About 95 percent of the energy released by earthquakes originates in a few relatively narrow zones, shown in Figure 30. These zones of earthquake activity are located along fault surfaces where tectonic plates interact along one of the three types of plate boundaries—convergent, divergent, and transform plate boundaries.

Earthquakes Associated with Plate Boundaries

The previous section described the relationship between plate tectonics and seismic activity. The zone of greatest seismic activity, called the circum-Pacific belt, encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian Islands (refer to Figure 5.30). Most of the large earthquakes in the circum-Pacific belt occur along convergent plate boundaries where one plate subducts beneath another at a comparatively low angle. Recall that the contacts between the subducting and overlying plates are called megathrust faults (refer to Figure 5). There are more than 40,000 kilometers (25,000 miles) of subduction boundaries in the circum-Pacific belt where displacement is dominated by thrust faulting. Ruptures occasionally occur along segments that are nearly 1000 kilometers (600 miles) long, generating catastrophic earthquakes with magnitudes of 8 or greater.

Another major concentration of strong seismic activity, referred to as the Alpine–Himalayan belt, runs through the mountainous regions that flank the Mediterranean Sea and extends past the Himalaya Mountains (refer to Figure 30). Tectonic activity in this region is mainly attributed to collisions of the African plate and the Indian subcontinent with the vast Eurasian plate (refer to Figure 18). These plate interactions created many thrust and strike-slip faults that remain active.

Transform fault boundaries, which are large strike-slip faults, are also a source of strong earthquakes. Examples include California’s San Andreas Fault, New Zealand’s Alpine Fault, Turkey’s North Anatolian Fault, and Turkey’s East Anatolian Fault, which produced two catastrophic earthquakes in 2023.

Figure 30 shows another continuous earthquake belt that extends thousands of kilometers through the world’s oceans. This zone coincides with the oceanic ridge system—a divergent plate boundary—which is an area of frequent but weak seismic activity.

Damaging Earthquakes East of the Rockies

When you think about earthquakes in the United States, you probably think of the western part of the country. However, six major earthquakes and several others that inflicted considerable damage have occurred in the central and eastern United States since the early seventeenth century (Figure 31).

Three of these quakes, which occurred as a cluster, had estimated magnitudes of about 7.0 and destroyed what was then the frontier town of New Madrid, Missouri, located in the Mississippi River valley. It has been estimated that if an earthquake the size of the 1811–1812 New Madrid event were to strike in the same location in the next decade, it would result in casualties in the thousands and damages in the hundreds of billions of dollars.

The greatest historical earthquake in the eastern states occurred on August 31, 1886, in Charleston, South Carolina. This earthquake resulted in 60 deaths, numerous injuries, and great economic loss over an area extending 200 kilometers (120 miles) from Charleston. Within 8 minutes, effects were felt as far away as Chicago and St. Louis, where strong vibrations shook the upper floors of buildings, causing people to rush outdoors. In Charleston alone, more than 100 buildings were destroyed, and 90 percent of the remaining structures were damaged (Figure 32).

Earthquakes in the central and eastern United States occur far less frequently than in California, yet history indicates that the East is vulnerable. Further, these shocks east of the Rockies have generally produced structural damage over a larger area than earthquakes of similar magnitude in California (refer to Figure 16). This is because the underlying bedrock in the central and eastern United States is older and more rigid. As a result, seismic waves can travel greater distances with less attenuation (loss of strength) than in the western United States.

Earthquakes that occur away from plate boundaries are called intraplate earthquakes. Intraplate earthquakes can be caused by a variety of factors. For example, stress can rejuvenate ancient fault systems that formed when crustal fragments collided to generate the continent billions of years ago (refer to Figure 12). In addition, the process called fracking, in which a solution is injected into the ground under high pressure to enhance oil and gas production, has contributed to the recent increase in earthquake activity east of the Rockies, especially in Oklahoma. Most of these intraplate earthquakes are weak, although a few have been moderately strong.

• Most earthquake energy is released in the circum-Pacific belt, the ring of megathrust faults rimming the Pacific Ocean. Another earthquake belt is the Alpine–Himalayan belt, which runs along the zone where the Eurasia plate collides with the Indian subcontinent and African plates.

• Earth’s oceanic ridge system produces another belt of earthquake activity. Seafloor spreading and active transform faults that separate ridge segments generate many frequent small-magnitude quakes. Transform faults in the continental crust, including the San Andreas Fault, can produce large earthquakes.

• Although most destructive earthquakes are produced along plate boundaries, some occur at considerable distances from plate boundaries. Examples include the 1811–1812 New Madrid, Missouri, earthquakes and the 1886 Charleston, South Carolina, earthquake.

• Circum-Pacific belt: An area approximately 40,000 kilometers (24,000 miles) in length surrounding the basin of the Pacific Ocean where oceanic lithosphere is continually subducted beneath the surrounding continental plates, causing most of Earth’s largest earthquakes. Also referred to as the Ring of Fire.