Zones of T-wave excitation in the NE Indian ocean mapped using variations in backazimuth over time obtained from multi-channel correlation of IMS hydrophone triplet data

Ground-coupled hydro-acoustic waves (so called T waves) observed at International Monitoring System (IMS) hydrophone triplet stations in the Indian ocean were analysed for backazimuth and apparent wave speed by applying a progressive multi-channel correlation method (PMCC). For T waves, which propagate along topographically unblocked geodesic paths, the three triplet channels generally show good signal correlation over sliding windows and various frequency bands. A characteristic change in backazimuth over time, in some cases combined with signs of dispersion, is observed for many T waves generated by earthquakes from the margin of the Indo-Australian Plate between the Sunda Islands to the southeast and the Andaman Islands to the northwest. Backazimuths and arrival times of those PMCC detections are used to map point-by-point the areas where ground-to-water coupling takes place. Such areas are mostly linked to large gradients in the bathymetry, predominantly at approximately the depth of the axis of the SOund Fixing And Ranging (SOFAR) channel, along island arcs, edges of continental shelves and at comparatively small features like isolated islands or sea mountains. Six examples are discussed in this paper. For the strongest events, the area of detectable ground-to-water coupling often spans distances of more than 15, in one case even more than 26, and a range in backazimuths of up to 35. Information on the accuracy of backazimuth determination, on the average hydro-acoustic wave speed along the wave path and on topographic blockage of waves can also be inferred from the results owing to the consistency and great detail of the mapped coupling zones. For the study area, it can be shown that it is predominantly seismic energy from P and/or Pn waves that couples efficiently into the SOFAR channel. Particularly for deep focus events, the backazimuths determined for the highest amplitude segments of the signals can deviate considerably from the great-circle azimuth between source and receiver, and may therefore lead to inaccurate epicentre locations based on backazimuth measurements of T waves. In some cases, laterally reflected T waves are observed.