Time-Reverse Imaging for the Tsunami Source

Many tsunami source inversion techniques have already been developed to derive source models with the assumption that tsunami generation is due to slip on a single large fault. Therefore, these inversion techniques cannot determine to what extent subsidiary phenomena - such as submarine landslides, block movement, or slip on splay faults - have contributed to the tsunami generation. We are proposing a new method that can be used to derive source models without requiring the assumption of slip on a fault of pre-determined geometry, but rather inverts directly for sea surface displacement. The proposed method is based on ''Time Reverse Imaging (TRI)'' technique, which has been used in underwater acoustic and medical imaging. We have applied TRI to recover the initial sea surface displacement associated with the tsunami source. This approach requires observations with good azimuthal coverage around the source area. It also requires a numerical model that will be run backward with a collection of point sources that coincide with observation locations. Synthetic numerical experiments show that if a good enough coverage of observations is available, TRI yields a good approximation to the spatial distribution of the initial source model. To show the application of this method we have chosen the tsunami triggered by the March 11, 2011 Tohoku-Oki earthquake, for which an unprecedented number of high-quality observations are available. We use both near- and far-field tsunami observations in our study. We will compare the findings of the TRI result with other more conventional methods of source inversion.