Fig. 8
From: On the observation of magnetic events on broad-band seismometers
Collocated electric and seismic signals related to subway and tramway activity. a Electric field (V/m, red line) and seismic vertical acceleration (nm/s2, black line) filtered between 0.005 and 0.05 Hz recorded between October 18 and 23, 2018. Intervals without subway activity are marked with blue boxes. The upper panel shows the envelope of the same signals, downsampled to one sample every 20 min. Dotted circle highlights the difference between electric field and seismic data during the Saturday to Sunday night (see text). b Spectrograms of the seismic acceleration (right) and electric field (left) expressed in dB