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Mitigating artifacts in back-projection source imaging with implications on frequency-dependent properties of the Tohoku-Oki earthquake

Lingsen Meng, Jean-Paul Ampuero, Yingdi Luo, Wenbo Wu, & Sidao Ni

Published 2012, SCEC Contribution #1651

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, back-projection studies extended to intermediate frequencies, down to about 0.1 Hz, 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" in high frequencies (1 s to 4 s). At lower frequencies, this modification yields notable but significantly smaller artifacts than time-domain stacking. 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, regardless of the back-projection methods, 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.

Meng, L., Ampuero, J., Luo, Y., Wu, W., & Ni, S. (2012). Mitigating artifacts in back-projection source imaging with implications on frequency-dependent properties of the Tohoku-Oki earthquake. Earth, Planets and Space, 64(12), 1101-1109. doi: 10.5047/eps.2012.05.010.