# How is the shaking of a building related to the magnitude of an earthquake?

In general, the greatest intensity of the shaking is related to the magnitude of the earthquake and inversely proportional to the distance from the epicenter. The "average" building in a given location will have been constructed in accordance with the minimum building code at the time of construction. Thus, it can be said that most buildings are not strong enough to withstand high winds or earthquakes. However these are rare events for most buildings.

The strength of a building to resist wind loads depends on many factors such as type of construction (frame vs. brick), size of the building, orientation of the building, proximity of the building to deep ravines, etc. As far as earthquake resistance is concerned, the key factor is the design load index (DLC) of the structure. The DLC is a number that indicates how much the design load of a structure would exceed what it actually does receive during an earthquake. A structure with a high DLC is more likely to survive an earthquake without damage.

For example, consider a simple frame house with a 20-foot clear span and a 2-story section. The house has an estimated value of \$100,000 and a DLC of 40. This means that the design load on this structure would be 40 times its actual weight. For example, if the house weighed 10,000 pounds, then the design load would be 400,000 pounds.

## What are the main factors that affect the amount of destruction caused by seismic shaking?

The degree of the damage caused by an earthquake is determined by the strength of the shaking. The key elements influencing earthquake shaking intensity, in turn, are earthquake depth, closeness to the fault, underlying soil, and building attributes, notably height. The more any of these factors increase the energy released by an earthquake, the greater will be the shaking intensity.

Other factors such as the location of the earthquake, its proximity to population centers, and the type of construction used for buildings can also influence the severity of damage but to a much lesser extent than the four factors listed above. For example, strong ground motions are likely at sites near active faults or deep earthquakes, while weak motions are expected at remote locations or shallow earthquakes.

Seismic shaking can cause damage to buildings through two primary mechanisms: vertical motion (jumping) and horizontal displacement (rolling). Factors such as building height and quality control measures during construction can reduce both types of damage significantly. However, if a building's foundation is not sufficiently strong to resist these forces, then it will suffer structural damage.

Damage to physical structures and loss of life result from seismic activity whether the event is shallow or deep. Seismically-induced damage occurs even at distances far away from the source region, due to the propagation of elastic waves through Earth's solid materials.

## How does an earthquake affect the area around the epicenter?

Because the faults that rupture during the earthquake are so deep, the seismic wave energy that they emit spreads across a considerably broader region than it would in a shallow quake. As a result, the region directly above the epicenter experiences less severe shaking than a shallow earthquake of the same size, but the shaking reaches a wider area. The distance over which this effect can be felt depends on the severity of the earthquake. In general, large earthquakes tend to spread their energy over a greater distance.

The strongest shaking is usually near the surface, but deeper waves also reach the ground and these can cause damage hundreds of miles away from the center.

Damage from faraway quakes is possible because the energy of earthquakes travels through Earth's solid core as well as its fluid outer layer. The core acts like a giant spring by which energy is transmitted from one part of the planet to another. Because most cities and towns are built on solid ground, the majority of damage caused by distant earthquakes is due to falling objects such as roofs, windows, and walls. This kind of damage occurs even if the earthquake takes place thousands of miles away from any town or city.

Damage within a few hundred kilometers of the epicenter is mainly caused by strong vibrations that travel through solid structures such as buildings and roads. These vibrations can weaken or destroy buildings by causing parts of them to collapse or by shifting different components of a structure apart.

##### Charles Lindemann

Charles Lindemann is a man of many passions; among them are building, architecture, and engineering. He has studied each of these fields extensively, and now spends much of his time designing buildings and working on technical projects. Charles has been able to use his knowledge of architecture and engineering to create some of the most unique and creative structures around.