Earthquakes

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 10.1). The plane defined by the rupture is known as a fault, and the surrounding rock layers become offset along it.

Figure 10.1: Elastic rebound.

Many earthquakes, including most of those that occur in the Northwest Central 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).

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.

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 earth quake 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 or 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 Northwest Central occurred in 1959 at Hebgen Lake, Montana (M7.3), near Yellowstone National Park.

The magnitude of an earthquake, however, 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 10.2 shows the intensities felt in surrounding areas after the 1983 Earthquake at Borah Peak, Idaho, which is the largest earthquake known to have occurred in the state.

Figure 10.2: Intensity map of the 1983 Borah Peak earthquake.

The Rocky Mountain and Columbia Plateau regions of the Northwest Central, including western Montana, northwestern Wyoming, and most of Idaho, compose one of the most seismically active areas in the United States (Figure 10.3), with as many as 3000 earthquakes occurring each year (although most are too small to feel). Most of these earthquakes are caused by a combination of two phenomena: the magmatic activity of the Yellowstone hot spot, and the (possibly related) tectonic activity of the Basin and Range region. The resulting crustal movements cause most earthquakes to be localized in particular areas, either around the Yellowstone area or along linear seismic belts or zones (Figure 10.4).

Figure 10.3: Seismic hazard map of the Northwest Central US, based on data in 2014.

Figure 10.4: Major seismic belts and zones of the Northwest Central US.

The Intermountain Seismic Belt is a major zone of earthquake activity that extends from the Flathead Lake region in the northwest corner of Montana, southward through Yellowstone Park, along the Idaho-Wyoming border, through Utah, and into southern Nevada. A branch of the Intermountain Seismic Belt, called the Centennial Tectonic Belt or Central Idaho Seismic Zone, extends west from the northwest corner of Yellowstone National Park through southwestern Montana and into central Idaho. This zone includes at least eight major active faults, and was the site of the two most severe earthquakes in the Rocky Mountains: the Hebgen Lake and Borah Peak earthquakes (Figure 10.5). The M7.3 Hebgen Lake earthquake, which occurred near the Montana-Wyoming Border in 1959, caused a major landslide that resulted in 28 fatalities as well as damming a river and destroying roads and buildings (Figure 10.6). The M6.9 Borah Peak earthquake occurred in Idaho in 1983, and caused extreme surface faulting as well as $12.5 million worth of damage to infrastructure in the surrounding Challis-Mackay area. A 34-kilometer-long (21-mile-long) fault scarp formed along the slopes of the Lost River Range; in other areas, the ground was shattered into huge blocks up to 100 meters (330 feet) in width.

Figure 10.5: Earthquake occurrences in the Central Idaho Seismic Zone and Western Idaho Seismic Zone between 1973 and 2009. Earthquake epicenters are shown in red. The locations of Borah Peak and Hebgen Lake are marked by stars.

Figure 10.6: Earthquake damage to State Highway 287 and the Hilgard Lodge near Hebgen Lake, Montana.

A geologically distinct region called the Western Idaho Seismic Zone lies between McCall and Boise. It is characterized by prominent north-south-trending basins and ranges that contrast strikingly with the surrounding area. A complex suture zone between accreted terranes and the ancient North American tectonic plate underlies the region and may influence the north-south orientation of the Zone’s faults. Major active faults in the Western Idaho Seismic Zone include the Squaw Creek fault and the Long Valley fault zone, which is notable for earthquake swarms. During a swarm, thousands of small shallow earthquakes occur over several weeks to months within a relatively small region.

The Lewis and Clark Zone is a megashear in the Earth’s crust, up to 48 kilometers (30 miles) wide, which runs some 386 kilometers (240 miles) through north Idaho and northwestern Montana. Geologic studies have shown that the North American plate has been sheared along this zone repeatedly over the past billion years, meaning that the rocks have been continuously fractured due to compressive stress. The most obvious manifestation of the zone is a set of parallel valleys that follow brittle fault zones across the grain of the northern Rocky Mountains from Helena and Missoula, Montana to Coeur d’Alene, Idaho. These valleys provided a natural transportation corridor through the mountains used in part by Lewis and Clark in 1806 and the Mullan Trail of the 1850s, and today by Interstate 90. Along the Lewis and Clark Zone in Idaho, many mining-related seismic events, called rockbursts, have occurred. Rockbursts are spontaneous, violent fractures of rock in deep mines. The sizable magnitudes of these events, their alignment with the direction of horizontal strain, and their location within the Lewis and Clark Zone suggest that tectonic stress release may be involved in causing them.

Earthquakes have many different effects on the rocks in which they occur, including breaking and movement along faults, uplift, and displacement. Earthquakes around Yellowstone National Park have altered the area’s extensive hydrothermal systems and may help to keep open the fractures and conduits that supply hot water to the surface. For example, both the 1959 Hebgen Lake and 1983 Borah Peak earthquakes caused measurable changes in the output of Old Faithful geyser and other hydrothermal features. Yellowstone is one of the most active seismic zones in the United States, and commonly experiences earthquake swarms (Figure 10.7). The largest swarm occurred in 1985, with more than 3000 earthquakes recorded on the northwest side of the park during a three-month period. Scientists believe these swarms are caused by shifting and changing pressures in the crust due to the migration of hydrothermal fluids, a common occurrence around volcanoes.

Figure 10.7: Earthquakes in Yellowstone National Park, 2014. Approximately 2000 earthquakes occurred during the course of the year.