Sinkhole risk in Whatcom County, WA: Not karst, but voids possible

Not karst, but voids possible

No soluble bedrock; some pseudokarst (piping or lava-tube ground).

On soluble rock: under 1%
At or near the surface: under 1%
Buried under cover: 0%

What this means before you buy

Not true karst, but soil piping or lava tubes can still open voids - mention it to an inspector.

That works out to roughly 13 of Whatcom County's 2,108 square miles on soluble rock, 13 of them with the rock at or near the surface. With about 234,954 residents (111 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 344 map points about 2.5 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: how ground with no limestone still opens up, and how these percentages are measured.

Counties around Whatcom County

The nearest county rated sinkhole country is Emery County, UT, about 874 miles southeast. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Skagit County, WA 24 mi south · under 1% at surface
Snohomish County, WA 55 mi south · 0% at surface
Island County, WA 61 mi southwest · 0% at surface
San Juan County, WA 62 mi west · 4% at surface
Chelan County, WA 88 mi southeast · 0% at surface
Kitsap County, WA 91 mi southwest · 0% at surface

Which formation it is

The largest mapped soluble unit here is the Carboniferous-Permian sedimentary and volcanic rocks (graywacke), Devonian to Permian with minor Mesozoic in age, covering about under 1% of the county.

Against the rest of Washington

Whatcom County ranks 7 of 39 in Washington for soluble rock at the surface, and sits above 53% of US counties on that measure.