When an earthquake occurs, many people naturally assume that the closer a building is to the epicenter, the greater the risk will be. Distance is certainly an important factor, but it does not tell the whole story.
A large earthquake can affect buildings hundreds of kilometers away, particularly when the earthquake generates long-period ground (seismic) motion. High-rise buildings can be especially sensitive to this type of movement because their natural periods are longer than those of many low-rise buildings. This means that a building can experience significant, prolonged movement even when it is far from the earthquake source.
Understanding why this happens requires looking at three things together: the characteristics of the seismic waves, the ground beneath the city, and the characteristics of the building itself.
Earthquakes generate ground motion with a range of periods. Some seismic waves produce relatively rapid movement, while others contain longer-period motion.
Long-period ground motion matters a lot in big earthquakes. This is because long-period waves can travel very far without losing much energy. Shorter waves lose energy faster.
The Japan Meteorological Agency (JMA) says that big earthquakes can create long-period ground motion. This motion can travel over long distances. JMA also says that soft ground — like flat plains made of sand and soil — can make this motion even stronger.
Because of this, knowing how far a city is from the earthquake is not enough. It does not tell you how much a tall building there will shake.
A city can be far from the earthquake and still shake a lot. This happens when the earthquake’s waves match well with the local ground and the buildings in that city.
The ground itself plays an important role in earthquake response. Seismic waves do not necessarily behave the same way everywhere. When they travel through different geological layers, their characteristics can change.
Soft sedimentary deposits can amplify certain types of long-period motion. Large metropolitan areas are often located on broad plains or sedimentary basins, making this an important consideration for urban earthquake preparedness.
The JMA notes that long-period ground motion tends to be amplified and last longer in soft strata such as those found on sedimentary plains.
This means that two cities at a similar distance from the same earthquake can experience different levels and characteristics of shaking.
Even within the same metropolitan region, local ground conditions can vary. This is one reason why earthquake risk cannot be evaluated simply by looking at an earthquake’s magnitude and distance.
Mexico City provides one of the clearest examples of how distance, geology, and building characteristics can interact.
During the 1985 Michoacan earthquake, the earthquake source was approximately 400 kilometers from Mexico City. Despite that distance, the city experienced severe damage. Research by the U.S. Geological Survey found that long-period ground motion was amplified by the thick sediments associated with the former lakebed beneath Mexico City. The dominant ground motion had periods that were particularly significant for many buildings in the city.
The result was a powerful interaction between the incoming seismic waves, the local ground conditions, and the natural periods of buildings.
This event became an important example of why earthquake risk cannot be assessed by distance alone. A building may be far from the source and still experience damaging movement if the characteristics of the seismic waves and the local ground are unfavorable.
The characteristics of a building are another important part of the equation. Every building has a natural period, which describes how long it takes the structure to complete one cycle of its natural vibration.
Generally, taller buildings have longer natural periods than lower buildings.
The JMA explains that high-rise buildings have longer natural periods and can therefore respond strongly when long-period ground motion contains motion close to those periods. This is the basic principle behind resonance.
Imagine pushing a swing.
If the timing of each push is unrelated to the swing’s movement, the swing may move only slightly. If the pushes are timed close to the swing’s natural rhythm, however, the movement can become much larger. A building can behave in a similar way when the period of incoming ground motion is close to the building’s natural period.
The building does not have to be located close to the earthquake source for this interaction to occur.
Another important characteristic of high-rise buildings is that the movement can become larger toward the upper floors. The JMA states that higher floors of high-rise buildings generally experience greater shaking than lower floors during long-period ground motion.
This creates practical risks inside the building.
These effects are particularly important for offices, hospitals, laboratories, hotels, commercial buildings, and other facilities where people and equipment are concentrated on upper floors.
The 2011 Great East Japan Earthquake demonstrated how far-reaching long-period effects can be. The JMA reports that high-rise buildings in areas such as the Tokyo metropolitan region experienced long and significant shaking during the earthquake. It also notes that the effects on high-rise buildings cannot always be understood from conventional seismic intensity alone.
Research has also documented the response of a tall building located hundreds of kilometers from the earthquake source.
The important lesson is not that every distant earthquake will cause severe damage to every high-rise building. Rather, the event demonstrates that distance from the epicenter does not automatically mean that long-period movement will be insignificant.
After an earthquake, people often look at two different numbers. One is magnitude — a single number that describes the size of the earthquake itself, at its source. The other is seismic intensity — a separate number that describes how strongly the ground shakes at a specific place. The same earthquake can have one magnitude, but many different intensity values, because places closer to the earthquake usually shake more than places farther away.
Japan uses its own intensity scale for this second number. It is useful information, but it does not fully describe what people on the upper floors of a high-rise building actually experience. JMA explains that this normal intensity scale is not enough to describe how strongly upper floors shake during long-period ground motion. For this reason, Japan created a separate four-level scale just for long-period ground motion.
This scale looks at things like how hard it is to walk or stand, and whether furniture and fixtures move or fall over. This matters for facility managers. Even if the intensity at ground level is only moderate, that does not mean every floor of a high-rise building will shake only moderately. Upper floors can shake much more.
Understanding distant earthquake effects should lead to practical questions. For example:
These questions are more useful than simply asking whether the building is “earthquake resistant.” Structural safety is essential, but the contents of a building also require attention.
A Broader Approach to Earthquake Risk
Long-period ground motion shows why earthquake risk is more complicated than distance from an epicenter.
These factors interact. For businesses located in high-rise buildings, this means that earthquake preparedness should consider not only the earthquake hazard itself but also the way the facility may respond.
A distant earthquake can still produce significant movement in a tall building when the right combination of seismic waves, ground conditions, and building characteristics exists. The practical lesson is simple:
Do not judge earthquake risk by distance alone.
Understanding how seismic waves travel and interact with buildings provides a better foundation for preparing people, equipment, and business operations.
The next question is how this interaction actually occurs inside a building. To understand that, it is necessary to look more closely at natural periods and resonance.
