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Distant Earthquake Effects Felt in Bangkok: Seismic Waves and Skyscraper Sway
Bangkok, Thailand, a sprawling metropolis renowned for its towering skyscrapers, experienced unsettling tremors as buildings swayed despite being over 600 miles from the epicenter of Friday’s earthquake in Myanmar. Notably, a 33-story high-rise under construction even suffered a collapse. This raises the question: how could seismic activity so far away influence the Thai capital?
Low-Frequency Seismic Waves: The Culprit
The phenomenon is attributed to low-frequency seismic waves, powerful energy waves capable of traversing immense distances and inducing sway in tall buildings. Earthquakes generate a spectrum of shaking frequencies simultaneously. Some manifest as rapid, jarring vibrations, while others present as slower, undulating movements.
High vs. Low Frequency Waves
During the Myanmar earthquake, intense, high-frequency seismic waves wreaked havoc on structures near the epicenter, located just outside Mandalay, Myanmar’s second-largest city. Low-rise buildings, ancient pagodas, and other structures constructed from brittle materials like brick and masonry were particularly vulnerable to this type of intense shaking and sustained significant damage or collapse.
High-frequency seismic waves tend to dissipate as they propagate through the Earth’s interior. Conversely, low-frequency waves travel along the Earth’s crust, enabling them to cover greater distances with less energy loss.
Long-Distance Impact Examples
The 2002 Denali earthquake in Alaska, a significant 7.9 magnitude event, provides a compelling example. Low-frequency waves from this quake traveled vast distances, causing water bodies as far away as Texas and Louisiana to splash for nearly half an hour, according to NASA, demonstrating their long-range reach, albeit harmlessly in that instance.
Resonance and Tall Buildings
These low-frequency seismic waves exhibit a particular resonance with tall structures. Buildings, much like tuning forks, possess unique resonant frequencies based on their size and design, especially their height. This means they respond differently to earthquake vibrations.
For instance, a 10-story building might complete a sway cycle in approximately one second during an earthquake, whereas a 50-story building could take up to five seconds for the same motion – a slow, potentially nauseating back-and-forth oscillation.
Mexico City: A Case Study in Soil Amplification
The devastating 1985 Mexico City earthquake underscores the significant impact of low-frequency waves on distant urban centers. In this event, nearly 900 buildings in Mexico City, the nation’s capital, experienced partial or total collapse despite the 8.0-magnitude quake’s epicenter being over 200 miles to the west. This extensive destruction initially perplexed seismologists and structural engineers due to the distance.
Investigations revealed that the seismic waves resonated with exceptional force through the clay and silt soils underlying Mexico City. This geological factor significantly amplified the earthquake’s effects and subsequent damage.
Bangkok’s Vulnerable Soil
A similar dynamic played out during the recent Myanmar earthquake in Bangkok. As low-frequency shaking radiated across mainland Southeast Asia, it experienced amplification within and around the Thai capital. This is because Bangkok is situated on the unconsolidated sediments of the Chao Phraya River delta, characterized by soft soil conditions.
Experts suggest a historical underestimation of the potential for these soft soils to exacerbate earthquake hazards. Engineers often draw an analogy, comparing constructing buildings on such terrain to building on a bowl of Jell-O, highlighting the amplified movement and instability.
Cities Susceptible to Basin Effects
Beyond Bangkok and Mexico City, other major urban centers including Los Angeles, downtown San Francisco, Seattle, and Tokyo are also vulnerable to these “basin effects.” This phenomenon can dramatically multiply the destructive potential of earthquakes, especially at lower frequencies, posing a significant risk for densely populated areas.
Height and Seismic Wave Frequency in Building Damage
The 1985 Mexico City event emphasized the critical role of seismic wave frequencies in understanding earthquake damage patterns. A team of American scientists, in a 1987 report published by the Department of Commerce, concluded that the most severe damage was “confined to buildings in the height range of seven to 18 stories.” They attributed this concentrated damage to a combination of the incoming lower frequency seismic waves and the construction practices prevalent for buildings of those heights, making them particularly susceptible to resonance at those frequencies.
Paradox of Building Vulnerability
Intriguingly, the report also noted that “older, low-rise masonry buildings generally performed well, as did the massive stone masonry colonial churches and government offices.” This outcome presents a paradox, as these are precisely the types of structures engineers typically deem most vulnerable to intense shaking in close proximity to an earthquake’s epicenter.
Evolution of Building Design: From Stiff to Flexible
Until the mid-20th century, many American engineers avoided constructing tall buildings in seismically active regions. According to Thomas H. Heaton, an emeritus professor at the California Institute of Technology with extensive experience studying earthquake effects on buildings, the prevailing approach was to construct stronger, more rigid buildings.
However, this philosophy shifted over time. Modern skyscrapers now incorporate flexible designs. Dr. Heaton explains that this flexible approach is effective for more frequent earthquakes with magnitudes around 6.
Concerns Regarding Major Earthquakes
Despite advancements in flexible design, Dr. Heaton expresses significant concern about the consequences of less frequent, but more powerful earthquakes. These major seismic events can expose vulnerabilities even in well-engineered tall buildings, as tragically illustrated by the 7.8 magnitude earthquake in Turkey two years prior, which resulted in over 50,000 fatalities.
Devastating Impact of a Direct Fault Rupture
Dr. Heaton warns that a massive fault rupture directly beneath a modern city – a “direct hit” scenario – would be catastrophic for tall buildings, regardless of engineering precautions. The sudden, violent ground displacement along the fault line, termed “slip” by seismologists, would cause the base of a high-rise to shift abruptly. This rapid movement could potentially decouple the upper floors, leaving them unsupported.
He emphasizes the limitations of structural engineering in such extreme events: “When you displace the base of a building by several meters in under a few seconds, there’s essentially nothing a structural engineer can implement to ensure the building remains standing. I would certainly not want to be inside a very tall building during a large magnitude earthquake.”