Abstract. To define a background in the electromagnetic emissions above seismic
regions, it is necessary to define the statistical distribution of the wave
energy in the absence of seismic activity and any other anomalous input (e.g.
solar forcing). This paper presents a completely new method to determine both
the environmental and instrumental backgrounds applied to the entire DEMETER
satellite electric and magnetic field data over L'Aquila. Our technique is
based on a new data analysis tool called ALIF (adaptive local iterative
filtering, Cicone et al., 2016; Cicone and Zhou, 2017; Piersanti et al.,
2017b). To evaluate the instrumental background, we performed a multiscale
statistical analysis in which the instantaneous relative energy
(ϵrel), kurtosis, and Shannon entropy were
calculated. To estimate the environmental background, a map, divided into
1∘×1∘ latitude–longitude cells, of the averaged
relative energy (ϵrel‾), has been constructed,
taking into account the geomagnetic activity conditions, the presence of
seismic activity, and the local time sector of the satellite orbit. Any
distinct signal different (over a certain threshold) from both the
instrumental and environmental backgrounds will be considered as a case event
to be investigated. Interestingly, on 4 April 2009, when DEMETER flew exactly
over L'Aquila at UT = 20:29, an anomalous signal was observed at 333 Hz
on both the electric and magnetic field data, whose characteristics seem to
be related to pre-seismic activity.
Electromagnetic field observations by the DEMETER satellite in connection with the 2009 L'Aquila earthquake