Mitigating Artifacts in Back-Projection Source Imaging with Implications on Frequency- Dependent Properties of the Tohoku-Oki Earthquake

Comparing teleseismic array back-projection source images of the 2011 Tohoku-Oki earthquake to results from static and kinematic finite source inversions has revealed little overlap between the regions of high and low frequency slip. Motivated by this interesting observation, backprojection studies extended to intermediate frequencies, down to about 0.1Hz, have proposed that a progressive transition of rupture properties as a function of frequency is observable. Here, by adapting the concept of array response function to non-stationary signals, we demonstrate that the “swimming artifact”, a systematic drift resulting from signal non-stationarity, induces significant bias on beamforming back-projection at low frequencies. We introduce a “reference window strategy” into the multitaper-MUSIC back-projection technique and significantly mitigate the “swimming artifact”. We perform extensive synthetic tests that include a 3D regional velocity model for Japan. We analyze the recordings of the Tohoku-Oki earthquake at the USArray and at the European array at periods from 1 s to 16 s. The migration of the source location as a function of period, in particular has characteristics that are consistent with the expected effect of the “swimming artifact”. In particular, the apparent up-dip migration as a function of frequency obtained with the USArray can be explained by the “swimming artifact”. This indicates that the most substantial frequency-dependence of the Tohoku-Oki earthquake source occurs at periods longer than 16 s. Thus, low frequency back-projection needs to be further tested and validated in order to contribute to the characterization of frequency-dependent rupture properties.