An earthquake is the sudden release of energy in the Earth’s crust that creates seismic waves, causing the ground to shake. This ground movement is the direct source of destruction, threatening both natural landscapes and the human-built environment. The resulting damage is a complex series of cascading failures, categorized by immediate physical impact, the failure of the earth beneath, and subsequent hazards that follow the primary shaking.
Direct Damage to Built Structures
The most immediate concern during an earthquake is the violent shaking, which translates into powerful horizontal and vertical forces acting on built structures. Buildings are often designed to handle vertical (gravity) loads, making them highly vulnerable to the lateral motion of seismic waves. When the natural sway period of a building matches the frequency of the ground motion, a destructive phenomenon known as resonance occurs, leading to vastly amplified oscillations and structural failure.
Building design and age determine vulnerability. Older structures built before modern seismic codes often lack ductility—the ability to deform without fracturing.
Reinforced concrete structures that use non-ductile detailing can suffer brittle failure when columns and beams crush under intense shear stress. A common failure point is the “soft story,” a ground floor that is significantly less stiff than the stories above it, such as an open-plan parking garage or commercial lobby. This stiffness irregularity concentrates all the movement and damage onto the weakest floor, frequently leading to pancaking or total collapse.
Damage extends beyond buildings to critical non-building infrastructure like highway overpasses and bridges. These structures are often damaged by the differential movement of the ground, where one pier or abutment moves differently or at a different time than another. This movement can cause the bridge deck to “pound” against adjacent sections at expansion joints, or, more catastrophically, lead to span unseating. Unseating occurs when the bridge deck slides off its support bearings because the span’s seating length is insufficient to accommodate the large relative displacement caused by the shaking.
Structural damage is quantified using the Modified Mercalli Intensity (MMI) scale, which measures the effects of an earthquake on people, buildings, and the environment. An MMI of VIII, for instance, describes considerable damage to ordinary buildings, including partial collapse.
Damage from Ground Failure
Ground failure represents a distinct category of destruction, where the earth loses stability, causing massive damage to even well-engineered structures. One of the most destructive forms of ground failure is liquefaction, which occurs in loose, water-saturated, granular soils like sands and silts. Seismic shaking increases the water pressure in the pores between soil particles, temporarily reducing the soil’s shear strength until it behaves like a dense liquid.
When the ground liquefies, structures founded on it can sink or tilt dramatically as their foundations lose support. Underground objects, such as utility vaults or empty storage tanks, can float to the surface due to buoyancy. Liquefaction also causes lateral spreading, where large blocks of surface soil slide horizontally on a liquefied layer toward a free face, such as a riverbank, causing bridges to fail and utility lines to rupture.
In mountainous or hilly terrain, seismic vibrations destabilize slopes, leading to mass wasting events like landslides and rockfalls. These can bury entire communities, block transportation corridors, and temporarily dam rivers, creating a flood hazard when the natural dam fails.
Fault rupture is the most direct form of ground failure, involving the tearing of the Earth’s surface along the fault line. This rupture creates a visible displacement, often forming a vertical step known as a fault scarp. Any structure built directly over this line is subject to extreme, permanent deformation; a fence or road crossing the fault can be sliced in two and offset by meters. This shearing action causes catastrophic failure to foundations, pipelines, and utility conduits that cannot accommodate the movement.
Secondary and Water-Related Hazards
Following the initial ground shaking, a series of secondary hazards often cascade, extending the disaster’s impact. The most common secondary hazard is fire, frequently ignited by broken natural gas lines, ruptured fuel tanks, or electrical shorts from downed power lines. This hazard is exacerbated by the concurrent failure of the water supply network.
The disruption of utilities and infrastructure is a major consequence of seismic activity. Earthquakes commonly rupture water mains and sewer pipes, which can undermine roads and buildings and leave rescue personnel without water to fight fires. Communication networks and power grids are also vulnerable to failure, hampering response and recovery efforts.
Water-related hazards pose a significant threat, particularly in coastal and inland areas. An undersea earthquake can cause large vertical displacement of the seafloor, generating a tsunami—a series of massive waves that flood coastal areas with devastating force. Less commonly, shaking can trigger seiches, which are standing waves that oscillate in enclosed bodies of water like lakes or reservoirs, posing a localized flooding risk.
A final water-related hazard is the failure of water retention structures. Strong shaking or fault rupture can damage dams and levees, leading to a sudden, uncontrolled release of water. This failure can cause widespread, destructive flooding in downstream areas, impacting communities hours or even days after the initial seismic event.