More than 600 miles shook into the vast metropolitan skyline, separating the epicenter of Friday's Myanmar earthquake from Bangkok's skyscrapers. The 33-storey skyscraper under construction has collapsed. How will the shaking in Bangkok, the capital of Thailand, lead to previous earthquakes?
The answers include low-frequency seismic waves that can travel vast distances and shake high-rise buildings.
When a large earthquake collides, it simultaneously emits different frequencies of shaking. Some produce fast front and rear rattles, while others also have low frequency shaking.
That was true on Friday when Myanmar's earthquake produced violent, high-frequency seismic waves. They destroyed low-rise buildings, Buddhist pagodas and other structures near the earthquake epicenter just outside Mandalay, Myanmar's second largest city. Many of the destroyed sites were made of bricks and masonry, and were brittle materials susceptible to such shaking.
When released from an earthquake, high frequency seismic waves spread within the Earth, where they dissipate. In contrast, low-frequency waves travel farther along the Earth's crust.
Low-frequency waves have so far been moved during the 2002 7.9 magnitude Denali earthquake in Alaska, with water from swimming pools and ponds scattering as far as Texas and Louisiana.
These types of seismic waves resonate particularly with tall buildings.
Like tuning forks that produce different sounds depending on size, buildings react differently to earthquakes depending on their design, particularly height.
A 10-storey building can take 1 second to sway left and right during an earthquake, while a 50-storey building can take 5 seconds to complete the same movement.
Low-frequency seismic waves were an important factor in the 1985 earthquake. This has partially or completely destroyed nearly 900 buildings in Mexico City, the country's capital. The epicenter of the massive earthquake was relatively long distances over 200 miles west of the city, so massive destruction first baffled seismologists and engineers.
They concluded that seismic waves resonated with specific forces through urban clay and silt soils, which are the aggravating factors for earthquakes and their aftermath.
A similar dynamic was played last Friday. The city is built on the soft soil of the Chao Phraya River Delta, which amplified the low frequencies pulsate across Southeast Asia and around the Thai capital.
In recent years, scientists say they have underestimated the possibility that these soft soils could make earthquakes even more dangerous. Engineers compare it to the dynamic building of Jell-O's bowls.
In addition to Bangkok and Mexico City, Los Angeles, downtown San Francisco, Seattle and Tokyo are all cities subject to these so-called basin effects, and can increase the destructive forces of earthquakes, particularly at low frequencies.
In Mexico City in 1985, seismic wave frequency was important to understand the damage caused by earthquakes. A team of American scientists concluded in a 1987 report published by the Commerce Department that most of the serious damage was “confined to buildings that are 7-18 stories tall.” The reason for this, they said, is a combination of seismic waves at lower frequencies reaching cities and construction that is vulnerable at those frequencies.
The report said, “old, low-rise masonry buildings generally worked well, and worked well as large masonry colonial churches and government agencies did.” Paradoxically, these are the types of buildings engineers consider to be the most vulnerable to shaking near the epicenter of an earthquake.
Until the 1950s, many American engineers avoided building skyscrapers in earthquake-prone areas, according to Thomas H. Heaton, professor emeritus at California Institute of Technology.
According to him, the general wisdom was to build stronger, stiffer buildings. But it has changed over decades, and today's skyscrapers are more flexible.
Dr. Heaton says the flexible design of the latest skyscrapers works well with the 6 o'clock earthquake and occurs more frequently. However, he is very concerned about the consequences of less frequent, larger earthquakes that highlight the vulnerability of tall buildings. The roster includes a 7.8 magnitude earthquake that killed more than 50,000 people in Türkiye two years ago.
Massive faults under modern cities — direct hits — will be devastating to tall buildings, regardless of engineering precautions, Dr. Heaton says.
He said violent movement of the ground on faults, the slips that seismologists call them, could quickly move high ascent bases and potentially leave the upper floors unsupported.
“If you take the base of a building and move it in just a few seconds and then there's very little that a structural engineer can do to get the building upright,” he said. “I definitely don't want to be a really tall building during a massive earthquake.”
