earthquakes & bridges

Have you ever wondered how earthquakes affect bridges? I must admit that I had not thought about that subject until

quite recently.

In the last few years in the North Texas area, we have been experiencing a significant increase in the frequency and

magnitude of earthquakes.

There are many who say that this is the result of the exploration for underground gas reserves, which has

become widespread in this area.

No matter the root cause, the subject of how earthquakes affect bridges is now on the minds of many.

When evaluating the affects of earthquakes the most important factors to take into account are the magnitude of

the earthquake, the distance from the epicenter of the earthquake and the geologic conditions at the site of the

earthquake itself.

When it comes to how earthquakes can affect bridges, the main concern is the damage that can be caused by the

ground motion that the earthquake creates.

Going deeper into this subject, one must take a look at the local geology and type of soil that the bridge has been built

upon.

When soft soil lies on top of hard bedrock the ground motion is amplified, this is known as ground-motion

amplification.

This type of amplification can result in much greater damage, even from bridges that are quit a distance from

the epicenter of the earthquake itself.

In fact, soft sandy soils can actually be liquefied by strong ground motion in process called liquefaction.

When liquefaction occurs, the foundation of structures, like bridges, can be completely compromised.

The general rule of thumb is that earthquakes below a magnitude of 4.0 do not cause infrastructure damage.

Since 2013 there have been over fifty reported earthquakes in a two mile area alone just west of the Dallas city limit.

None of these earthquakes have been recorded to be above 4.0., But there have been earthquakes in the North Texas

area in the past that have been above 4.0.

The largest recorded earthquake in the State of Texas occurred in 1931, when a magnitude 5.8 earthquake was recorded in far West Texas near the town of Valentine.

Highway engineers have devised standards in an attempt to earthquake-proof bridges that have been built since the

early 1990's.

Most bridges that we drive on daily were built before these higher standards were implemented however.

Today, the subject of how earthquakes affect bridges is something that all of us must pay attention to avoid a

potential catastrophe.