An earthquake is the sudden release of strain energy in the Earth’s crust, resulting in waves of shaking that radiate outwards from the earthquake source. When stresses in the crust surpass the strength of the rock, it breaks along either a pre-existing or new fault line.

The point where an earthquake starts is called the focus or hypocenter and may be many
kilometres deep within the earth. The point at the surface directly above the hypocenter is termed the earthquake epicenter.

An earthquake’s effects depend on the softness of the soil. The National Earthquake Hazards Reduction Program (NEHRP) defined six different site classifications, based on the type of soil and rock in the area and their shear-wave velocity: Type A, hard rock, Type B, rock, Type C, very dense soil and soft rock, Type D, stiff soil, Type E, soft soil and Type F, soils requiring site-specific evaluations.

Seismic waves that travel through the ground move faster through hard rock than soft soil – when waves transition from hard to soft earth, they increase in amplitude. Soft soil means bigger waves and stronger amplification.

Bedrock absorbs more wave energy than sandy soils, therefore buildings on solid rock are much less affected than those built on softer soils. When an earthquake occurs, shockwaves travel throughout the ground in short rapid intervals that extend in all directions.

Even though buildings are usually equipped to handle vertical forces from their weight and gravity, they cannot handle horizontal forces emitted by earthquakes. This horizontal
movement shakes walls, columns, floors, beams and the connectors that hold them together.

The most dangerous type of earthquakes are ones that trigger horizontal movements, since tall buildings are better at resisting vertical loads than horizontal ones. These ground motions can damage building foundations in a matter of seconds.

The difference in movement between the bottom and top of buildings leads to extreme stress, causing the supporting frame to rupture and afterwards the entire structure to collapse.

Earthquake-Resistant Materials

For a material to resist stress and vibration, it must be able to withstand large deformations and tension, also known as ductility. Modern buildings are usually constructed with structural steel, a component that can be found in a variety of shapes and allows
buildings to bend without breaking.

Wood is a ductile material as well, because of its high strength relative to its lightweight structure. Scientists and engineers have been developing new building materials with even greater shape retention.

Innovations such as shape memory alloys can both endure heavy strain and revert to their original shape. In addition, fiber-reinforced plastic wrap, made by a variety of polymers, can be wrapped around columns and provide up to 38% added strength and ductility.

Recently, engineers have also been turning to natural elements to help reinforce buildings. The sticky yet rigid fibers of mussels and the strength-to-size ratio of spider silk have promising capabilities in creating new structures. Bamboo and 3D printed materials can also function as lightweight, interlocking structures with limitless forms that can offer even a greater resistance for construction.

Earthquake-resistant Construction Technologies

Shear walls are a useful building technology that can help transfer earthquake forces. Made of multiple panels, these walls help a building keep its shape during movement. Shear walls help keep buildings stable by transferring lateral loads to the foundation, preventing the structure from leaning or collapsing.

While all shear walls serve the same purpose, the material they’re made of (like concrete, steel, and wood), thickness, length, and where they’re positioned can vary based on several factors in building construction and must conform to specific construction codes.

Shear walls are often supported by diagonal cross braces made of steel. These beams can support compression and tension, helping to counteract pressure and push forces. One of the leading causes behind the collapse of buildings during an earthquake is the failure of the foundation.

When the foundation cannot withstand the seismic stresses imposed, it fails, thereby causing the falling of the building. One way to resist ground forces is to lift the building’s foundation above the earth through a method called base isolation.

Base isolation involves constructing a building on top of flexible pads made of steel, rubber and lead. When the base moves during an earthquake, the isolators vibrate while the structure itself remains steady. This effectively helps to absorb seismic waves and prevent them from traveling through the building.

Retrofitting old buildings to improve their seismic performance is as important as using earthquake resistance technologies. Engineers have found that adding base-isolation systems to structures is both feasible and cost-effective.

Another promising solution, much easier to implement, requires a technology known as fiber-reinforced plastic wrap, or FRP. Manufacturers produce these wraps by mixing carbon
fibers with binding polymers, such as epoxy, polyester and vinyl ester to create a lightweight, but incredibly strong, composite material.

In retrofitting applications, engineers wrap the material around concrete support columns of buildings and then pump pressurized epoxy into the gap between the column and the material. It is important to note that even earthquake-damaged columns can be repaired
with carbon-fiber wraps.

Another technology to help buildings stand up to earthquakes is shock absorbers. It has been found that shock absorbers can be useful when designing earthquake-resistant buildings. Engineers generally place dampers at each level of a building, with one end attached to a column and the other end attached to a beam.

Each damper consists of a piston head that moves inside a cylinder filled with silicone oil. When an earthquake occurs, the horizontal motion of the building causes the piston in each damper to push against the oil, transforming the earthquake’s mechanical energy into heat.

In many modern high-rise buildings, core-wall construction is used to increase seismic performance at lower cost. In this design, a reinforced concrete core runs through the heart of the structure, surrounding the elevator banks.

For extremely tall buildings, the core wall can be quite substantial. While corewall
construction helps buildings stand up to earthquakes, it is not a sufficient technology. Researchers have found that fixed-base buildings with core-walls can still experience significant inelastic deformations, large shear forces and damaging floor accelerations.

A better solution for structures in earthquake zones calls for a rocking-core wall combined
with base isolation. A rocking core-wall rocks at the ground level to prevent the concrete in the wall from being permanently deformed. To accomplish this, engineers reinforce the lower two levels of the building with steel and incorporate post-tensioning along the entire height.

In post-tensioning systems, steel tendons are threaded through the core wall. The
tendons act like rubber bands, which can be tightly stretched by hydraulic jacks to increase the tensile strength of the core-wall.