Review of unprecedented ULF electromagnetic anomalous emissions possibly related to theWenchuan MS = 8.0 earthquake, on 12 May 2008
Abstract
This work presents ground based ultra low frequency
(ULF) electromagnetic field measurements in the
frequency range 0.1–10 Hz from January 2007 to December
2008. In this time period a strong earthquake series
hits the Wenchuan region with a main shock of magnitude
MS =8.0 on 12 May 2008. The Hebei ULF electromagnetic
observation network includes eight observation stations in
north China and the observation system named E-EM is employed
to record the electric potential difference between two
electrodes with an analog automatic real-time continuous pen
recorder. First, weak electric signals appeared on 11 October
2007 at Ningjin station, most of which are with relative
long periods 0.4–3 s and unequal amplitudes 0.5–20 mm.
Then, similar signals appeared at Gaobeidian station at the
end of October. Abnormal behavior with various time intervals
appeared randomly and not every day. At the beginning
of April 2008, one and a half months before the Wenchuan
MS =8.0 earthquake, the anomalies were gradually subject
to an intensive increase mainly in Gaobeidian SN direction
and Ningjin EW direction. The abnormal behavior appeared
almost every day and the amplitudes of electric signals, with
short periods of 0.1–0.3 s, enhanced to 3–30 mm. Qingxian
station started to record marginally high frequency signals
in SN and EW components in the middle of April. On
9 May, 3 days before the main shock, the amplitude of high
frequency information increased sharply at the same time
in two components at Gaobeidian station and the maximum
amplitude was up to 70 mm, i.e. 1.3mVm−1 for the electric
field. This situation did not stop until 17 May, 5 days after the
main event. However, this kind of climax phenomena did not
happen at Ningjin station and Qingxian station. Then weak
anomalous information lasted about four months again, and
strong signals appeared again for a short time before several
powerful aftershocks. It is the first time that an abnormity
with so large an amplitude and so long a duration time in
the observation history of this network though several strong
earthquakes were recorded. Furthermore, no obvious interferences
have been found during this period. So this event is
possibly related to this shock although all these three stations
are more than 1300 km away from the Wenchuan earthquake
epicenter.
Origin : Publication funded by an institution
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