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Sasai, Y.
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- PublicationRestrictedTime dependent piezomagnetic fields in viscoelastic medium(2008-02)
; ; ; ;Currenti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Del Negro, C.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Sasai, Y.; Disaster Prevention Division, Tokyo Metropolitan Government, Japan; ; We investigated time dependent piezomagnetic fields due to volcanic sources embedded in a viscoelastic, homogeneous half-space. Especially in volcanic areas, the presence of inhomogeneous materials and high temperatures produce a lower effective viscosity of the Earth’s crust that calls for considering anelastic properties of the medium. Piezomagnetic properties are carried by grains of titano-magnetite, which occupy only a small fraction of ordinary rock volume and are supposed to be elastic, while the non-magnetic surrounding matrix is assumed to be viscoelastic. From all the possible rheological models, we investigated two cases in which the bulk modulus is purely elastic and the shear modulus relaxes as: (i) a Maxwell solid and (ii) a standard linear solid (SLS). We applied the Correspondence Principle to the analytical elastic solutions for pressurized spherical sources and dislocation sources in order to determine the time dependent piezomagnetic fields in a viscoelastic medium. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset value for a SLS rheology. These different behaviours provide helpful hints in understanding the temporal evolution of piezomagnetic anomalies in volcanic regions.117 17 - PublicationOpen AccessDrag-out effect of piezomagnetic signals due to a borehole: the Mogi source as an example(2007-02)
; ; ; ; ; ; ; ; ; ; ;Sasai, Y.; Disaster Prevention Division, Bureau of General Affairs, Tokyo Metropolitan Government, Tokyo, Japan ;Johnston, M. J. S.; US Geological Survey, Menlo Park, CA, U.S.A. ;Tanaka, Y.; Graduate School of Science, Kyoto University, Japan ;Mueller, R.; US Geological Survey, Menlo Park, CA, U.S.A. ;Hashimoto, T.; Graduate School of Science, Hokkaido University, Japan ;Utsugi, M.; Graduate School of Science, Kyoto University, Japan ;Sakanaka, S.; Faculty of Engineering and Resource Science, Akita University, Japan ;Uyeshima, M.; Earthquake Research Institute, The University of Tokyo, Japan ;Zlotnicki, J.; Observatoire de Physique du Globe de Clermont-Ferrand, France ;Yvetot, P.; Observatoire de Physique du Globe de Clermont-Ferrand, France; ; ; ; ; ; ; ; ; We show that using borehole measurements in tectonomagnetic experiments allows enhancement of the observed signals. New magnetic dipoles, which vary with stress changes from mechanical sources, are produced on the walls of the borehole. We evaluate such an effect quantitatively. First we formulate a general expression for the borehole effect due to any arbitrary source models. This is valid everywhere above the ground surface as well as within the cylindrical hole. A first-order approximate solution is given by a line of horizontal dipoles and vertical quadrupoles along the central axis of the borehole, which is valid above the ground surface and a slightly away (several tens of cm) from the top of the borehole. Selecting the Mogi model as an example, we numerically evaluated the borehole effect. It turned out that the vertical quadrupoles produce two orders of magnitude more intense magnetic field than the horizontal dipoles. The borehole effect is very local, i.e. detectable only within a few m from its outlet, since it is of the same order or more than the case without a borehole. However, magnetic lines of force cannot reach the ground surface from a deeper portion (>10 m) of a borehole.127 162 - PublicationRestrictedTime dependent piezomagnetic fields in viscoelastic medium(2007)
; ; ; ;Currenti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Del Negro, C.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Sasai, Y.; Disaster Prevention Division, Tokyo Metropolitan Government, Japan; ; We investigated time dependent piezomagnetic fields due to volcanic sources embedded in a viscoelastic, homogeneous half-space. Especially in volcanic areas, the presence of inhomogeneous materials and high temperatures produce a lower effective viscosity of the Earth’s crust that calls for considering anelastic properties of the medium. Piezomagnetic properties are carried by grains of titano-magnetite, which occupy only a small fraction of ordinary rock volume and are supposed to be elastic, while the non-magnetic surrounding matrix is assumed to be viscoelastic. From all the possible rheological models, we investigated two cases in which the bulk modulus is purely elastic and the shear modulus relaxes as: (i) a Maxwell solid and (ii) a standard linear solid (SLS). We applied the Correspondence Principle to the analytical elastic solutions for pressurized spherical sources and dislocation sources in order to determine the time dependent piezomagnetic fields in a viscoelastic medium. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset value for a SLS rheology. These different behaviors provide helpful hints in understanding the temporal evolution of piezomagnetic anomalies in volcanic regions.123 28 - PublicationRestrictedClose temporal correspondence between geomagnetic anomalies and earthquakes during the 2002–2003 eruption of Etna volcano(2007)
