Effects
Ground shaking leads to landslides and other soil movement. These are the main damage-causing events that occur during an earthquake. Primary effects that can accompany an earthquake include property damage, loss of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of food and clean water, also occur after a large earthquake.
A Ground Shaking and Landslides
Earthquake waves make the ground move, shaking buildings and causing poorly designed or weak structures to partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or further property damage.
B Fire
Another post-earthquake threat is fire, such as the fires that happened in San Francisco after the 1906 earthquake and after the devastating 1923 Tokyo earthquake. In the 1923 earthquake, about 130,000 lives were lost in Tokyo, Yokohama, and other cities, many in firestorms fanned by high winds. The amount of damage caused by post-earthquake fire depends on the types of building materials used, whether water lines are intact, and whether natural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and fire fighting cannot be effective if the water mains are not intact to transport water to the fires. Fires can be significantly reduced with pre-earthquake planning, fire-resistant building materials, enforced fire codes, and public fire drills.
C Tsunami Waves and Flooding
Along the coasts, sea waves called tsunamis that accompany some large earthquakes centered under the ocean can cause more death and damage than ground shaking. Tsunamis are usually made up of several oceanic waves that travel out from the slipped fault and arrive one after the other on shore. They can strike without warning, often in places very distant from the epicenter of the earthquake. Tsunami waves are sometimes inaccurately referred to as tidal waves, but tidal forces do not cause them. Rather, tsunamis occur when a major fault under the ocean floor suddenly slips.
The displaced rock pushes water above it like a giant paddle, producing powerful water waves at the ocean surface. The ocean waves spread out from the vicinity of the earthquake source and move across the ocean until they reach the coastline, where their height increases as they reach the continental shelf, the part of the Earth’s crust that slopes, or rises, from the ocean floor up to the land. Tsunamis wash ashore with often disastrous effects such as severe flooding, loss of lives due to drowning, and damage to property.
Earthquakes can also cause water in lakes and reservoirs to oscillate, or slosh back and forth. The water oscillations are called seiches (pronounced saysh). Seiches can cause retaining walls and dams to collapse and lead to flooding and damage downstream.
D Disease
Catastrophic earthquakes can create a risk of widespread disease outbreaks, especially in underdeveloped countries. Damage to water supply lines, sewage lines, and hospital facilities as well as lack of housing may lead to conditions that contribute to the spread of contagious diseases, such as influenza (the flu) and other viral infections. In some instances, lack of food supplies, clean water, and heating can create serious health problems as well.
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