If you are in a building during an earthquake, the way the building is constructed and your
position in the building can have an effect on the shaking you feel. This means you may
experience an earthquake differently from someone only a few buildings away.
The type of construction also has a big effect on whether a building is damaged in an earthquake.
Many different types of materials are used in building construction, and the type of
material and how it is used affects how a building performs during an earthquake.
These materials have a wide range of physical properties.
For example, brick masonry is very stiff, but can fracture under earthquake forces,
whereas wood can’t support as much weight, but is more flexible. Depending on how much steel
reinforcement is used, reinforced concrete can be made to be either very stiff or somewhat flexible.
Architects and structural engineers can design buildings to resist deformation during an
earthquake. If the building is stiff and strong and only a few floors tall, the shaking you feel
might not be very different from someone standing outside. At the opposite end of the spectrum,
a tall building that is designed to absorb the energy of an earthquake by flexing like a tree,
may have a significant amount of movement, particularly in the upper stories. For example, in
the 2019 M 7.1 Ridgecrest earthquake, some people in high rises in LA experienced motion sickness.
A wood-framed home will flex and creak so you may feel the earthquake more than if you are outside,
but if the home is tied to its foundation, and has shear-wall support,
it’s one of the safest buildings to be in during an earthquake.
However not all buildings, especially older buildings,
are properly designed to minimize earthquake damage, so there are additional construction
issues that can affect the shaking you feel. When the ground shifts violently
in an earthquake, [1] weak materials can fail, [2] shear forces can bend or break unbraced walls and
bridges, [3] untethered houses can slide off their foundations, and [4] heavy roofs can collapse.
Earthquakes release seismic wave energy that enters all structures,
and depending on the quality of the materials, it can cause the construction materials to break,
crack, or even fail completely. Wood or steel structures can generally withstand shaking, but
brick, stone, or adobe are more prone to failure. A devastating example of that happened during
the tragic 2010 Haiti earthquake where inferior mortar played a major role in structural failure.
Centuries-old unreinforced masonry buildings are among the most vulnerable throughout the world.
And heavy tile roofs can fail catastrophically.
Also susceptible to failure are structures with mortared-brick, or stone facades.
The flexible wood and/or steel frames constructed during the rapid expansion of the early 1900’s
weren’t designed to hold these heavy facades during earthquake shaking, which is why we
often see them peel away during an earthquake. For example, the Nisqually earthquake in Washington
witnessed the collapse of brick walls in Seattle and Olympia. Brick chimneys also tend to be
brittle and can collapse in earthquakes, even if the rest of a wood-framed house is undamaged.
Older houses that aren’t attached to their foundations sometimes slide off and are damaged
or even collapse. And buildings that are tied to their foundations, but lack sufficient side to
side strength, particularly in the lowest floors, can shear sideways with strong or
prolonged shaking. Homes can be made safer by ensuring they are tied to their foundations,
and by installing plywood sheeting or diagonal bracing on the first floor,
and all upper floors where possible. Prime examples of weak, or soft stories
are the wood-frame apartment buildings built above carports, that collapsed during the 1989
Loma Prieta and 1994 Northridge earthquakes. These open carports lacked the bracing needed to resist
the side-to-side forces, called sheer forces, that occur during an earthquake
A number of techniques can be used to help reduce the damage to structures during an earthquake.
When designing larger buildings, engineers must first calculate the earthquake forces that
are likely to be exerted on a building during the lifetime of the building. To do that they
use building codes that are based on seismic hazard estimates developed by geoscientists,
such as the National Seismic Hazard Map (image). Then they consider the strategies
appropriate for that expected level of shaking. For example, diagonal shear bracing is important
for larger buildings and while the bracing is usually hidden once the building is complete,
it is sometimes on display on the outside of the building.”
For large buildings there are additional techniques to reduce the potential damage
to buildings and their contents. Base isolation essentially puts a building on horizontal shock
absorbers, which reduces the side-to-side sway of a structure. This reduces the forces that the
building and its contents experience during an earthquake. Base isolation allows the foundation
to move but protects everything above the foundation when the ground moves suddenly.
“Tuned mass dampers” can counteract the amount of sway during prolonged ground shaking.
Here we see two buildings, one with a fixed damper. They appear to oscillate similarly
following an earthquake. If the damper on the right is free to move, we see that the mass
counteracts the force from the seismic waves by absorbing kinetic energy from the system.
Dampers like this are engineered specifically to accommodate the height, weight, and stiffness
of the structure. An unusual example is the iconic structure at the LAX Airport,
which had a 1200 ton mass damper installed on the roof to counteract the expected
building response during an earthquake. In closing, earthquake-resistant buildings
save lives by using some of the design principles we’ve talked such as cross bracing, shear walls,
and shock absorbers. Still, if you feel the bump of an earthquake, it is important to know that
the first waves may not be the strongest. Take immediate actions to "Drop, Cover, and Hold On",
before strong shaking arrives, which can save lives and reduce the risk of injury.