Movie Summary

The Earthquake Fault is a popular stop for many skiers on the way to Mammoth Mountain. Although much has been written about this “fault” its origin remains the subject of much debate. The fissure trends north-south like many of the known faults in the Mammoth area. Some geologists have suggested that it represents the southern extension of the Hartley Springs fault. In fact, as early as 1936 it was proposed that the west side of the fault had dropped as much as three feet. Nonetheless, no slickensides are apparent on outcrop and the postulated offset is problematic at best. Even the suggestion of west-side down motion is inconsistent with the typical east-side down pattern of range front faulting. This has led to a suggestion that the fault is merely a fissure formed during cooling of the black, glassy, rhyolitic lava flows. This, however, is difficult to rationalize as the Rhyodacites of Mammoth Mountain are thought to be at least 50000 years old and a crack of this nature is almost certainly Holocene in age. A more intriguing explanation is that the crack represents the head scarp of a large landslide block or that it is a tension crack formed during warping, similar to those those at the Big Pumice Cut. Reportedly the “fault” was reactivated during the 1980 Mammoth earthquake and the trail down into the fissure was closed. It is now thought the apparent motion at the bottom of the fissure resulted from settling of unconsolidated material. Some researchers have also …