Generation of hydroacoustic signals by oceanic subseafloor earthquakes: a mechanical model

For more than 15 yr, the recording of hydroacoustic signals with hydrophones moored in a minimum sound-velocity channel, called the SOFAR (SOund Fixing And Ranging) channel, has allowed for detection and localization of many small-magnitude earthquakes in oceanic areas. However, the interpretation of these hydroacoustic signals fails to provide direct information on the magnitudes, focal mechanisms, or focal depths of the causative earthquakes. These limitations result, in part, from an incomplete understanding of the physics of the conversion, across the seafloor interface, from seismic waves generated by subseafloor earthquakes to hydroacoustic T waves. To try and overcome some of these limitations, we have developed a 2-D finite-element mechanical model of the conversion process. By computing an exact solution of the velocity field of the waterborne T waves, our model shows that a double-couple source mechanism of a subseafloor earthquake generates T waves, whose take-off angles are adequate to allow penetration into the SOFAR channel and efficient trapping by this waveguide. Furthermore, our model confirms that a double-couple source with a high S-wave content produces higher-amplitude T waves than a simple explosive source, which only generates P waves.