TITLE:
An Integrated Study of ULF Magnetic Field Variations in Association with the 2008 Sichuan Earthquake, on the Basis of Statistical and Critical Analyses
AUTHORS:
Masashi Hayakawa, Alexander Schekotov, Stelios M. Potirakis, Konstantinos Eftaxias, Qi Li, Tomokazu Asano
KEYWORDS:
2008 Sichuan EQ, ULF Magnetic Field Variations, Statistical Analysis, Critical Analysis, EQ Precursor
JOURNAL NAME:
Open Journal of Earthquake Research,
Vol.4 No.3,
July
24,
2015
ABSTRACT: The ultra-low frequency (ULF) magnetic field data at a station very close to the 2008 Sichuan
earthquake (EQ) (on 12 May, 2008; M = 8.0) are extensively studied on the basis of combined statistical
and natural time analyses. Two effects in ULF are treated: one is the well-known ULF radiation
from the lithosphere, and the other is the non-conventional depression of ULF horizontal
magnetic field. The simple statistical analysis has yielded: 1) no clear evidence of the presence of
precursory ULF radiation, and 2) a significant effect of depression of ULF horizontal field a few
days before the EQ (as a signature of ionospheric perturbations). The recently introduced natural
time analysis has also been performed in order to study the critical features of the lithosphere and
essentially new information has been brought about. The parameter “polarization”, as the ratio of
vertical to horizontal components, showed critical features in the time period of 17 - 27 April,
about one month to two weeks before the EQ as a signature of lithospheric radiation. Then, the
natural time analysis has reconfirmed the presence of ionospheric perturbations a few days before the EQ, together with an additional time window found on 19 - 23 April, about one month before
the EQ, exhibiting critical features in the ULF depression.