| Radon activity index | Expression of fault activity | |
| KRn | Level | |
| KRn > 20 | Ultra-high | Fault having strong activity expression |
| 20 ≥ KRn > 15 | High | Fault having clear activity expression |
| 15 ≥ KRn > 10 | Increased | Fault having activity expression or existence of strong tectonic fracture zone |
| 10 ≥ KRn > 5 | Medium | Fault having not clear activity expression or existence of fracture zone |
| KRn ≤ 5 | Low | Fault having no activity expression |