Earth Fissures in South-Central Arizona

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.

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.

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.

Some of the more common damage associated with earth fissures.

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).

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.