The same 2,868 earthquakes peak at 149.7 per 1,000 km² with a 50 km radius and at 6.5 with 300 km. Only the radius changed.

Kernel density spreads every point over a circle and adds the circles up. It makes a smooth surface, but the radius you choose and the top of the color scale decide how hot it looks. Optimized hot spot analysis asks a different question: is a cluster stronger than chance would give, after correcting for testing thousands of cells?