Arizona does experience earthquakes, a fact that often surprises both residents and visitors who associate seismic risk primarily with coastal states. While the state is generally characterized by a low-to-moderate seismic hazard profile, hundreds of earthquakes occur within its borders each year. This activity means that Arizona is not immune to the potential for significant ground shaking. The state’s seismic history includes events capable of causing localized damage.
Arizona’s Seismic Activity Profile
The vast majority of seismic events in Arizona are micro-quakes that are too small for people to feel. Earthquakes large enough to be felt by the public, typically magnitude 3.0 or higher, happen more frequently than many people realize. The state experiences a moderate-sized earthquake, magnitude 5.0 or greater, approximately once every five to ten years.
The largest earthquake recorded with an epicenter entirely within Arizona measured magnitude 5.6, occurring in 1959 near Fredonia. This event caused items to fall from shelves and resulted in minor damage to chimneys and walls. Arizona also experienced widespread shaking from the magnitude 7.6 Sonora, Mexico, earthquake in 1887, which was felt as far north as Phoenix.
The early 20th century saw a cluster of activity in northern Arizona, including three quakes between 1906 and 1912 that ranged from magnitude 6.0 to 6.2 near Flagstaff. These historical events demonstrate that the state is capable of generating significant ground motion. While the overall rate of seismic activity is lower compared to highly active regions like California, the potential for damaging earthquakes remains a hazard.
Tectonic Forces and Underlying Fault Systems
Arizona’s seismic activity is not caused by a major boundary between tectonic plates, but rather by internal deformation of the North American Plate. The state sits at the transition between two provinces: the relatively stable Colorado Plateau to the northeast and the actively stretching Basin and Range Province to the south and west. This boundary is the primary driver of current earthquake activity.
The Basin and Range Province is characterized by extensional tectonics, where the Earth’s crust is being slowly pulled apart. This stretching causes large blocks of the crust to drop and tilt along a series of fractures known as normal faults. Stress builds up over long periods until it exceeds the strength of the rock, causing a sudden slip along a fault and generating an earthquake.
The extensional forces and resulting faulting have been migrating slowly eastward and northward into the margin of the Colorado Plateau. This creates a wide transition zone where the crust is under significant stress and is prone to fracturing. Earthquakes in Arizona typically originate at depths of around 10 kilometers or deeper. Major faults in the northwest, such as the Grand Wash fault zone, are examples of the large, west-dipping normal faults accommodating this regional extension.
Regional Hotspots for Earthquake Activity
Seismic activity is not uniformly distributed across Arizona, with a distinct belt of higher activity known as the Northern Arizona Seismic Belt (NASB). The NASB is considered an extension of the larger Intermountain Seismic Belt, which stretches through Utah and into other western states.
Northern Arizona, particularly the area around Flagstaff and the Mogollon Rim, is a primary area of concern. The Lake Mary Fault, located south of Flagstaff, has the potential to generate earthquakes up to magnitude 7. Further north, the area near the Grand Canyon experienced the Cataract Creek earthquake sequence in 1993, which included events of magnitude 5.4 and 5.0.
Central Arizona, within the transition zone, also sees measurable activity, including the region near Prescott and the Chino Valley, which experienced a magnitude 5.0 quake in 1976. The Big Chino Fault, situated north of Prescott, is highly active. The Yuma area frequently feels the effects of strong earthquakes originating in Southern California and Baja California, Mexico. Southeastern Arizona is also a concern due to its proximity to the Pitaycachi fault in Mexico, the source of the devastating 1887 magnitude 7.6 event that caused significant damage in the Douglas area.
Risk Mitigation and Safety Measures
Given Arizona’s low-to-moderate seismic risk, preparedness efforts focus on mitigating the most common hazards associated with ground shaking. The primary dangers in a moderate earthquake are falling objects and non-structural damage, not ground ruptures. Securing items inside the home is an effective preventative measure.
Homeowners should focus on securing heavy items to wall studs:
- Bookcases
- Dressers
- Water heaters
- Refrigerators
In older homes, particularly those built with unreinforced masonry, the risk of structural damage is higher. It is especially important to secure interior items and be aware of exterior hazards like parapets.
The standard, universally recommended action during an earthquake is to “Drop, Cover, and Hold On.” This involves taking cover under a sturdy desk or table and holding on until the motion stops. Arizona promotes this practice through the annual Great Arizona ShakeOut drill. Preparing a small emergency kit with water, non-perishable food, and a first-aid kit is also important for any potential hazard.