The bizarre waveform is what scientists call “monochromatic”. Earthquakes normally produce waves of so many different frequencies, the wave readings appear more jumbled.
But the mystery waveform from Mayotte was a crisp zigzag, which repeated after steady 17-second intervals.
“They're too nice. They're too perfect to be nature,” joked the University of Glasgow’s Helen Robinson, who is study for a PhD in applied volcanology.
https://www.independent.co.uk/news/world/earthquake-seismic-waves-mayotte-madagascar-volcanic-activity-science-a8659236.html