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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Poster presentation 2004
Analytical study on piezomagnetic effect due to 3-D magnetic structure
and it's applications to the earthquakes in Japan
Prof. Yasunori Nishida
Institute of Seismology and Volcanology Faculty of Science IAGA
Mitsuru Utsugi, Toru Mogi
We developed an analytical form of the piezomagnetic field due to a non-uniformly magnetized medium.
In order to represent the non-uniform distribution of the crustal magnetization, we divide the crust into
a number of blocks assuming that each block has its own magnetic properties. The piezomagnetic
potential derived from a single block is expressed by a definite surface integral of an integrand, which
includes the magnetization change and displacement of the medium, over the boundary surface of the
block (the representation theorem by Sasai, 1991). We show the surface integral is reduced analytically
to the line integral which has the integrand including elliptic integrals. The line integral is calculated
numerically to represent the piezomagnetic potential. This method of analysis is advantageous to fast
computation of the piezomagnetic fields. No co-seismic geomagnetic changes related with the 2003
Tokachi-oki earthquake (M8.0) and the 2004 Kushiro-oki earthquake (M7.1) in Hokkaido, Japan were
observed at a magnetic station whose epicentral distances were about 120 km and 50 km, respectively.
Applying the above-mentioned analytical method, model calculations showed that co-seismic
piezomagnetic fields did not amount to 1 nT at the station in both cases when we assumed the relevant
fault parameters, in-situ Curie temperature depth, subsurface 3-D magnetic structure and stress
sensitivity of rocks. Therefore, it may be reasonable that we could not detect the piezomagnetic signals
at the station. We also made model calculations to forecast the piezomagnetic amplitudes caused by
M7.9 and M8.5 earthquakes predicted to occur along the southern Kurile trench and by M7.8
earthquake in the central Japan (Tokai earthquake) in the future. The model calculations reveal the
piezomagnetic fields up to about 4 nT and 7 nT are expected in the eastern part of the Hokkaido island
for the M7.9 and M8.5 earthquakes and +25 nT on the Sagami trough for the M7.8 earthquake,
encouraging magnetic observations hereafter.
Keywords: piezomagnetism, theory, observations