Normal faults offset a bedrock surface scoured by Pleistocene ice in several areas within and south of Yellowstone National Park. Recurrent earthquake shocks and fresh appearance of some scarps suggest that movement is continuing along some faults. Four systems of faults are described.
Quaternary movement occurred along more than 60 faults on the Mirror Plateau, 15 miles northeast of Yellowstone Lake. Faults trend northwest, and several are more than 6 miles long. Maximum displacement exceeds 250 feet. The majority have northeast blocks downdropped, but some grabens and horsts are present. Eocene to Pliocene igneous or pyroclastic rocks are displaced. Ice moved southwest and south from the Beartooth and Absaroka ranges, nearly at right angles to the fault trends. Drainage in many ice-scoured valleys was disrupted by faulting, and small lakes (such as Mirror Lake) formed on downthrown blocks. Thermal activity occurs along some of these faults.
Directly east of Mirror Plateau, the Lamar normal fault has a displacement of 1300 + feet; perhaps 1000 feet of this may have occurred during Quaternary time.
The Yellowstone Falls fault system cuts Pliocene rhyolite southeast of the Upper Falls of the Yellowstone River. Faults trend northwest; maximum displacement exceeds 200 feet.
The Solfatara fault system trends north-northwest, cuts Pliocene rhyolite, and has a maximum Quaternary displacement of about 200 feet. The Hering Lake fault system is a northern extension of the Teton fault, trends northward, and cuts Pliocene rhyolite and rhyolitic welded tuff. Maximum displacement is about 200 feet. West-flowing streams established on bedrock scoured by ice were disrupted, and Beula, Hering, and South Boundary lakes formed on the downthrown (east) blocks.
The sharp angular unstepped appearance of fault scarps 50 to 200 feet high in these fault systems suggests that each scarp of this type was formed by one continuous movement. The displacement along faults associated with the Hebgen earthquake of August 1959 is commonly less than 20 feet. The abundance of Quaternary faults and the record of 18 earthquakes in historic time suggest that additional faulting and earthquake activity can be expected in the future. Recognition of this probability should influence the location and type of construction of buildings and other facilities.