Land Subsidence in Arizona

Land subsidence has been occurring across Arizona since the 1950’s. Millions of people live in active land subsidence areas and are unaware. Most of the time, there is no clear and identifiable sign that land subsidence has occurred in an area. Areas in Maricopa and Pinal Counties have subsided more than eighteen feet since 1950.

Land subsidence in the basins of Arizona is generally due to compaction of the alluvium caused by lowering of the water table. As the water table declines, pores in the alluvium once held open by water pressure are no longer supported and collapse. Collapse and subsequent lowering in elevation of the land surface is defined as land subsidence. This subsidence is generally not recoverable. If this subsidence occurs over areas of bedrock, differential subsidence can occur.

Differential subsidence is when adjacent areas subside at different rates. Bedrock will not compress like the surrounding alluvium, creating a subsurface platform. Differential subsidence occurs where shallow bedrock and deep bedrock are adjacent to each other, creating a zone of differential change in surface elevation. Because of these different amounts of subsidence, tension can build in the alluvium layer at this differential subsidence zone, forming an earth fissure.

Earth Fissures

The Y Fissure in the Queen Creek Area
after a Monsoon Storm in August 2005

Earth fissures are cracks at or near the earth’s surface that are the result of differential subsidence. Earth fissures start out as small cracks and may not be visible on the surface. They grow and widen from surface water flowing into the crack, eroding material from the sides.

Earth fissures have caused millions of dollars in property and infrastructure damage, damaging pipelines, roads, canals, flood retention structures, bridges, buildings, and private property. There are unanswered questions about how earth fissures interact with groundwater pollution.

Impacts of Land Subsidence and Earth Fissures

There are many consequences of land subsidence and earth fissures: Elevation and slope change affecting the
natural flood plain drainage, flow within canals and drains, damagingintermodal infrastructure (roads, bridges,
railways, etc), damaging water retention andretarding structures (dams, levees, floodways, etc), and damaging
private property(homes, driveways, fences, etc).

Impacts of Land Subsidence and Earth Fissures

Earth fissures are identified by using on the ground and aerial monitoring techniques.
The Arizona Geological Survey (ASGS) started an intensive earth fissure mapping program
in 2006. The goal of this program is to survey and record each known earth fissure around
the State and provide this data to the public.

Land Subsidence Monitoring

earth fissures photo

Hawk Rock Area 2004- 2008 Interferogram

Land subsidence has been detected over the years using surveying techniques such as differential leveling and high accuracy Global Positioning System (GPS) surveying. In the early 1990’sscientists began to use Synthetic Aperture Radar (SAR) and interferometric processing (InSAR) to detect land surface elevation changes. InSAR has been developed into a highly reliable land subsidence monitoring technique that has been utilized by ADWR since 2002. ADWR has identified numerous subsidence features around the State and continues to monitor the extent and rates of these features on an annual basis.

Land subsidence maps were developed by ADWR using archived and current InSAR data with the goal of providing hydrologists, geologists, water managers, and the public the most accurate land subsidence data.

ADWR is also part of the Arizona Land Subsidence Group which was jointly created by the geological and engineering communities as a venue for discussing the state of knowledge regarding subsidence of Arizona’s alluvial basins.

ADWR is also part of the Arizona Land Subsidence Group which was jointly created
by the geological and engineering communities as a venue for discussing the state
of knowledge regarding subsidence of Arizona’s alluvial basins.

Arizona Land Subsidence Areas Based on ADWR InSAR Data

Scottsdale Subsidence Feature

Counties: Maricopa

Cities: Scottsdale, Phoenix

Maps: 1992 – 2000, 2004 –2008 , 2006 –2008, 2007-2008

West Valley Subsidence Feature

Counties: Maricopa

Cities: Sun City, Surprise, Peoria, Glendale, Avondale

Maps: 1992 – 2000, 2004 – 2008, 2006 – 2008, 2007 - 2008

Hawk Rock Subsidence Feature

Counties: Maricopa, Pinal

Cities: Mesa, Apache Junction, andUnincorporated County Land

Maps: 1992 – 2000, 2004 – 2008, 2006 – 2008, 2007 - 2008

Harquahala Valley Subsidence Feature

Counties: Maricopa

Cities: Unincorporated County Land

Maps: 1992 – 1997, 2004 – 2008, 2007 - 2008

Buckeye Subsidence Feature

Counties: Maricopa

Cities: Buckeye and Unincorporated County Land

Maps: 2006 – 2008, 2007 - 2008

Gila Bend Subsidence Feature

Counties: Maricopa

Cities: Gila Bend and Unincorporated County Land

Maps: 2006 – 2008, 2007 - 2008

McMullen Valley Subsidence Feature

Counties: Maricopa, La Paz

Cities: Aguila, Wenden, Salome,Unincorporated County Land

Maps: 1992 – 1997, 2004 – 2008, 2007 - 2008