Sinkhole risk in Orange County, CA: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 6%
At or near the surface: 6%
Buried under cover: 0%

The next step for one property

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 51 of Orange County's 793 square miles on soluble rock, 51 of them with the rock at or near the surface. With about 3,170,435 residents (3,997 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 528 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and how these percentages are measured.

Counties around Orange County

The nearest county rated sinkhole country is Emery County, UT, about 538 miles northeast. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

Los Angeles County, CA 45 mi northwest · 1% at surface
San Diego County, CA 73 mi southeast · 0% at surface
Ventura County, CA 91 mi northwest · 9% at surface
Riverside County, CA 102 mi east · under 1% at surface
San Bernardino County, CA 122 mi northeast · 2% at surface
Kern County, CA 128 mi northwest · under 1% at surface

Which formation it is

The largest mapped soluble unit here is the Oligocene nonmarine rocks, unit 2 (Central and Southern California) (sandstone), middle Eocene to early Miocene in age, covering about 6% of the county.

6% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

Against the rest of California

Orange County ranks 6 of 58 in California for soluble rock at the surface, and sits above 64% of US counties on that measure.