Abstract

Geologic hazards are an important planning consideration in the St. George area, and include rock falls, landslides, problem soils, and earthquakes. Rock-fall debris at the bases of many slopes indicate the hazard, and a rock fall in 1984 along West Black Ridge prompted temporary evacuation of a building until studies were performed and mitigating measures taken. Most active and older landslides are in the Chinle Formation (Petrified Forest Member) which is found in slopes bordering the Santa Clara and Virgin Rivers. Modern landslides in this unit have occurred in the Santa Clara Bench and Green Valley areas, and large older slide complexes occur at the southern tips of West, Middleton, and Washington Black Ridges. Expansive clays derived from the Petrified Forest Member have also caused foundation problems. Other soil foundation problems have resulted from gypsiferous and calcium carbonate-rich soils, piping, and shallow ground water.

St. George is in a moderately active earthquake zone. The largest earthquake in the area in historical time was a magnitude 6.3 event in 1902 near Pine Valley to the north. The only known potentially active fault in the area is the Washington fault in Washington, but earthquakes generated on the nearby Hurricane and Grand Wash faults or on unknown secondary structures may also affect the area. The principal earthquake hazards include ground shaking, rock fall, and liquefaction, but surface fault rupture and landsliding may also occur. It is recommended that geologic hazards studies be performed, and mitigation measures taken prior to development in hazard areas.