Fig. 8

Source migration of HF bursts. a Logarithm of the envelope of the seismic energy (filtered between 5 and 15 Hz) arriving at each seismic stations. Data are smoothed using a 10 min sliding median filter. b Ratios of seismic energy, \(\text {station}_1/\text {station}_2\), recorded across the network. Considering vertical or horizontal migration, an increase in this ratio corresponds to a source getting closer to \(\text {station}_1\), while a decrease correspond to a source getting closer to \(\text {station}_2\), see text for more details