Large earthquakes are not focused on a point, they are actually a shear zone that lets go and shifts. The 2/25/2018 Papua New Guinea earthquake just happened to have a very well defined shear zone when you looked at the aftershocks.
The large red dot in the center is the primary earthquake the other dots are earthquakes sorted by depth, yellow <15km
dk. orange 25km-35km
Not sure why the primary is Red, should be Orange, unless the downloaded list had the depth wrong. The main thing is that if you look at the earthquakes they are all concentrated in one well defined area. On top of that the depth of the earthquakes slope downward towards the Northeast.
This is even better defined when you look at only the larger earthquakes, which are going to have more precise depth calculations. The yellow dots are mostly to the SW, and the Red are mostly to the NE.