On April 25, 2015, Nepal experienced its worst natural disaster in over 80 years, when an Mw 7.8 earthquake struck this mountainous region of South Asia. Geologists had anticipated a significant earthquake for decades because of Nepal’s location high in the Himalayas, above the collisional boundary where India is being thrust into Asia. Nepal’s capital, Kathmandu, reportedly shifted 3 meters (10 feet) to the south during the event. The earthquake killed nearly 9000 people across Nepal, India, China, and Bangladesh, resulted in injuries to more than 22,000 others, and left more than 2.8 million people without housing or shelter. With a hypocenter at a depth of only 12 kilometers (8 miles), the shallow nature of the 50-second quake resulted in widespread destruction. Over a broad area, buildings were severely damaged or destroyed, especially those made of rock, brick, and unreinforced concrete (Figure 20). On Mt. Everest, the shaking triggered avalanches, claiming the lives of 19 people, including international hikers and Sherpa guides. Numerous landslides throughout the region blocked roads and delayed relief efforts.

Destruction from Seismic Vibrations

Many factors determine the degree of destruction that accompanies an earthquake. The most obvious is the magnitude of the earthquake and its proximity to a populated area. During an earthquake, areas near the epicenter tend to experience the most ground shaking and the vibrations lessen with distance. This is because propagating surface waves lose energy and diminish in amplitude as they travel away from their source. The nature of the materials through which the waves travel can cause them to impact small or very large areas. As described earlier, earthquakes that occur in the stable continental interior, such as the 1811–1812 earthquakes that hit New Madrid, Missouri, are generally felt over a much larger area than those in earthquake-prone areas, such as California (refer to Figure 16). This is because the amplitudes of the seismic waves decrease more slowly in the old, dense bedrock of the Eastern United States as compared to the younger, heavily faulted rock of California.

Shaking 

As the energy released by an earthquake travels along Earth’s surface, it causes the ground to vibrate in a complex manner involving up-and-down as well side-to-side motion. The amount of damage to human-made structures attributable to the shaking depends on several factors, including (1) the intensity and duration of the vibrations, (2) the building materials and construction practices of the region, and (3) the nature of the material on which structures rest. Strong earthquakes (those with the greatest intensity) generally have a longer duration of shaking than weaker earthquakes. Recall that fault rupture begins at the hypocenter and then propagates (travels) along the fault surface. The larger the area of rupture, the longer the shaking, and the more potential for structural damage. For example, the Great Alaskan Earthquake of 1964—the most violent ever recorded in North America at a magnitude of 9.2—shook the ground for 3 to 4 minutes. In comparison, the shaking of the Mw 7.0 earthquake that impacted Anchorage, Alaska, in 2018 lasted only about 30 seconds.

When the earthquake struck Alaska in 1964, all of the multistory structures in Anchorage were damaged by the shaking. The more flexible wood-frame residential buildings fared best. A striking example of how construction variations affect earthquake damage is shown in Figure 21. You can see that the steel-frame Shanteri’s building (lower left) withstood the shaking, whereas the poorly designed JCPenney building (center) was badly damaged.

Structural engineers have learned that buildings constructed of blocks and bricks that are not reinforced with steel rods are the most serious safety threats in earthquakes. Wood-framed structures tend to be more earthquake resistant because they are able to flex during shaking and, therefore, not collapse. Unfortunately, most of the buildings in low to middle income nations are constructed of unreinforced concrete slabs and bricks made of dried mud—a primary reason the death toll of earthquakes in Haiti, Nepal, and Mexico is usually higher than earthquakes of similar size in Japan, New Zealand, and the United States.

Amplification of Seismic Waves

Although the region near an earthquake’s epicenter experiences seismic waves of about the same energy, destruction may vary considerably in the area due to the nature of the ground on which the structures are built. Loose or unconsolidated sediments and rock, for example, amplify the seismic vibrations more than solid bedrock. In the 2018 earthquake that struck Anchorage, buildings and roadways situated on saturated, unconsolidated sediments experienced the most damage (Figure 22).

Liquefaction

The intense shaking of an earthquake can cause loosely packed water-logged materials, such as sandy stream deposits, to be transformed into a substance that acts like a fluid. The phenomenon of transforming a somewhat stable soil into mobile material capable of rising toward Earth’s surface is known as liquefaction. When liquefaction occurs, the ground may not be capable of supporting buildings, causing them to sink (Figure 23), and underground storage tanks and sewer lines may literally float toward the surface.

During the 1989 Loma Prieta earthquake, in San Francisco’s Marina District, more than 70 structures were severely damaged as foundations failed and geysers of sand and water shot from the ground, evidence that liquefaction had occurred (Figure 24). During the 2011 earthquake that hit Japan, liquefaction caused entire buildings to sink several feet. And a 2018 Mw 7.5 earthquake in Indonesia caused large-scale liquefaction and subsequent mudflows, destroying thousands of buildings and causing hundreds of deaths.

