THE POTENTIAL FOR EARTHQUAKE TRIGGERED LANDSLIDES IN THE GRAND CANYON, ARIZONA

The role of earthquakes as a trigger for landslides of a variety of types is a well established relationship. Regions with rugged terranes have numerous susceptible slopes that show repeated movements through time. The Grand Canyon of Arizona is such an area. An initial study of the hazard at Grand Canyon from mass-movements triggered by earthquakes required two data sets: a record of mass-movements from all causes, and a record of regional seismicity.

Data for mass-movements in the Grand Canyon were available for the years 1968-2006. Over this time period the average of mass-movements varied from a low of 1.8/year up to 3.4/year.Many of these events were directly attributable to rain storms as a trigger. The data base for earthquakes in the region was from 1906 to 2006 and included events ranging from Modified Mercalli V (M=4.0) up to MMVIII (M=6.2). Four was chosen as the low end of the size range because this is the lowest magnitude attributable to mass-movements. The twin M4.0 tremors of March 1989 caused rock falls that were noted by observors.

The recurrence interval for the Grand Canyon region indicates that for events likely to cause mass-movement (M4.0 or greater) that on average such triggering should occur at least once every ten years. This poses an obvious hazard to hikers and campers in the inner canyon as well as to river rafters.