LANDSLIDES ARE SURPRISINGLY LARGE AND WIDESPREAD IN ARIZONA

In 2015 the Arizona Geological Survey (AZGS) began compiling AZ landslide data to update the Landslide Profile in the upcoming 2018 State of AZ Hazard Mitigation Plan. The AZ Statewide Landslide Inventory Database (AzSLID) contains landslide and mass movement data from published maps and reports as well as newly documented slides identified while building the database. AzSLID currently contains approximately 5,000 landslide and mass movement polygons covering 1800 km² and 1,400 debris flow observation points. New data is added as it becomes available.

Approximately 10% of the polygons in the database are previously undocumented landslides. Many of these landslides lie in Tribal Lands in northern AZ. Most landslides have occurred where consolidated units overlie fine-grained erodible sediment such as the Chinle Fm. or Mancos Shale. Vast landslide complexes are nearly continuous along the steep margins of Black and Balakai Mesa in the Navajo and Hopi Reservations. Large rotational slides were also identified along Fossil Creek and dissected basalt-capped mesas in central AZ. Based on the AzSLID compilation, AZGS geologists recently conducted a field assessment of a previously mapped landslide near the Mormon Flat Dam on the Salt River in Maricopa County, AZ. Mapped slide boundaries were verified, low-altitude air photos were collected, and a new geological assessment of the landslide was prepared.

AZGS collaborates with the AZ Department of Water Resources (ADWR), which monitors land subsidence throughout AZ using InSAR. InSAR data are typically used to quantify land subsidence on a regional scale, but slope movement has been identified along the Vermillion Cliffs. InSAR data also detected recent movement of a large slump block near the February 2013 Bitter Springs landslide that heavily damaged US Route 89. InSAR has proven to be a useful tool for recognizing new and ongoing slide zones where subsidence is evident in InSAR data but not in aerial imagery or on the ground.

Landslide hazards are widely distributed in AZ. The proximity of landslides to critical infrastructure, utilities, and populated areas is important information not easily accessible prior to creation of the database. The AzSLID database will be useful to local, county, and tribal governments for hazard mitigation and planning purposes.