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Johnston, M. J. S.
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- 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 - 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