California volcanic activity risk
Siskiyou County has the highest volcanic activity risk in California, scoring 98 out of 100 — rated "Relatively High" by FEMA's National Risk Index.
| # | County | NRI score | Rating |
|---|---|---|---|
| 1 | Siskiyou County | 98 | Relatively High |
| 2 | Lake County | 96.7 | Relatively Moderate |
| 3 | Shasta County | 94.8 | Relatively Moderate |
| 4 | Tehama County | 92.5 | Relatively Low |
| 5 | Plumas County | 88.9 | Relatively Low |
| 6 | Mono County | 88.3 | Relatively Low |
| 7 | Lassen County | 87 | Relatively Low |
| 8 | Imperial County | 86 | Relatively Low |
| 9 | Sonoma County | 85 | Relatively Low |
| 10 | Modoc County | 81.4 | Very Low |
| 11 | San Joaquin County | 79.8 | Very Low |
| 12 | Butte County | 79.5 | Very Low |
| 13 | Sacramento County | 79.2 | Very Low |
| 14 | Alameda County | 78.8 | Very Low |
| 15 | Stanislaus County | 78.2 | Very Low |
| 16 | Contra Costa County | 77.9 | Very Low |
| 17 | Mendocino County | 77.2 | Very Low |
| 18 | Tulare County | 73.3 | Very Low |
| 19 | Solano County | 70 | Very Low |
| 20 | Placer County | 67.4 | Very Low |
| 21 | Riverside County | 66.8 | Very Low |
| 22 | Inyo County | 66.4 | Very Low |
| 23 | Marin County | 63.5 | Very Low |
| 24 | Madera County | 62.9 | Very Low |
| 25 | Yolo County | 62.2 | Very Low |
| 26 | Tuolumne County | 61.6 | Very Low |
| 27 | El Dorado County | 60.6 | Very Low |
| 28 | Napa County | 60.3 | Very Low |
| 29 | Kern County | 59.6 | Very Low |
| 30 | Calaveras County | 59.3 | Very Low |
| 31 | Sutter County | 59 | Very Low |
| 32 | Mariposa County | 58.6 | Very Low |
| 33 | Nevada County | 58 | Very Low |
| 34 | Yuba County | 56 | Very Low |
| 35 | Trinity County | 55.7 | Very Low |
| 36 | Fresno County | 54.1 | Very Low |
| 37 | Glenn County | 51.5 | Very Low |
| 38 | Merced County | 50.2 | Very Low |
| 39 | Amador County | 46.9 | Very Low |
| 40 | Sierra County | 45.9 | Very Low |
| 41 | Colusa County | 39.4 | Very Low |
| 42 | San Diego County | 33.9 | Very Low |
| 43 | San Bernardino County | 31.6 | Very Low |
| 44 | San Francisco County | 28.3 | Very Low |
| 45 | Alpine County | 15.3 | Very Low |
| 46 | Santa Clara County | 15 | Very Low |
| 47 | Humboldt County | 0 | No Rating |
Source: FEMA National Risk Index v1.20.0, December 2025. Scores are 0–100 percentile rankings among all US counties. See the methodology page for definitions. Download the full CSV for all counties.
Volcanic Activity risk nationally:
Other hazard risks in California:
Frequently asked questions
What is the highest volcanic activity risk county in California?
Siskiyou County, CA has the highest volcanic activity risk in California, with a FEMA National Risk Index score of 98 out of 100 (rated "Relatively High").
What causes volcanic activity risk in California?
Volcanic activity risk reflects expected losses from lava flows, tephra fall, lahars (volcanic mudflows), and volcanic gases. The highest-risk counties are in Washington (Mt. Rainier), Oregon (Mt. Hood), and Hawaii (Kilauea, Mauna Loa). Alaska's Aleutian chain contains many active volcanoes affecting nearby communities and trans-Pacific air traffic. Volcanic ashfall events are reported at forecast-zone level in NOAA Storm Events and are not included in county-level counts.