Inversion of tsunami waveforms for the seismic moment tensor

Abstract:

Focal mechanism determination is an important content in modern seismology. Currently, it is mainly realized by inverting seismic and geodetic data. For large earthquakes, however, there are some limitations for these two data sets. In seismic data inversion, the point-source approximation demands both epicentral distance and wavelength much larger than the source dimension. Thus very low frequency need to be used, and sometimes they may exceed the response range of broadband seismometers. For geodetic data, since the surface deformation attenuates with the square of epicentral distance, it is hard to use the far-field data to deduce the point-source mechanism.In this work, we introduce the tsunami wave data into mechanism determinations. Because of the long-periods and slow attenuation, tsunami wave can to some extent overcome the above problems existing in seismic and geodetic data. For point source model, by ignoring the source volume change, the focal mechanism can be represented by a linear combination of five characterized focal mechanisms (Kikuchi and Kanamori, 1991). So if we calculate the theory tsunami wave (TTW) of the characterized focal mechanisms, we can get the focal mechanism with the observed tsunami wave (OTW) through a linear optimization.In the application to the 2011 Tohoku Mw9.0 earthquake, the mechanism determined by tsunami wave inversion is in a good agreement with our current knowledge of the earthquake, further confirming that it was a thrust event. In addition, the tsunami wave inversion shows an advantage in constraining the centroid depth, which is relatively difficult in some traditional studies.