Earthquakes are a sudden release of energy in the Earth’s crust that generates seismic waves, causing the ground to shake. This motion is transmitted to the structures built upon that ground. The common belief that earthquakes are felt more intensely on the higher floors of a building is a frequent question. This article explores the physical reality behind the sensation of increased movement and the scientific reasons why altitude plays a significant role in earthquake perception.
Understanding Building Dynamics During an Earthquake
The way a building responds to ground motion is governed by its physical characteristics, especially its height, mass, and stiffness. Every structure has what is known as a natural period, which is the specific amount of time it takes for the building to sway back and forth one time when set into motion. The actual movement a person feels inside a building is the result of the ground shaking being transferred through the foundation and up into the structure.
When a seismic wave hits, the ground motion introduces forces that cause the building to oscillate. Generally, shorter, stiffer buildings have a shorter natural period, meaning they vibrate more quickly, and are more affected by high-frequency earthquake waves. Conversely, taller, more flexible buildings possess a longer natural period, making them susceptible to the slow, long-period waves generated by distant or large earthquakes.
This structural response is what differentiates the experience between a person on the ground floor and someone high above. The foundation of the building moves with the ground, but the floors above begin to move independently as the structure absorbs and distributes the energy. This difference in movement sets the stage for the phenomenon of dynamic amplification.
The Amplification Effect: Why Movement Increases with Height
The increased sensation of shaking on higher floors is a direct result of dynamic amplification, which is the structural exaggeration of the initial ground motion. This amplification is particularly pronounced when the building’s natural period aligns closely with the dominant period of the seismic waves, a condition known as seismic resonance. When resonance occurs, the energy input is prolonged, and the amplitude of the building’s sway significantly increases.
As the structure oscillates, each floor above the ground experiences greater displacement than the one below it. This effect is analogous to the tip of a whip, where the base moves very little, but the end exhibits a much larger, more dramatic motion. In a high-rise, this means the top floors can experience lateral movements, or sway, that are several times greater than the movement measured at the foundation.
Studies of reinforced concrete buildings have shown dynamic amplification factors that can be close to three times the ground acceleration, depending on the structure’s height and design. This increase in motion is felt as a much more intense and prolonged shaking experience for occupants on the upper levels. Taller buildings also tend to have lower damping, which means they take longer to dissipate the energy, resulting in a swaying motion that can persist for a longer duration after the ground shaking has stopped. While the risk of structural collapse is often lower in modern, well-designed high-rises, the perception of danger is intensified due to the significantly increased displacement and acceleration at altitude.
Safety and Preparedness for High-Rise Occupants
The primary hazards for people in high-rise buildings during an earthquake are not typically structural collapse, but rather non-structural items and falling debris. Occupants on upper floors must be especially aware of risks from unsecured furniture, heavy objects, and broken glass from windows. The intense swaying motion can send objects flying across a room, creating immediate danger.
The recommended action during the shaking is to immediately Drop, Cover, and Hold On (DCH) under a sturdy piece of furniture, such as a strong desk or table. It is important to stay put, as attempting to evacuate during the shaking is extremely hazardous; stairwells can be damaged, and elevators should never be used.
Safety Actions During Shaking
- Drop, Cover, and Hold On under a sturdy piece of furniture, such as a strong desk or table.
- Stay put until the shaking has fully stopped.
- Move away from exterior walls and windows to avoid injury from glass or falling facade elements.
Once the shaking has fully stopped, occupants should check for injuries and assess the immediate area for potential hazards. Evacuation should only be considered if there is clear evidence of structural damage, like severe cracking, significant water leaks, or fire, or if instructed by emergency personnel. If evacuation is necessary, the stairs must be used with caution, as power outages may disable lighting and elevators. Aftershocks may occur minutes or hours later, requiring a repeat of the DCH procedure.