If you look into it, it's the rounding errors from the lat/lon being rounded off to two decimal places. Each of those dots will land on 1/100 of a degree. The actual quakes are best guesses from the seismographs so this level of precision is beyond the instrumentation despite some spitting out what seems like very high precision estimates.
Any more granular and it just creates a lot more processing overhead for no benefit. So, this is the out on a high precision map, where the map is able to resolve finer than the data is granular.
It's just an artifact of a system of things.