Earthquakes

Earthquakes occur when a critical amount of stress is applied to the Earth’s crust. 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 rock layers become offset along it.

Many earthquakes, including most of those that occur in the South Central US, arise along ancient, 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, which measures the amplitude of a seismic wave at a defined distance from the earthquake. Unfortunately, the Richter scale proved incapable of accurately measuring large earthquakes, so the Moment Magnitude scale (Mw) was introduced in 1979 as an alternative. Both the Richter and Moment Magnitude 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 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 quake would have 100 times the amplitude and 1024 times the energy of a M7.0 earthquake. The largest recorded earthquake in US history was the 1964 Alaskan earthquake, which had an Mw of 9.2. By comparison, the largest recorded earthquakes in the South Central were a cluster of four M7.5-M7.0 earthquakes that were centered around the New Madrid fault region in Missouri and Arkansas.

Figure 10.1: Elastic rebound.

The magnitude of an earthquake, however, does not tell us how much damage is done by the seismic waves in a particular area. 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 1931 Earthquake in Valentine, Texas, which is the largest earthquake known to have occurred in the state.

Earthquakes have occurred in each of the South Central states (Figure 10.3), but the greatest hazard potential is in the area of the New Madrid seismic zone (NMSZ), a 240-kilometer (150-mile) set of subsurface faults thought to have formed during the breakup of the supercontinent Rodinia in the late Precambrian (about 750 million years ago). Although this rift did not split the continent, it remains an underground weak point. The bedrock that makes up most of the central US is colder, drier, and less fractured than rocks on the East or West Coast. As a result, the earthquakes here can release the same amount of energy as other earthquakes, but the shaking affects a much larger area because the seismic waves travel through denser, more solid bedrock.

Figure 10.2: Intensity map of the 1931 Valentine earthquake.

Four of the largest earthquakes in North American history—the New Madrid Sequence—occurred in the NMSZ on three days over a period of three months: December 16, 1811, January 23, 1812, and February 7, 1812. The quakes, with estimated magnitudes between 7.0 and 8.0, occurred along the Mississippi River in southeastern Missouri and northern Arkansas, and shook the Mississippi Valley and much of the eastern United States. The tremors destroyed buildings and warped the ground, causing landslides along the Mississippi River bluffs and ground subsidence brought on by soil liquefaction across the Mississippi River flood plain. Shaking was felt as far away as New Orleans and Boston, where it is said to have caused church bells to ring, and the waters of the Mississippi River appeared to flow backwards for several days due to local uplift and waves flowing upstream. Hundreds of aftershocks followed for a period of a several years, and were felt regularly until 1817.

The next largest quake to have occurred along the NMSZ was a 6.6-magnitude quake that occurred on October 31, 1895. The quake, centered in Charleston, Missouri, damaged almost every building in the city. Even today, areas in the NMSZ continue to experience earthquake activity, which is closely monitored by seismologists. There are ancient, seismically inactive subsurface faults in many other parts of the country, and it is unclear why seismic activity remains so high along the faults in the NMSZ, which are now far from North America’s plate margins. Most of the dozens of annual earthquakes that occur in the NMSZ (Figure 10.4) are very small—too small to notice except with sensitive equipment. If a major earthquake were to occur there, it could be expected to produce landslides, fissures, soil liquefaction, and bridge and road failures. Interstate 55 in Arkansas could become impassable; flooding of farmland could contaminate rivers and streams with mud, sand and agricultural chemicals; and the failure of levees and riverbanks could make the Mississippi River and its tributaries difficult to navigate for many weeks.

Figure 10.3: Intense earthquakes in the South Central between 1750 and 1996, measured using a Modified Mercalli Intensity of VI to XI. The large red squares represent the New Madrid earthquakes of 1811 and 1812.

Another area that presents modest seismic risk is the Nemaha Uplift in northern Oklahoma and eastern Kansas (Figure 10.5). The seismic activity around the Nemaha Uplift is associated with faulting known as the Humboldt fault zone, which, like the NMSZ, lies along a Precambrian basement and ancient rift system.

Recently, Oklahoma has experienced an unusual amount of earthquake activity, with numerous earthquakes of magnitude 3 or 4 and a few above magnitude 5 (Figure 10.6). Only 89 earthquakes occurred in the state between 1970 and 2009, but since then the incidence has increased dramatically, rising from 48 earthquakes in 2010 to 611 in 2014 alone. The seismic activity in these instances has been 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. Concerns exist that additional activity along offshoot faults from the Nemaha Uplift near Oklahoma City might be even more serious. Similar instances of induced seismic activity have occurred elsewhere in the South Central, perhaps most famously with cases associated with injection wells near Dallas-Fort Worth. These wells have been used to dispose of wastewater from the extraction of natural gas in the Barnett Shale.

Figure 10.4: The epicenters of over 4000 earthquakes recorded from 1974 to 2006, with the highest density of events falling along the New Madrid seismic zone.

Networks of seismograph stations have improved geologists’ ability to detect and accurately locate earthquake hazards (Figure 10.7), and specific fault zones are being studied throughout the South Central. 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.

Figure 10.5: Earthquakes recorded between 1977 and 1989, with Richter magnitudes from 0.8 to 4.0, relative to the position of the Nemaha Uplift and Humboldt fault zone.

Figure 10.6: Seismic activity in Oklahoma. Greatly increased seismic activity in 2013 - 2015 has been linked to injection wells.

Figure 10.7: Seismic hazard map of the South Central US, based on data in 2014.