Earth Fissures in Arizona
Mitigating risk to communities through geohazard modeling
A unique hazard
Every year, the booming population of the Phoenix metro area pushes development further into what was once desert wilderness or agricultural land.
As communities build outward at a rapid pace in response to high regional demand for housing and commercial development, vulnerability to natural hazards is often overlooked in favor of this prioritized expansion.
But Arizona does have a unique and little-known geohazard: earth fissures.
What are earth fissures?
Earth fissures are cracks that appear in desert basin areas where ground subsidence, aquifer depletion, alluvial soils, and monsoon rains combine. Fissure openings can be sudden, dynamic events, with the potential threat to life for humans and livestock, as well as extensive and irreparable property damage.
At their largest, earth fissures can be miles long and nearly one hundred feet deep, but even a modest fissure greater than five feet deep is considered a life-threatening hazard.
The video on the right shows an Arizona Geological Survey drone flight at an earth fissure in Pinal County.
What causes earth fissures?
Distinct from seismic fault lines or desiccation cracks, earth fissures are essentially human-caused geohazards.
Groundwater pumping in desert basins lowers the water table and causes land subsidence. Around the margins where land is not sinking at the same rate, this in turn puts tensional forces on the earth, causing fissures.
Fissures often become visible after heavy monsoon rains or irrigation runoff; when surface water is introduced to the compromised area, the ground opens up.
The map below illustrates a few of Arizona's notable earth fissure sites. Fissures are surveyed, catalogued, and mapped by the Arizona Geological Survey (AZGS) Earth Fissure Program .
To date, the AZGS has identified over 170 miles of fissures throughout the state. Almost all are in areas of measured land subsidence.
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1
Chandler Heights "Y-Crack"
In July 2007, a heavy monsoon thunderstorm inundated the Chandler Heights area of northern Pinal County, and an earth fissure opened up amidst homes, roads, and livestock pastures. The fissure was more than 40 feet deep in some places and up to 15 feet wide (Diaz, 2007); this event was deadly for a 13-year-old horse named Cash who fell into the crack.
2
Apache Junction Fissure
In 2018, an earth fissure destroyed a business parking lot and a section of Houston Ave while a van was driving over it; the driver was uninjured but the road damage was extensive. The fissure created an undercut of about 15 feet beneath the road.
3
Rodgers Fissure
One of the largest and most dramatic earth fissures in the state, this feature is located in western Maricopa County near the Harquahala Valley, which is also a known area of agriculture-related land subsidence. The Rodgers fissure is about one mile long and more than 45 feet deep in places.
4
Picacho Basin
This fissure system made news when it suddenly expanded in 2017, with depths of 30+ feet and a length of several miles. The area near Picacho Peak is now considered extremely hazardous for popular outdoor recreation activities such as horseback riding and off-roading.
Modeling the risk
Like many geological hazards, the exact when and where of earth fissure formation is unpredictable, with serious impacts to life and property, but we do know how they form and the factors associated with them.
Rather than predicting the next earth fissure, the relative likelihood of an area's conditions for earth fissures can be modeled. And if we know where they're more likely, builders and developers can be proactive to mitigate the risk.
The study area
Maricopa and Pinal counties contain some of the fastest growing communities in the country.
Some of these areas are also prone to earth fissures, including Casa Grande, the Phoenix East Valley (Queen Creek, Gilbert, Apache Junction, and San Tan Valley) and the West Valley (Buckeye and Surprise).
Given the potential impact on both existing populations and new growth and the fact that Maricopa and Pinal already have a large number of mapped fissures as inputs to the model, these two counties together made an ideal study area.
Weights of Evidence Model
Likelihood modeling brought in several known geological characteristics and factors that contribute to earth fissure formation:
- Land subsidence
- Proximity to surface bedrock or mountains
- Alluvial soils
"Weights of Evidence" is a type of data-driven modeling methodology first adapted for geology by the USGS, and has since been used successfully in emergency management for identifying potential hazard risk areas.
It generates favorable vs. not favorable areas for a given phenomenon by comparing known occurrences (existing earth fissures) with these input variables.
Results
By calculating the weight of each variable (i.e. how much it contributes to earth fissure formation) based on how many known earth fissures are in those areas, a likelihood map was generated.
The resulting efficiency was 84%, indicating a successful model.
In this example (right) of the model output near Picacho, in Pinal County, the earth fissure occurrences trained the model to recognize the most favorable conditions, resulting in the likelihood areas (red and orange).
Earth fissure likelihood zones
The areas mapped during the modeling process were classified based on probability, with high likelihood, potential likelihood, and potential due to the presence of a land subsidence zone.
The likelihood may be less than modeled in areas that are heavily urbanized with extensive hardscape, pavement, and drainage systems.
Zoom, search, and explore the map below to see the model results. Search for an address on the top left, and expand the legend on the bottom left.
Please note: these maps are for educational use and mitigation awareness. The areas depicted are the result of a likelihood model only and do not constitute a prediction of future earth fissure activity. This is not intended as a legal or other authoritative source on earth fissure hazard risk at any particular location. In no event will the author be liable for any direct or indirect damages resulting from the use or misuse of these data.
Earth Fissure Likelihood Zones
Mitigation measures
An important principle in emergency management is to "mitigate as you grow." This means that it is much easier (and less expensive) to manage hazard risk prior to and during the construction process.
Hazard Mitigation Awareness:
Illustrating Potential Impacts to New Development
The map on the right shows planned major developments that are also in earth fissure likelihood zones of High or Potential. The planned developments are symbolized by density: number of future dwelling units per acre.
In addition to the likelihood zones, the map also shows the existing fissures mapped by the AZGS.
Pan, zoom, and explore the map to see where it's important to raise awareness of earth fissure mitigation strategies for builders, developers, and municipal planners.
Reducing impacts through good mitigation practices
In areas where earth fissures are more likely, mitigation measures are necessary during planning, development, and construction. With proper management of the potential risk, new construction can proceed safely.
According to a report by the National Institute of Building Sciences, for every $1 invested in disaster mitigation, society saves $6.
During planning and permitting:
Well-engineered drainage for stormwater and/or irrigation
- This is especially important if building near a known, established earth fissure, since the introduction of surface water can cause additional erosion and expansion
Soil tests, trenches, and geophysical surveys
- This will help identify subsurface fissures that have not yet opened
Planning for sustainable water use at the community level
- Addressing the excessive groundwater pumping that causes land subsidence will go a long way in preventing fissures from forming
During construction of roads, buildings, and utilities:
Backfill and compaction
- Adding 50%+ to standard depth backfill with layered compaction
Post-tensioned foundations
Continually Reinforced Concrete Pavement (CRCP)
Geosynthetic mesh or geotextiles
Flexible/ductile/chain-structured water and wastewater lines
- Should an earth fissure form, this will protect the integrity of both city water supply and the aquifer/groundwater beneath
While earth fissures can be dangerous and damaging when they appear in populated areas, they can also be well-mitigated with thoughtful planning and building practices.
Preventing earth fissures in the first place through sustainable water resource management at a policy level is the ultimate goal, but in the meantime, awareness of how and where this geohazard can occur will help lessen the impacts to humans and animals.