Surface Rupture

When slippage along a fault affects the Earth’s surface, visible offsets may occur (refer to Figure 5.3). These breaks at the surface are dependent on the nature of the surface materials and the way the fault moves. Ruptures may be simple horizontal offsets, cracks in the ground, or may involve large blocks of material moving up or down. For example, the 1959 M 7.3 earthquake near Hebgen Lake, Montana, resulted in the lake-side of the fault dropping more than 6 meters (20 feet) in a rupture that is still visible today. The impacts of surface ruptures to human-made structures, include buckling pavement, breakage of utility lines, and extensive building damage.

Landslides and Ground Subsidence

The greatest earthquake-related damage to structures is often caused by landslides and ground subsidence triggered by earthquake vibrations. Landslides occur when masses of rock or sediment move down a slope due to gravity, a topic we will explore more later this semester. During the 1964 Great Alaska earthquake in the port cities of Valdez and Seward, violent shaking caused coastal sediments to slump, carrying away both waterfronts. And in Turnagain Heights, an area of Anchorage, Alaska, homes were destroyed when a layer of clay lost its strength and over 200 acres of land slid toward the ocean. Downtown Anchorage was also disrupted as sections of the main business district dropped by as much as 3 meters (10 feet). During the 2015 Nepal earthquake, more than 20,000 landslides buried houses and villages and damaged roads and utility lines. The mountainous area, weakened by the shaking, continued to have landslides in the periods of heavy rain and aftershocks following the main earthquake. The largest and most devastating landslide completely submerged the village of Langtang (Figure 25), located in a popular trekking (backpacking) region. This slide began as a snow and ice avalanche that gathered rock debris from the mountain slope, moving 2 million cubic meters (70 million cubic feet) of debris downslope. The landslide buried or destroyed all the houses of the village and killed as many as 200 villagers, guides and porters, and international trekkers.

Fire 

From the mid-nineteenth century to the start of the twentieth century, San Francisco was the economic center of the western United States, largely because of gold and silver mining. Then, at dawn on April 18, 1906, a violent earthquake struck, triggering an enormous firestorm (refer to Figure 2). Much of the city was reduced to ashes and ruins. It is estimated that 3000 people died, and more than half of the city’s 400,000 residents were left without housing.

Figure 2: Earthquakes can trigger fires

The historic San Francisco earthquake reminds us of the formidable threat of fire, which started when the quake severed gas and electrical lines. The initial ground shaking broke the city’s water lines into hundreds of disconnected pieces, which made controlling the fires virtually impossible. The fires, which raged out of control for 3 days, were finally contained when expensive houses along Van Ness Avenue were dynamited to provide a fire break, similar to the strategy used in fighting forest fires.

While few deaths were attributed to the San Francisco fires, other earthquake-initiated fires have been more destructive, claiming many more lives. For example, the 1923 earthquake in Japan triggered an estimated 250 fires, devastating the city of Yokohama and destroying more than half the homes in Tokyo. More than 100,000 deaths were attributed to the fires, which were driven by unusually high winds. And in 2024, shaking from the Mw 7.5 Noto earthquake in Japan damaged electrical wires and sparked a fire that raged for 24 hours, devastating a residential area and the historic Wajima marketplace.

Tsunamis

Major undersea earthquakes may set in motion a series of large ocean waves that are known by the Japanese name tsunami (“harbor wave”). Most tsunamis are generated by displacement along a megathrust fault that suddenly lifts a large slab of seafloor (Figure 26). Once generated, a tsunami resembles a series of ripples formed when a pebble is dropped into a pond. In contrast to ripples, however, tsunamis advance across the ocean at amazing speeds, about 800 kilometers (500 miles) per hour—equivalent to the speed of a commercial airliner. Despite this striking characteristic, a tsunami in the open ocean can pass undetected because its height (amplitude) is usually less than 1 meter (3 feet), and the distance separating wave crests ranges from 100 to 700 kilometers (60 to 425 miles). However, upon entering shallow coastal waters, these destructive waves “feel bottom” and slow, causing the water to pile up (refer to Figure 26). A few exceptional tsunamis have approached 20 meters (65 feet) in height. As the crest of a tsunami approaches the shore, it appears as a rapid rise in sea level with a turbulent and chaotic surface; it usually does not resemble a breaking wave.

The first warning of an approaching tsunami is often the rapid withdrawal of water from beaches, which is the result of the trough of the first large wave preceding the crest. Some inhabitants of the Pacific basin have learned to heed this warning and quickly move to higher ground. Approximately 5 to 30 minutes after the retreat of water, a surge capable of extending several kilometers inland occurs. In a successive fashion, each surge is followed by a rapid oceanward retreat of the sea. Therefore, people experiencing a tsunami should not return to the shore when the first surge of water retreats.

