Back-projection of circular rupture synthetics. The synthetic seismograms of a circular rupture scenario recorded at the USArray (left two columns) and European array (right two columns) are processed by the MUSIC and linear stacking techniques separately at periods of 1~2 s, 2~4 s, 4~8 s and 8~16 s. The location of the aftershock is denoted by the black asterisk. The black circle is the hypothetical circular path of the rupture scenario. The solid circles are the peak of the back projection images color-coded by time and sized by their relative amplitude with respect to the global maximum. The blue asterisks are the “temporal local maximums” of the stacking result. The MUSIC recovers the rupture reasonably well at high frequency but loses the resolution beyond 4 s. The time-domain stacking method shows a bias resulting from the swimming artifact.