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T‐phase and tsunami pressure waveforms recorded by near‐source IMS water‐column hydrophone triplets during the 2015 Chile earthquake

Underwater acoustic signal waveforms recorded during the 2015 Chile earthquake (Mw 8.3) by the hydrophones of hydroacoustic station HA03, located at the Juan Fernandez Islands, are analyzed. HA03 is part of the Comprehensive Nuclear‐Test‐Ban Treaty International Monitoring System. The interest in the particular data set stems from the fact that HA03 is located only approximately 700 km SW from the epicenter of the earthquake. This makes it possible to study aspects of the signal associated with the tsunamigenic earthquake, which would be more difficult to detect had the hydrophones been located far from the source. The analysis shows that the direction of arrival of the T phase can be estimated by means of a three‐step preprocessing technique which circumvents spatial aliasing caused by the hydrophone spacing, the latter being large compared to the wavelength. Following this preprocessing step, standard frequency‐wave number analysis (F‐K analysis) can accurately estimate back azimuth and slowness of T‐phase signals. The data analysis also shows that the dispersive tsunami signals can be identified by the water‐column hydrophones at the time when the tsunami surface gravity wave reaches the station.