Tsunami Damage from the 2004 Indonesia Earthquake

A massive M 9.1 undersea earthquake occurred near the island of Sumatra on December 26, 2004, sending tsunami waves racing across the Indian Ocean and Bay of Bengal (Figure 27). It was one of the deadliest natural disasters of any kind in modern times, claiming more than 230,000 lives. As water surged several kilometers inland, cars and trucks were flung around like toys in a bathtub, and fishing boats were rammed into homes. In some locations, the backwash of water dragged bodies and huge amounts of debris out to sea. The destruction was indiscriminate, destroying luxury resorts as well as poor fishing hamlets along the Indian Ocean. Damage was reported as far away as the coast of Somalia in Africa, 4100 kilometers (2500 miles) west of the earthquake epicenter.

Japan Tsunami of 2011

Because of Japan’s location along the circum-Pacific belt and its extensive coastline, it is especially vulnerable to tsunami destruction. The most powerful earthquake to strike Japan in the age of modern seismology was the M 9.0 earthquake that in 2011 struck Tōhoku. This historic earthquake and devastating tsunami resulted in nearly 20,000 deaths, more than 2500 people missing, and 6127 injured. Nearly 400,000 buildings, 56 bridges, and 26 railways were destroyed or damaged. The tsunami was generated when a slab of seafloor located 60 kilometers (37 miles) off the east coast of Japan was suddenly “thrust up” an estimated 5 to 8 meters (16 to 26 feet).

The majority of human casualties and damage after the Tōhoku earthquake were caused by a Pacific-wide tsunami that reached a maximum wave height of 40 meters (131 feet) in the Iwate Prefecture on Japan’s main island, Honshu. In the region of Sendai, Japan, tsunami waves reached heights of 19.5 meters (64 feet) and traveled as far as 10 kilometers (6 miles) inland (Figure 28). In addition, the tsunami disabled the power supply and cooling mechanisms, which caused the meltdown of three inundated nuclear reactors in Japan’s Fukushima Daiichi Nuclear Complex. Across the Pacific in California, Oregon, Peru, and Chile, a tsunami generated by the Tōhoku earthquake caused some loss of life and the destruction of several houses, boats, and docks.

Tsunami Warning System

In 1946, a large tsunami struck the Hawaiian Islands without warning. A wave more than 15 meters (50 feet) high left several coastal villages in shambles and caused 165 deaths. This destruction motivated the establishment of the first official tsunami warning system for coastal areas of the Pacific Ocean. Today, the National Tsunami Warning Center provides warnings for the continental United States, Alaska, and Canada, and the Pacific Tsunami Center provides warnings for the Hawaiian Islands, the U.S. Pacific and Caribbean territories, and the British Virgin Islands. These centers also collaborate internationally with other warning centers. Scientists at the centers use deep-sea buoys equipped with pressure sensors to detect energy released by an earthquake. In addition, tidal gauges measure the rise and fall in sea level that accompany tsunamis, and warnings are issued within an hour. Although tsunamis travel very rapidly, collected data is processed quickly by models providing sufficient time to warn all except those in the areas nearest the epicenter. For example, a tsunami generated near the Aleutian Islands would take 5 hours to reach Hawaii, and one generated near the coast of Chile would travel 15 hours before reaching the shores of Hawaii (Figure 29).

• Factors influencing how much destruction an earthquake might inflict on a human-made structure include (1) intensity of the shaking, (2) how long shaking persists, (3) the nature of the ground that underlies the structure, and (4) building construction.

• Buildings constructed of unreinforced bricks and blocks are more likely than other types of structures to be severely damaged in a quake. In general, bedrock-supported buildings fare best in an earthquake, as loose sediments amplify seismic shaking.

• Liquefaction may occur when water-logged sediment or soil is severely shaken during an earthquake. Liquefaction can reduce the strength of the ground to the point that it may not support buildings.

• Earthquakes may also trigger landslides or ground subsidence, and they may break gas lines, which can initiate devastating fires.

• Tsunamis are large ocean waves that form when water is displaced, usually by a megathrust fault rupturing on the seafloor. Traveling at the speed of a jet aircraft, a tsunami is hardly noticeable in deep water. However, upon arrival in shallower coastal waters, the tsunami slows down and piles up, producing a wall of water sometimes more than 30 meters (100 feet) in height. Tsunamis cause major destruction in coastal areas if they strike the shoreline. Tsunami warning systems have been established in most of the large ocean basins.

• liquefaction: A phenomenon, sometimes associated with earthquakes, in which soils and other unconsolidated materials containing abundant water are turned into a fluid-like mass that is not capable of supporting buildings.

• tsunami: A series of very large ocean waves generated by the displacement of a large volume of water in association with earthquakes, underwater volcanic eruptions, or underwater landslides.