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Earth Planets Space, Vol. 64 (No. 12), pp. 1101-1109, 2012
doi:10.5047/eps.2012.05.010

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

Lingsen Meng1, Jean-Paul Ampuero1, Yingdi Luo1, Wenbo Wu2, and Sidao Ni3

1Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, U.S.A.
2Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
3CAS Key Laboratory of Dynamical Geodesy, Institute of Geodesy and Geophysics, Wuhan 430077, China

(Received December 27, 2011; Revised May 4, 2012; Accepted May 17, 2012; Online published January 28, 2013)

Abstract: Comparing teleseismic array back-projection source images of the 2011 Tohoku-Oki earthquake with 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 suggested 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" at 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.
Key words: Back-projection, Tohoku-Oki earthquake, frequency-dependent rupture properties, MUSIC, swimming artifact.


Corresponding author E-mail: lsmeng@gps.caltech.edu


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