Seismological Observatory|
|Chichibu||CCB||36.00°N, 139.12°E||Induction||85Hz||Now stopped|
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