Hydroacoustic observations of sub‐Indian ocean earthquakes using arrays of the International Monitoring System

Until recently, there have been few public hydroacoustic recordings of earthquakes beneath the Indian Ocean. Now, the International Monitoring System, which includes three Indian Ocean station sites, is providing excellent data for study. These stations consist of three‐element horizontal hydrophone arrays floated to the middle of the sound channel axis, and can record low‐level signals from distant sources. System bandwidth is approximately 2–100 Hz. We have accumulated a waveform database of 150 sub‐Indian Ocean earthquakes and surrounding continental events recorded by this system. These data allow us to research T‐wave generation and propagation. The recordings include seismic body‐wave conversions beneath the arrays, T waves, and long‐range reflections. Recordings of main shock/aftershock series have allowed us to compute scaling relationships for T‐wave amplitudes versus magnitude and examine T‐wave excitation versus source mechanism. When combined with distance corrections, we can scale T waves recorded at a given station to a fixed magnitude and distance. Plotted versus azimuth around the station, these scaled T‐wave amplitudes provide insight into hydroacoustic blockage, which is essential for understanding detection capability. Seismic body‐wave conversions beneath the arrays can be timed to 1 second or better, and provide important location constraints for subocean events occurring far from continental stations.