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Earth Fissures & Subsidence

Earth Fissures in South-Central Arizona

Some arid valleys of central and southeastern Arizona are home to earth fissures: pervasive cracks that occur on valley floors that result from basin subsidence associated with extensive groundwater withdrawal. Earth fissures are an  anthropogenic geologic hazard that threaten people, property, infrastructure (e.g., roads, gas lines, canals), and livestock. 

subsidence

Earth fissures and subsidence threatens people and livestock, damages homes, roads, dams, and canals, and reduces property values

Earth Fissure Facts

  • Fissures crop out in Cochise, La Paz, Maricopa, Pima, and Pinal Counties.
  • Twenty-six earth fissure study areas encompass ~ 1,400 sq. miles
  • Aggregate of mapped earth fissures is 169 miles
  • Aggregate of unconfirmed fissures is 180 miles
  • First fissure appears near Eloy in 1929    

Fissures range from discontinuous hairline fractures to open ground cracks up to two miles long, as much as 15- to 25-feet wide, and up to 90 feet deep (see the photo gallery images below). The observed fissure floor, however, does not reflect its true depth; geoscientists believe that fissures extend to the top of the groundwater table, which can be several hundred feet below the ground surface. Torrential monsoon rains can rapidly widen and deepen fissures resulting in hazardous conditions to people, livestock and infrastructure. Fissures can provide a ready and open conduit to basin aquifers, which could facilitate delivery of runoff and contaminated waters to area groundwater.  Rapid population growth in southern Arizona has increasingly juxtaposed population centers and fissures.

2-mile Long Earth Fissure

Bird's-eye view of 2-mile long earth fissure, Pinal County, Arizona. Drone video captured January 2017 by AZGS's Brian Gootee and Joe Cook.

Impacts on People, Property, and Infrastructure

Ground subsidence and resulting earth fissures impact more than 3,000 square miles in Arizona, including expanding areas of Phoenix and Tucson. The cost to the Arizona economy is undetermined, but probably reaches the millions of dollars annually. Repairs to an irrigation canal near Scottsdale Airpark in 2007 were estimated at $820,000, and that's a single incident involving one canal. During construction of the Red Mountain Highway (Loop 202) in Phoenix, the cost of mitigating an earth fissure that impinged on the road bed was $200,000 (Arizona Land Subsidence Group, 2007).

Some of the more common damage associated with earth fissures.

  • Cracked or collapsing roads
  • Broken pipes & utility lines
  • Damaged or breached canals
  • Cracked foundation/separated walls
  • Loss of agricultural land
  • Livestock & wildlife injury or death
  • Damaged well casing or wellhead
  • Disrupted drainage
  • Contaminated groundwater aquifer
  • Sudden discharge of ponded water
  • Human injury or death

What should you do if you find an earth fissure on or near your property?

  • Keep children, pets and livestock away from the fissure.
  • E-mail AZGS (azgs-info@email.arizona.edu); alert your municipal or county emergency managers office.
  • Prevent runoff or flood irrigation waters from entering the fissure. Erosion can rapidly increase the fissure width, length, and depth.
  • Consult a geotechnician regarding the extent of fissuring and how to minimize damage to your home and property.

Geoscience at Work for Arizona - Monitoring Earth Fissures

  • To reduce the societal risk of earth fissures, the AZGS couples earth fissure mapping with an aggressive educational outreach and fissure map dissemination program.
  • Since the 1930s, the number and distribution of earth fissures in Arizona has increased dramatically. In 2006, the
  • Arizona Legislature charged AZGS with establishing an earth fissure mapping and monitoring program (Earth Fissure Mapping program). Over the past decade, our fissure mapping team has mapped hundreds of earth fissures in 28 study areas in five counties.

AZGS Earth Fissure Mapping Procedures

In each earth fissure study area, we begin by reviewing existing maps and technical reports, examining new and historical aerial photographs, and consulting with geoscientists and engineers in agencies (e.g. Salt River Project, AZ Department of Water Resources) or consulting firms with expertise or information on fissures.

Once we identify known or suspected fissures, we head to the field to examine, characterize, and map each fissure using a high-precision global positioning system (GPS) receiver. Along the fissure, we collect data (fissure width & depth) every 5-15 feet (about 2- to 4-meters), yielding 300 to1,000 data points per mile (250 to 500 data points per kilometer) of fissure!

Back in the office, the data go through post-processing and uploaded into a geographic information system (GIS) map environment. Since 2007, this process has been repeated 100s of times until all known or suspected earth fissures were mapped. The fissure map data are then added to study area field sheets, which are reviewed before release at the AZGS Online Document Repository.

Each study area map is constructed on an aerial photographic base map and includes a map legend denoting the status of individual fissures – continuous, discontinuous, or reported but unconfirmed.

Following review, new fissure data are compiled with existing data and published at the Natural Hazards in Arizona Viewer.

AZGS's earth fissure monitoring and mapping program puts geoscience to work for Arizona.

Earth Fissure Resources: Public

Earth Fissure Resources: Geotechnical

Earth Fissure Resources: Geosurveys

Glossary of Terms

Earth Fissures are open ground fractures that form in unconsolidated sediments as the result of tensional stresses associated with land subsidence.

Giant desiccation cracks form in fine-grained sediments, i.e., clays, as the result of drought. By way of example see Arizona's Willcox Playa or Red Lake.

Land subsidence  is the downward movement or sinking of the Earth's surface caused by removal of underlying support, e.g., pumping out groundwater.

From the AZGS Photo Library