著者

尾池 和夫 山田 聡治

出版者

独立行政法人防災科学技術研究所

雑誌

防災科学技術研究所研究資料 (ISSN:0917057X)

巻号頁・発行日

vol.166, pp.161-175, 1995-03-31

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The temporal variation of the number of LF and VLF noises has been compared with the occurrence of near shallow earthquakes. The results have been described by Oike et al.(1992), Oike and Murakami(1993) and Oike et al.(1993). In the case of 70 per cent of large earthquakes with magnitude larger than or equal to 6.0 anomalous increase of LF noises is observed within one day before the main shock whose epicentres are located in the land or shallow sea region. Similar phenomena are found in the case of near shallow earthquakes with magnitude larger than or equal to 5.0. Also inthe case of the largest earthquake (M7.8) during the observation a similar increase of LF and VLF noises as above mentioned was recorded. New observation system has been developed to record the wave forms of such phenomena and wave forms of co-seismic radiations from hypo-centers of large earthquakes. We have developed observation systems of electromagnetic radiations (EMR) related with the occurrence of earthquakes. Observing EMR at many and various observation points is important to detect EMR related with earthquakes. It is also important to observe them in low and wide frequency ranges for understanding the characteristics of the waves. We chose the ball antenna as a sensor which has sensitivity in low and wide frequency range (Ogawa et al., 1966). It is difficult to cover all frequency ranges by only one recorder, so we divided signals into several frequency bands and designed the most suitable filter and recording system for each band. We have developed handy observation system for EMR in the frequency range from 80Hz to 20kHz. Using this system, we can easily record EMR at any points and at any time. For example, going to the region where earthquake swarm is occurring, we can record EMR phenomena in the hypo-central region. Observation system for basic stations can record EMR in frequency range from DC to 20kHz broader than the handy system. By this system, we can get and make detailed analysis of records of EMR. By these two systems, we can expect to discuss EMR phenomena related with earthquakes. Such phenomena are possible to be caused by various mechanisms. It is also important to observe and analyze EMR from lightnings, because there is a possibility of the physical relationship between earthquakes and lightnings (Oike et al, 1993). During the observation using the developed recording systems the large earthquake of magnitude 7.8 occurred in the southwestern off Hokkaido region. Before and after the earthquake the anomalous increase of the number of LF and ELF noises were observed by the usual system and coseismic anomalous signals were recorded by the newly developed handy recording system.