Secular variation of the geomagnetic field intensity during the past 2000 years in Japan

Abstract

We report the results of paleointensity determination with volcanic rocks of Izu-Oshima and Fuji Volcano, Japan. Samples used for the present study were basaltic rocks from nine lava flow units of Oshima Volcano, which erupted between AD 590 and 1950, and Aokigahara lava flow derived from Fuji Volcano in AD 864. Demagnetization and several rock magnetic experiments show that the samples have stable, single-component remanences, which are carried by magnetite or titanomagnetite with the PSD magnetic domain state. Thellier’s method was used for the paleointensity experiments, and reliable determinations for 10 eruptive units were obtained from 58 samples in total. The unit-mean values generally show good consistency with the previous archaeointensity and paleointensity results, allowing us to construct a reference curve of the field intensity variation for the past 2000 yr in Japan. The reference curve has a general trend of decreasing intensity since AD 500, showing two peaks of the field strength around AD 500 and 1400 and two troughs around AD 1200 and 1700. Since around AD 800, the field intensity predictions for Japan by the inversion spherical harmonic models show good consistency with the reference curve, suggesting that the peak and the troughs in this period have probably been produced mainly by non-dipole sources. The geocentric axial dipole variation extracted from the reference curve with the inversion model shows good agreement with the global averages of the virtual axial dipole moment, except that our reference curve yields a significant peak around AD 500. This suggests that a temporal strengthening of the dipole moment might have occurred over a few hundred years around AD 500.