Abstract The duration time of a mainshock’s wave increases with epicentral distance. Aftershocks that occur within the duration time are always merged in the coda wave and missing in existing catalog earthquake swarm. By comparing duration times of Ludian 6.5 seismic waves, we get at different epicentral distance. It is argued that strong acceleration recording near the source region is good for merged earthquakes detection because the duration time of the mainshock is shorter. Then twice aftershocks merged in coda wave of Ludian 6.5 are detected using strong acceleration recording near the source region| the magnitudes are M4.6 and M3.6 individually.