To spatter is to scatter or dash (a liquid) in small drops. The small drops are key. For example, a light rain might spatter the roof.
Splatter, which came later and was probably formed by blending splash and spatter, has a similar meaning, but it doesn’t necessarily involve small drops. A splatter of liquid might be large and messy. For example, paint from an upturned bucket might splatter on the floor. Think of spatter as a synonym of sprinkle or spit, and splatter as closer to splash.
Still, this conventional distinction notwithstanding, the words are often used interchangeably.
A CSI team found lots of blood spatter after spraying the attic with luminol. [Washington Post]
They have paid around £250 for an unrestricted view of Kylie’s bum, and the privilege of being spattered with chlorinated water at the end. [Guardian]
The rain continued to spatter against the window. [San Diego Reader]
[F]ive others head off on four-wheeled ATVs for a mud-splattered off-road trek. [Stuff.co.nz]
Not to mention the deficit-whipped New York City of the 1970s, with a ravaged, graffiti-splattered subway system. [Denver Post]
The paint splatters, art textbooks, empty beer bottles and unmade bed on the floor come together to show Winston’s desperation and creativity. [Montgomery News]