Back-projection of aftershock synthetics. The synthetic aftershock seismograms 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 asterisk. 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 beamforming results show a “swimming” artifact at all frequencies. The MUSIC estimates are reliable at periods shorter than 4 s. At lower frequencies, the spatial bias is notable but less severe than that of beamforming.