Abstract

The northwestern Deccan volcanic province in India is one of the most seismically active intraplate regions of the world. In addition, the region is associated with episodic swarm activity and reports of sounds, whose linkage hitherto remains elusive. During the month of January 2016, a swarm activity occurred in the Kachchh and Saurashtra regions and continued for about two months. Many of the events were accompanied by audible sounds, like blasting, that caused severe panic among local residents, prompting us to investigate the causative mechanism. The events were recorded by our seismic stations and an additional five stations that we were able to deploy at the onset of the swarm. The activity produced sounds with good energy in the audible frequency range of humans. Spectrogram analysis of the events with associated sounds revealed frequencies 20  Hz, in contrast to the lower frequencies for those that did not generate the sounds. In addition to the higher frequencies, we observed horizontal particle motion that was dominated by retrograde elliptical motion consistent with Rayleigh waves. These observations were not recorded from any of the events that did not generate sounds. Audible sounds generated by earthquakes are consistently reported from shallow earthquakes that generate high‐frequency surface waves.

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