Earthquakes occur less frequently in the Southwestern US than they do in some other regions, but modest-sized earthquakes nonetheless represent potential hazards for the Southwestern states. Earthquakes occur when a critical amount of stress is applied to the Earth’s crust and the crust responds by moving. According to the elastic rebound theory, rocks can bend elastically up to a point, until they finally break. The rocks then snap apart, releasing energy in the form of seismic waves (Figure 9.12). The plane defined by the rupture is known as a fault, and the surrounding rock layers become offset along it.

Many earthquakes, including most of those that occur in the Southwestern US, arise along pre-existing faults. In cases such as these, stress may accumulate from lateral compressive pressure, as the rocks are temporarily locked in position by friction and other constraints, until sufficient strain energy has built up to cause sudden slippage along the fault (i.e., an earthquake). Earthquakes have many different effects on the rocks in which they occur, including breaking and movement along faults, uplift, and displacement.

There are two common ways to measure the size of earthquakes: magnitude and intensity. Magnitude (M) is the measure of the energy released by the earthquake, whereas the intensity is what people actually experience. The first scale used to measure magnitude was the Richter scale (abbreviated M_L), which measures the amplitude of a seismic wave at a defined distance from the source of the earthquake. The Richter scale was designed to classify earthquakes at a local scale, but it does not do a very good job of describing the energy released by very large earthquakes. Geologists therefore developed another measurement, the Moment Magnitude scale (abbreviated M_w), which was introduced in 1979. The Moment Magnitude estimates the total energy released by an earthquake along an entire fault surface.

Figure 9.12: Elastic rebound.

Both the Richter and Moment Magnitude scales are logarithmic, meaning that an M9.0 earthquake has 10 times the amplitude, and releases 32 times the energy, of an M8.0 earthquake. Accordingly, an M9.0 earthquake would have 100 times the amplitude and 1024 times the energy of an M7.0 earthquake. Both scales may appear to reach maximum values of 10 (since the largest recorded earthquakes are slightly greater than 9), but technically there is no upper limit. The United States Geological Survey (USGS) describes earthquakes as minor (M3.0–3.9), light (M4.0–4.9), moderate (M5.0–5.9), strong (M6.0–6.9), major (M7.0–7.9), and great (M8.0 and higher). The largest recorded earthquake in US history was the 1964 Alaskan earthquake, which had an M_w of 9.2. By comparison, the largest recorded earthquake in the Southwest occurred in 1934 in Kosmo, Utah (M6.6). Notable earthquakes that have occurred just outside the Southwestern states, such as the 1887 Sonoran earthquake (M7.4) and the 1940 Imperial Valley earthquake (M7.1), have also caused extensive shaking and property damage, especially in Arizona.

The magnitude of an earthquake does not tell us how much damage it causes. The amount of shaking and damage is known as the earthquake’s intensity, and it can be measured by the Modified Mercalli Intensity (MMI) scale. This scale uses the Roman numerals I–XII to describe the effects of the earthquake in a particular location. For example, near the epicenter of a small earthquake, or at a location far from a large earthquake, the intensity may be described with an MMI of II: “Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing.” Unlike the Moment Magnitude scale, the MMI scale is a subjective gauge, and the USGS has attempted to improve the accuracy of MMI shake maps by soliciting data from the public. Figure 9.13 shows the intensities felt in surrounding areas after the 1934 earthquake near Kosmo, Utah, which is the largest earthquake known to have occurred in the state.

Large earthquakes are relatively uncommon in the Southwestern US, due to the area’s distance from current plate boundaries—the Southwest is located in the center of a tectonic plate rather than at an active plate margin. All earthquakes that occur in the Southwestern US are therefore referred to as “intraplate” earthquakes, and they are largely related to faults that localize earthquakes in particular areas, along linear seismic belts or zones. Many of the largest earthquakes in the Southwest, especially those in the Rocky Mountains, stem from activity along the Intermountain Seismic Belt (Figure 9.14). This linear zone of earthquake activity extends 1290 kilometers (800 miles) from northwestern Montana southward along the Idaho-Wyoming border, through Utah, and into southern Nevada. Many active fault lines occur along this belt; the largest and most active is the Wasatch Fault, which marks the eastern edge of Basin and Range extension (Figure 9.15). Geologic studies indicate that the Wasatch Fault has experienced 19 or more surface-faulting earthquakes in the last 600 years. Some of these prehistoric earthquakes displaced the land surface by as much as 3 meters (10 feet) in a 30- to 65-kilometer (20- to 40-mile) radius, while others formed fault scarps over 6 meters (20 feet) high. Because the Wasatch Front is such a desirable place to live—about 80% of Utah’s population resides along this mountain range, known for its spectacular views—the area is designated as having the greatest earthquake risk in the interior western US. Scientists estimate that the Wasatch Range has a 1-in-7 chance of being hit by a M7.0 earthquake sometime in the next 50 years.

Figure 9.13: Intensity map of the 1934 Kosmo earthquake.

Figure 9.14: Since 1850, there have been 35 earthquakes in Utah with a magnitude of 5 or higher. All have occurred in or near the Intermountain Seismic Belt.

Figure 9.15: The Wasatch Fault is one of many active faults in the Intermountain Seismic Belt, which runs from Montana through Utah. The largest earthquakes in the Southwest, both historical and prehistoric, have occurred in this seismic belt.

The Northern Arizona Seismic Belt is an offshoot of the Intermountain Seismic Belt that extends south into Arizona along the Colorado Plateau. Faults in this zone are Quaternary in age, and are thought in part to have formed due to stress between the Basin and Range and edge of the Colorado Plateau. Swarms of tiny earthquakes occur along these fault lines in Arizona every year, most too small to be felt. Major quakes can and do occur, though, and geologists are keeping a close eye on the Anderson Mesa Fault near Flagstaff, which could conceivably produce an earthquake between M5.0 and M6.5.

Earthquakes can also occur through human causes, or “induced seismicity.” These events are specifically linked to the high-pressure injection of wastewater from oil and gas extraction operations into the ground. The pressure of the water increases the likelihood that a rupture might occur along an otherwise locked fault. In early 2016, the US Geological Survey released a list of states considered to be at the highest risk for manmade earthquakes. Colorado and New Mexico rank fourth and fifth respectively due to the presence of the Raton Basin, an important source of coalbed methane and natural gas.

Networks of seismograph stations have improved geologists’ ability to detect and accurately locate earthquake hazards (Figure 9.16), and specific fault zones are being studied throughout the Southwest. This information on earthquake risk can lead to better designs for high-risk infrastructure like dams, high-rise buildings, and power plants—and it can also be used to inform the public of potential hazards to lives and property. The hazards associated with earthquakes are mainly related to collapsing buildings and other structures, as well as fire related to broken gas lines and other utilities (and broken water lines that prevent fire-fighting).

Figure 9.16: Seismic hazard map of the Southwestern US, based on 2014 data.