Abstract
Analysis of normal‐mode amplitudes excited by the Chilean earthquake of 27 February, 2010, shows that theoretical amplitudes computed for the Global Centroid Moment Tensor solution (GCMT) underestimate observation for frequencies below 1 mHz. Data are systematically larger and there is a hint that this ratio increases toward lower frequencies. Amplitude ratios for the three gravest modes, 0S2, 0S3 and 0S4, are 1.36, 1.29 and 1.22 respectively, with standard error (1σ) being about 0.1 for each of them. The most natural explanation for this observation is the occurrence of a slow afterslip. Assuming that this afterslip had occurred on a similar fault with dip angle 18 degrees, the moment of this afterslip is estimated to be 0.26 (26 percent) of the GCMT solution and the rise time of about 170 seconds.
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