Why Do Buildings Fall in Earthquakes? The Science Behind It

Earthquakes can cause catastrophic damage to buildings, leading to loss of life and economic devastation. While modern engineering has improved structural resilience, some buildings still collapse due to scientific and design-related factors. Understanding why this happens is crucial for improving safety measures and construction practices.

Several factors determine whether a building withstands an earthquake or crumbles under stress. These include ground movement, structural response to vibrations, material properties, and foundation stability.

Ground Shaking and Structural Loads

When an earthquake strikes, the ground moves in complex patterns, exerting powerful forces on buildings. These movements originate from seismic waves, which travel outward from the earthquake’s epicenter. The two primary types affecting structures are body waves, which move through the Earth’s interior, and surface waves, which travel along the ground. Surface waves—especially Love and Rayleigh waves—cause the most damage due to their horizontal and rolling motions, which buildings are not always designed to withstand. The intensity and duration of these waves determine the extent of structural stress, with prolonged shaking increasing the likelihood of failure.

As seismic waves pass through the ground, they generate dynamic loads on buildings, forcing them to sway, vibrate, and sometimes twist. Unlike static loads, such as a building’s weight, these dynamic forces change rapidly in direction and magnitude. Engineers categorize them into lateral loads, which push buildings side to side, and vertical loads, which can cause floors to buckle or collapse. The severity of these forces depends on the earthquake’s magnitude, depth, and proximity to the affected area. Strong lateral forces can destabilize structures, especially those with inadequate bracing or weak connections between floors and walls.

A building’s response to these forces depends on its mass and stiffness distribution. Structures with irregular shapes or uneven weight distribution are more susceptible to stress concentrations that lead to localized failures. For example, buildings with soft-story designs—where the ground floor has large open spaces for parking or commercial use—often suffer disproportionate damage due to insufficient lateral support. Similarly, high-rise buildings with flexible frames may sway excessively, increasing the risk of structural fatigue and collapse.

Resonance and Building Frequency

Every building has a natural frequency, the rate at which it vibrates when subjected to external forces. If seismic waves match this frequency, resonance occurs, amplifying the building’s motion and increasing the risk of failure. This phenomenon is similar to how a singer can shatter a glass by hitting the right note—the external vibrations align with the object’s inherent oscillations, causing extreme movement.

A building’s height and structural design influence its natural frequency. Taller buildings tend to have lower frequencies, meaning they sway more slowly, while shorter structures vibrate at higher frequencies. Seismic waves vary in frequency depending on the earthquake and distance from the epicenter. Generally, shorter buildings are more affected by high-frequency shaking, while taller structures respond more to long-period seismic waves. This explains why different buildings experience varying degrees of damage during the same earthquake.

Engineers mitigate resonance effects by adjusting structural stiffness and mass distribution. Features such as tuned mass dampers—large counterweights that move in opposition to sway—help absorb vibrational energy. Base isolators, which decouple a structure from ground motion using flexible bearings, also reduce destructive forces. Structural modifications like shear walls or bracing elements can shift a building’s natural frequency away from expected seismic wave frequencies. Managing resonance helps structures resist amplified forces that might otherwise lead to collapse.

Material Properties and Deformation

A building’s ability to endure seismic forces depends on its materials. Different construction materials exhibit varying degrees of strength, flexibility, and failure modes under intense shaking. Concrete, for example, is strong in compression but weak in tension, making it prone to cracking. Steel, on the other hand, is highly ductile, allowing it to bend without breaking. The interaction between these materials determines how a structure deforms when seismic energy is transferred through it.

During an earthquake, materials stretch, compress, and shear in response to shifting forces. Brittle materials, such as unreinforced masonry, fracture suddenly, leading to catastrophic collapses. Ductile materials absorb energy through controlled deformation, preventing abrupt failure. Reinforced concrete, which combines steel’s flexibility with concrete’s compressive strength, enhances seismic resilience. However, its effectiveness depends on proper reinforcement placement; insufficient rebar or poor bonding between steel and concrete can create weak points where cracks spread rapidly.

Another key factor is strain rate—the speed at which a material is stressed. Gradual loading allows materials to redistribute stress, but earthquakes impose rapid, high-intensity forces that can overwhelm structural components. Older buildings constructed before modern seismic codes are particularly vulnerable due to insufficient reinforcement. Retrofitting strategies, such as fiber-reinforced polymers or external bracing, can improve a building’s ability to deform safely without collapsing.

Reinforcement Gaps and Weak Points

Structural integrity during an earthquake depends on reinforcement quality. Poor design, construction errors, or material degradation can create weak points that compromise a building’s ability to withstand seismic forces. One common issue is insufficient rebar placement in concrete structures. When steel reinforcement is too sparse or improperly positioned, columns and beams become vulnerable to shear and bending stresses, leading to brittle failure and sudden collapse.

Joints and connections between structural components are another critical factor. Buildings rely on strong connections between floors, walls, and supporting columns to distribute seismic forces evenly. Weak or poorly anchored connections can cause sections of a structure to separate during shaking, leading to partial or total failure. This was evident in the 1995 Kobe earthquake in Japan, where many buildings collapsed due to inadequate column-to-beam connections, highlighting the importance of proper reinforcement detailing.

Soil Conditions and Foundation Behavior

A building’s stability during an earthquake is also influenced by the ground it sits on. Different soil types respond to seismic waves in unique ways, affecting how much shaking a structure experiences. Dense, well-compacted soils, such as bedrock or gravel, transmit seismic energy with minimal amplification, reducing ground motion intensity. In contrast, loose or water-saturated soils, such as clay or artificial fill, can amplify shaking, increasing the forces acting on a building.

One of the most destructive soil-related phenomena is liquefaction. This occurs when saturated, loose soil temporarily loses its strength due to intense shaking, behaving more like a liquid than a solid. As a result, buildings and infrastructure can tilt, sink, or even topple. Liquefaction was a major factor in the 1964 Niigata earthquake in Japan, where entire apartment buildings tipped onto their sides as the ground lost stability. Engineers mitigate this risk by improving soil conditions through compaction, deep foundations, or stone columns to enhance drainage and cohesion. Understanding ground behavior is essential for designing foundations that resist shifting forces and prevent catastrophic failures.