; ; ; ; ;Currenti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Del Negro, C.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Johnston, M.; U.S. Geological Survey, Menlo Park, California, USA. ;Sasai, Y.; Disaster Prevention Division, Tokyo Metropolitan Government, Tokyo, Japan.; ; ; The early stages of the 2002–2003 lateral eruption at Mount Etna were accompanied by slow changes (over some hours) and some rapid step offsets in the local magnetic field. At five monitoring locations, the total magnetic field intensity has been measured using continuously operating Overhauser magnetometers at a sampling rate of 10 s. The very unique aspect of these observations is the close temporal correspondence between magnetic field offsets and earthquakes that occurred in the upper northern flank of the volcano on 27 October 2002 prior to a primary eruption. Rapid coseismic changes of the magnetic field were clearly identified for three of the most energetic earthquakes, which were concentrated along the Northeast Rift at a depth of about 1 km below sea level. Coseismic magnetic signals, with amplitudes from 0.5 to 2.5 nT, have been detected for three of the largest seismic events located roughly midway between the magnetic stations. We quantitatively examine possible geophysical mechanisms, which could cause the magnetic anomalies. The comparison between magnetic data, seismicity and surface phenomena implies that piezomagnetic effects are the primary physical mechanism responsible for the observed magnetic anomalies although the detailed cause of the rapid high stress change required is not clear. The modeling of the observed coseismic magnetic changes in terms of piezomagnetic mechanism provides further evidence of the complex interaction between volcanic and tectonic processes during dike propagation along the Northeast Rift.164 21 - PublicationOpen AccessSelf-potential chenges associated with volcanic activity: Short-term signals associated with March 9, 1998 eruption on La Fournaise volcano (Reunion Island)(2001-04)
; ; ; ; ; ;Zlotnicki, J.; UMR6530, Clermont-Ferrand, France ;Le mouel, J. L.; Laboratoire de GéomagnèTisme, IPGP, Paris, France ;Sasai, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Yvetot, P.; UMR6524, Laboratoire de Geophysique d'Orleans, France ;Ardisson, M. H.; UMR6524, Laboratoire de Geophysique d'Orleans, France; ; ; ; After six years of quietness La Fournaise volcano entered into activity on March 9, 1998. Fissures opened gradually downwards on the northern flank of the cone. Two cones, Kapor and Krafft built, from which lava poured until September 1998. Several other vents opened during this eruption. Mappings, surveys, and continuous recordings of the Self-Potential have been performed on the volcano for twenty years. SP mappings disclose the variability of large scale SP anomalies due to the modification of the hydrothermal system over some ten years. Most of the eruptions take place along a Main Fracture Zone (MFZ) in which ground water flows prevail. SP measurements have also regularly been made on the northern flank of the cone, on a west-east profile crossing the MFZ. Between 1981 and 1992 an enlargement and a shift of the MFZ to the east are evidenced. In particular, the eastern fissural axis trending N35°E could be related to the possible collapse of the east flank of the volcano. After a decrease between 1992 and 1997, the SP anomaly was enhanced again by the 1998 eruption. Short scale, about 250 m wide, 750 mV amplitude anomalies were superimposed on a large scale one, 2500 m wide, and about 250 mV in amplitude. For several years, continuous stations have been measuring the electric field along two directions, with a 20 s sampling, in order to record the genesis of SP signals associated with the volcanic activity. Oscillations belonging to the ULF band were evidenced several days before the 1988 eruption, some of them at 9 km from the summit. Their amplitude reached several tens mV/km. These oscillations sometimes present a phase lag from one station to another; they progressively shift towards the location of the future effusive vents. The polarisation of the oscillations is similar to the polarisation of longer SP variations (1 h period or more) and are correlated with the structural anisotropy. Finally, during the last hours preceding the effusive activity, huge SP signals, up to a few Volts/km, appeared at the stations located on the MFZ, and especially on the branch where the magma migrated. We interpret these SP signals as due to electrokinetic effects generated by fluid flow in cracks opened by the stress field changes.154 251 - PublicationOpen AccessTectonomagnetic modeling based on the piezomagnetism: a review(2001-04)
; ;Sasai, Y.; Earthquake Research Institute, The University of Tokyo, JapanDevelopment of tectonomagnetic modeling on the basis of the piezomagnetic effect is reviewed for the period since the early 1990's. First, the basic theory is briefly summarized, in which the representation theorem or the surface integral representation for the piezomagnetic potential and the Green's function method are presented. In the 1990's, several field observations in earthquakes and volcanoes were interpreted with the aid of analytic solutions based on the Green's function method. A general formula was developed for an inclined rectangular fault with strike-slip, dip-slip and tensile faulting. The surface integral method has been applied to 2D and 3D models, as well as to fault models in the inhomogeneously magnetized crust. When the magnetic field is measured within a bore hole, the effect of magnetic poles around the hole should be taken into account. As a result, tectonomagnetic signals are much enhanced in a bore hole compared with on the ground surface. Finally, piezomagnetic field changes associated with the Parkfield fault model are introduced and the new aspect of the model is discussed.223 333 - 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.111 365 - PublicationOpen AccessEvaluation of electric and magnetic field monitoring of Miyake-jima volcano monitoring of Miyake-jima volcanomonitoring of Miyake-jima volcano (Central Japan): 1995-1999(2001-04)
; ; ; ; ; ; ; ; ;Sasai, Y.; Earthquake Research Institute, University of Tokyo, Japan ;Zlotnicki, J.; Observatoire de Physique du globe de Clermont-Ferrand, Universitè Blaise Pascal et UMR6524, Aubiére cedex, France ;Nishida, Y.; Graduate school of science, Hokkaiodo University, Sappoto, Japan ;Uyeshima, M.; Earthquake Research Institute, University of Tokyo, Japan ;Yvetot, P.; Observatoire de Physique du globe de Clermont-Ferrand, Universitè Blaise Pascal et UMR6524, Aubiére cedex, France ;Tanaka, Y.; Volcano Research Center; Graduate Schoool of science, Kyoto University, Aso, Kumamoto, Japan ;Watanabe, H.; Earthquake Research Institute, University of Tokyo, Japan ;Takahashi, Y.; Earthquake Research Institute, University of Tokyo, Japan; ; ; ; ; ; ; full abstract in pdf130 452 - 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 1994.148 668 - 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.162 528