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Ishikawa, Yoichi
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Ishikawa, Yoichi
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- PublicationOpen AccessGeomagnetic changes correlated with crustal movement in the north-eastern part of the Izu Peninsula, Japan(1997-03)
; ; ; ; ; ;Oshiman, N.; Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan ;Sasai, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Honkura, Y.; Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan ;Ishikawa, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Tanaka, Y.; Aso Volcanological Laboratory, Faculty of Science, Kyoto University, Aso, Kumamoto, Japan; ; ; ; After the 1989 sea-floor eruption off the east coast of Ito city, no remarkable activities of earthquake swarms were observed in the eastern part of the Izu Peninsula, Central Japan during the period from 1990 to 1992. However, a small swarm activity was again observed in January, 1993 and a remarkable one took place again in May-June 1993. Several months after the subsidence of the swarm activity, abrupt changes in the crustal movement in the inland of the peninsula were observed during the period from September 1993 to February 1994. At some continuous observation sites, well correlated changes in the geomagnetic total intensity were observed almost during the same period when the anomalous changes in the crustal movement were seen in the eastern part of the peninsula. The spatial patterns of negative changes of the total intensity in the northern half and positive ones in the south were seen in the north-eastern edge of the domed distribution of the upheaval. The changes in crustal movement and the geomagnetic field terminated when a small swarm activity occurred at the end of February 1994.148 666 - PublicationOpen AccessSeismomagnetic models for earthquakes in the eastern part of Izu Peninsula, Central Japan(1997-03)
; ; ;Sasai, Y.; Earthquake Research Institute, The University of Tokyo, Japan ;Ishikawa, Y.; Earthquake Research Institute, The University of Tokyo, Japan; Seismomagnetic changes accompanied by four damaging earthquakes are explained by the piezomagnetic effect observed in the eastern part of Izu Peninsula, Central Japan. Most of the data were obtained by repeat surveys. Although these data suffered electric railway noise, significant magnetic changes were detected at points close to earthquake faults. Coseismic changes can be well interpreted by piezomagnetic models in the case of the 1978 Near Izu-Oshima (M 7.0) and the 1980 East Off Izu Peninsula (M 6.7) earthquakes. A large total intensity change up to 5 nT was observed at a survey point almost above the epicenter of the 1976 Kawazu (M 5.4) earthquake. This change is not explained by a single fault model; a 2-segment fault is suggested. Remarkable precursory and coseismic changes in the total force intensity were observed at KWZ station along with the 1978 Higashi-Izu (M 4.9) earthquake. KWZ station is located very close to a buried subsidiary fault of the M 7.0 Near Izu-Oshima earthquake, which moved aseismically at the time of the M 7.0 quake. The precursory magnetic change to the M 4.9 quake is ascribed to aseismic faulting of this buried fault, while the coseismic rebound to enlargement of the slipping surface at the time of M 4.9 quake. This implies that we observed the formation process of the earthquake nucleation zone via the magnetic field.162 526 - PublicationOpen AccessLong-term geomagnetic changes observed n association with earthquake swarm activities in the Izu Peninsula, Japan(2001-04)
; ; ; ; ; ;Oshiman, N.; Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan ;Sasai, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Honkura, Y.; Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan ;Ishikawa, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Koyama, S.; Earthquake Research Institute, University of Tokyo, Japan; ; ; ; Anomalous crustal uplift has continued since 1976 in the Izu Peninsula, Japan. Earthquake swarms have also occurred intermittently off the coast of Ito since 1978. Observations of the total intensity of the geomagnetic field in the peninsula started in 1976 to detect anomalous changes in association with those crustal activities. In particular, a dense continuous observation network using proton magnetometers was established in the northeastern part of the peninsula, immediately after the sea-floor eruption off the coast of Ito in 1989. No remarkable swarm activities were observed there from 1990 to 1992. However, after the occurrence of a small swarm in January 1993, five large swarm activities were observed. At some observation sites, we observed a remarkable long-term trend in the total geomagnetic field in association with the change in the distribution pattern in the seismicity of the earthquake swarms.111 357