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UNESCO, AIPG and AGS examining subsidence and earth fissures in central Arizona

On the week of 5 November, AZGS staff contributed to two field trips examining the role of subsidence in forming earth fissures. The first trip was part of the 4-6 November IGCP Workshop co-sponsored by UNESCO, AEG – Association of Environmental and Engineering Geologists -, and Arizona State University. Workshop participants discussed land subsidence and earth fissure formation in Arizona, Mexico and San Joaquin Valley, California. About 20 geoscientists from Mexico, China, Italy, and the U.S. participated (Figure 1).

J.P. Cook, AZGS Earth Fissure Program Manager, was one of several field trip leaders. Joe’s role was to showcase the physical characteristics and recent activity at the Houston Ave. earth fissure in south Apache Junction. Field trip stops also included site visits to flood retention structures impacted by the presence of underlying fissures.

The second trip, ‘Graboids* are the least of our concerns: Earth Fissures of Apache Junction and Chandler Heights, Arizona’,  was co-sponsored by the Arizona section of the American Institute of Professional Geologists and the Arizona Geological Society. AZGS’ Mike Conway led 17 intrepid geologists on a short walk along several long fissures. The trip began at Houston Avenue with a quick study of failed efforts to mitigate the impact of the earth fissure there. Our second stop, at the Hunt Highway – Thompson Rd in the Chandler Heights Earth Fissure Study Area, was to examine north-south trending fissures that encroach on Hunt Highway.

Background Information - earth fissures

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.
Earth Fissure Facts

  •     Fissures crop out in Cochise, La Paz, Maricopa, Pima, and Pinal Counties.
  •     Twenty-seven earth fissure study areas encompass ~ 1,400 sq. miles
  •     Aggregate of mapped earth fissures ~175 miles
  •     Aggregate of unconfirmed fissures ~ 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.

Online Resources: Subsidence & Earth Fissures in south-central Arizona

Arizona Geological Survey, 2019, Natural Hazards of Arizona Online Viewer, Earth Fissure theme.

Arizona Geological Survey, 2017, Locations of Mapped Earth Fissure Traces in Arizona, v.11.06.2017. Arizona Geological Survey Digital Information (DI-39 v. 11.06.17), shapefile and Google Earth KMZ file

Arizona Land Subsidence Group, 2007, Land subsidence and earth fissures in Arizona—Research and information needs for effective risk management: Arizona Geological Survey Contributed Report CR-07-C, 24 p.

Conway, B.D., 2015, Land subsidence and earth fissures in south-central and southern Arizona, USA. Journ. Of Hydrogeology, 8 p. DOI 10.1007/s10040-015-1329-z

Conway, B., 2017, That sinking feeling: state-of-art technology at work on Arizona subsidence finds that you’re not imaging it. Arizona Geology Blog, 27 July 2017.

Cook, J.P., 2013, Revisiting earth fissures near Luke Air Force Base, central Maricopa County, Arizona. Arizona Geological Survey Open File Report, OFR-13-15, 10 p. 

Cook, J.P., 2017, Discovery of a large earth fissure in the Southern Picacho Basin, Pinal County, Arizona. Arizona Geological Survey Open-File Report, OFR-17-01, 7 p., 1 appendix.

Slaff, S. (1993). Land Subsidence and Earth Fissures in Arizona.  Arizona Geological Survey Down-to-Earth Series #3, 30 p.