An earthquake is the sudden, rapid release of stored energy within the Earth’s crust, which occurs when tectonic stresses overcome the friction along a fault line. This rupture generates seismic waves that radiate outward, causing the ground to vibrate violently. The destructive power of an earthquake is not limited to this initial shaking but manifests through a variety of complex physical processes.
Direct Damage from Seismic Wave Motion
The most immediate cause of damage is the intense ground shaking produced by Primary (P) waves, Secondary (S) waves, and Surface waves. The fastest P-waves cause vertical ground movement that buildings are generally best equipped to handle. However, S-waves and the slower-moving Surface waves cause intense horizontal and rolling motions, responsible for the most significant structural failure.
When the ground beneath a structure shifts violently, the building’s mass resists this change in motion due to inertia. This resistance creates immense shear forces throughout the structure as the foundation attempts to move with the ground while the upper floors lag behind. If the force exceeds the building materials’ strength, structural elements like columns and beams fail, leading to collapse.
A particularly damaging phenomenon is structural resonance, which occurs when the frequency of the incoming seismic waves matches a building’s natural frequency of vibration. Every structure possesses a natural frequency determined by its height, mass, and stiffness. When the two frequencies align, the shaking is dramatically amplified, causing the building to oscillate with increasing violence and leading to failure.
Low-rise, stiff buildings are more susceptible to the higher frequencies typically associated with P and S waves, while taller, more flexible structures are vulnerable to the lower frequencies of Surface waves. The horizontal, side-to-side motion of S-waves and Love waves is especially destructive because most buildings are engineered to resist vertical gravity loads, not the lateral forces of ground shear.
Ground Instability and Failure
Beyond the direct vibration of structures, the intense seismic shaking can fundamentally alter the ground beneath them, leading to damage independent of a building’s design. This is especially true for areas with loose, saturated soils susceptible to liquefaction. During liquefaction, seismic waves increase the water pressure within the soil, causing the granular material to lose strength and temporarily behave like a viscous fluid.
When the supporting soil loses stiffness, structures resting on shallow foundations can sink, tilt, or topple, as seen in past events like the 1964 Niigata earthquake. Liquefaction also causes underground infrastructure, such as pipelines and utility lines, to float upward and rupture due to the buoyancy of the surrounding liquefied soil.
Seismic movement also destabilizes slopes in hilly or mountainous regions, triggering widespread landslides and slumping. Ground shaking reduces the shear strength of the soil and rock masses, leading to massive downward movements of earth that can bury or sweep away homes, roads, and infrastructure. The 1994 Northridge earthquake, for example, triggered over 11,000 landslides.
Furthermore, direct fault rupture can cause damage when the ground surface permanently breaks and shifts along the fault line. Any structure built directly across this rupture zone is subject to massive displacement, which can tear foundations, roads, and utility lines apart. This permanent ground deformation is a direct physical tearing of the earth’s surface, a completely different mechanism from vibratory shaking or soil fluidization.
Secondary Environmental Hazards
Earthquakes often trigger secondary events that contribute significantly to the overall destruction and loss of life. One common post-earthquake hazard is fire, ignited by ruptured gas lines, downed power lines, and damaged electrical systems. The shaking can simultaneously break water mains, complicating firefighting efforts and allowing urban blazes to spread rapidly and uncontrollably, as seen after the 1906 San Francisco earthquake.
In coastal areas, a large undersea earthquake that causes a sudden vertical displacement of the seafloor can generate a tsunami. This massive displacement of water creates a series of powerful waves that travel across the ocean, leading to destructive coastal inundation and flooding when they reach land. The force of the water and debris it carries can flatten buildings and cause widespread infrastructure failure.
The failure of engineered infrastructure can also lead to secondary disasters. A seismic event can compromise large retaining structures like dams, potentially leading to catastrophic downstream flooding. Similarly, the rupture of storage facilities for hazardous materials, such as chemical plants or nuclear power facilities, can result in chemical spills or the release of radioactive materials, causing long-term damage.