The profound intensity of the shaking during the 1985 Mexico City earthquake was primarily due to two critical factors: the resonance of seismic waves within the soft sediments of an ancient lakebed beneath the city and the exceptionally long duration of the ground motion.
The 1985 Mexico City earthquake, which struck on September 19, was devastating, causing widespread destruction and loss of life despite its epicenter being hundreds of kilometers away off the Pacific coast. The city's unique geological setting played a crucial role in amplifying the seismic waves, turning a distant powerful earthquake into a local catastrophe.
1. Lakebed Resonance: A Geological Amplifier
Much of Mexico City is built upon the drained basin of what was once Lake Texcoco, an ancient lake in the Valley of Mexico. This geological reality meant that the city's foundation consists of incredibly soft, water-saturated clay and silt sediments.
- Soft Sediments: Unlike rigid bedrock, these loose sediments behave almost like a bowl of jelly when subjected to seismic waves. As shear waves from the distant earthquake traveled from the coastal subduction zone towards the city, their velocity decreased significantly upon entering these soft lakebed deposits.
- Wave Amplification: To conserve energy, the amplitude (the height of the wave, representing the intensity of shaking) of these seismic waves increased dramatically. This effect is known as site amplification.
- Resonance Effect: This amplification was significantly exacerbated by resonance. Just as a musical instrument vibrates at its natural frequency, the specific thickness and properties of the lakebed sediments have a natural period of vibration. The incoming seismic waves from the 1985 earthquake happened to have frequencies that closely matched this natural period. This resonance caused the ground shaking to be greatly amplified and sustained, making buildings sway violently and leading to their collapse. Structures with natural periods similar to the amplified ground motion were particularly vulnerable.
2. Prolonged Duration of Shaking
Beyond the amplification, another critical factor was the unusually long duration of the intense ground shaking. While many coastal earthquakes might produce strong shaking for a shorter period, the 1985 event subjected Mexico City to sustained, violent motion for an extended time.
- Sustained Stress: The prolonged shaking meant that buildings and infrastructure were subjected to repeated stress cycles for several minutes. This continuous back-and-forth motion exceeded the design limits of many structures, leading to fatigue and eventual structural failure, even for buildings that might have withstood shorter, equally intense bursts of shaking.
- Cumulative Damage: The cumulative effect of the long duration of ground motion meant that weaker structures had more time to degrade and fail, and even robust buildings were pushed beyond their elastic limits. This sustained stress is a significant factor in differentiating the damage caused by the 1985 earthquake from other major seismic events.
Consequences of Intense Shaking
The combination of extreme amplification due to lakebed resonance and the extended duration of shaking led to catastrophic consequences for Mexico City:
- Selective Damage: Taller buildings, particularly those between 6 and 15 stories, were especially susceptible to the resonant frequencies of the lakebed, leading to a disproportionate number of collapses in these structures.
- Liquefaction: In some areas, the intense and prolonged shaking of the water-saturated sediments also contributed to liquefaction, where the ground temporarily loses its strength and behaves like a liquid, further destabilizing foundations.
- Widespread Destruction: The result was the collapse of hundreds of buildings, severe damage to thousands more, and a tragic loss of life, primarily concentrated in the lakebed zones of the city.
Summary of Key Factors
| Factor | Description | Impact on Shaking Intensity |
|---|---|---|
| Lakebed Resonance | Mexico City is built on soft, water-saturated clay and silt sediments of an ancient lakebed. The natural vibration period of these sediments closely matched the frequencies of the incoming seismic waves from the distant earthquake. For more on site effects, refer to the Earthquake Engineering Research Institute. | Dramatically amplified the amplitude of seismic waves, turning relatively mild ground motion from the distant epicenter into violent oscillations as energy transferred from deep bedrock to soft surface layers. |
| Long Duration of Shaking | The ground motion lasted for an unusually extended period—several minutes of strong shaking—compared to many other coastal earthquakes in the last 50 years. This aspect is crucial for understanding the widespread damage of the 1985 Mexico City earthquake. | Subjected buildings to prolonged stress cycles, leading to cumulative fatigue and failure, even for structures that might have withstood shorter, equally intense bursts of shaking, pushing them beyond their design limits. |
The lessons learned from the 1985 Mexico City earthquake profoundly influenced building codes and urban planning, emphasizing the critical importance of understanding local geology in earthquake-prone regions.
