When a 7.4-magnitude earthquake rocked Taiwan on Wednesday, people inside the island’s tallest skyscraper, Taipei 101, were protected by a large yellow pendulum at the building’s center that helped absorb the shock.

Known as a “tuned mass damper,” the 730-ton steel sphere is suspended between several floors at the top of the building, where it can be viewed by the public. The observatory is a popular tourist attraction in the capital city.

The sphere moves back and forth during earthquakes or typhoons that regularly lash the island, absorbing the force of any “violent swinging,” according to the Taipei 101 website. The damper’s engineers say it can curtail the building’s movement by up to 40 percent, reducing the queasiness felt by its occupants.

Closed-circuit TV footage of the Taipei skyline at the moment the earthquake hit shows the pagoda-shaped skyscraper hardly moving. The security camera, mounted on another building, is shaking violently.

Live cam footage and security cameras catch the moment a deadly 7.4-magnitude earthquake struck off the east coast of Taiwan on April 3. (Video: Reuters)

Taipei 101 was the world’s tallest building when it was completed in 2004 — a title it held until 2009. As the name suggests, it is 101 stories tall, reaching a height of 1,667 feet, including its spire.

It has a number of other design features that increase its resilience to natural disasters — including 380 piles driven deep into the ground. The deepest is rammed some 30 meters, or almost 100 feet, into the bedrock, which, according to Taipei 101, “is similar to nailing the entire building onto a solid tectonic plate.” Power is supplied to the building via two substations, reducing the risk of an outage.

Taiwan, home to 23 million people, is vulnerable to earthquakes because of its location within the world’s most seismically active zone, known as the Ring of Fire. Nine people were killed and more than 900 injured in Wednesday’s earthquake, the strongest to strike the island in 25 years. A 7.6-magnitude tremor struck central Taiwan in 1999, killing more than 2,400 people.

Taipei 101 is not the only skyscraper in Taiwan and around the world to use damper systems for stability — although it is a rare case in which the engineering is on display.

A design flaw could have doomed a New York skyscraper in the 1970s — even though it had a tuned mass damper, an advanced design feature at the time. The building’s engineer had to hustle to resolve the issue — which was pointed out by a college student — as the hurricane season was approaching.

A video on Taipei 101’s website shows the sphere rocking gently during a cyclone in 2015, ranked by scientists as one of the strongest storms in the world that year.

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