This is the spectrogram of the acoustic signal received at hydroacoustic station HA03 on Chile’s Robinson Crusoe Island. The red patch with the very sharp cut-on is the P-phase, i.e. a seismic wave in the crust going past the hydrophone and radiating sound into the water as it goes. The loudest part of the signal is about 1000 seconds later and is the arrival of the T-phase, i.e. the noise made at the epicentre after it has travelled through the ocean. The horizontal lines are a mixture of ship noise and whale noise. The recording of the event has been sped up by a factor of 64 to make it audible. All IMS (International Monitoring System) stations in the area are operational.