When it comes to earthquakes, the impact on cities built on sedimentary basins can be a devastating and mysterious phenomenon. The recent research findings on the seismic echoes in these basins shed light on a long-standing enigma.
The Resonance Chamber Effect
Imagine a city built on a flat depression in the Earth's crust, a favored spot for urban development. During an earthquake, this basin becomes a natural resonance chamber, trapping and amplifying seismic waves. It's akin to sound waves echoing in an empty hall, but with far more destructive consequences.
A Case Study: Wellington's Experience
New Zealand's capital, Wellington, provides a stark example. Despite being 80 kilometers from the epicenter of the 2016 Kaikōura earthquake, the city's central business district experienced shaking beyond design predictions. The damage was extensive, with many multi-storey buildings suffering severe structural issues.
Historical records further emphasize the vulnerability. During the 1942 Wairarapa quake, some 10,000 chimneys in Wellington were destroyed, even though the quake's epicenter was about 80 kilometers away.
Unveiling the Basin's Secrets
New research has revealed a deeper understanding of the Wellington basin. It is almost twice as deep as previously thought, measuring about 500 meters, and its shape is significantly different from earlier models. These revelations help explain the unexpectedly strong shaking experienced during earthquakes.
The Deadliest Example: Mexico City, 1985
The most devastating instance of seismic echoes occurred in Mexico City in 1985. With an epicenter 350 kilometers west of the city, the earthquake's moderate-amplitude waves became trapped and amplified in the basin's low-wave-speed sediments. This resulted in specific zones of extreme destruction, highlighting the risk posed by distant earthquakes to cities built on sedimentary basins.
Understanding Amplification
Seismic waves become trapped and amplified due to two main factors. Firstly, as waves move from a fast wave-speed medium to a low wave-speed medium, their amplitude increases to compensate for the drop in speed. This is similar to a tsunami wave, which gains amplitude as it approaches the shore.
The second factor is resonance. When the wavelengths of incoming seismic waves match the vertical and horizontal dimensions of the basin, amplification occurs. Additionally, if the basin has steep sides, an edge effect generates strong amplification near the basin's edge due to the buildup of different wave types.
Surprising Findings: The Shape of Wellington's Basin
One of the most surprising discoveries is the shape of the basin under Wellington. Its western edge is not the Wellington Fault, as previously assumed, but instead cuts across the basin at a high angle to the fault, following the lines of two low-activity faults: the Terrace and Lambton faults.
These differences between the new and old basin models have significant implications for predicting the shaking Wellington might experience during future earthquakes. The newly described edge and the deeper basin will result in higher predicted amplification.
Computer Simulations and Future Predictions
The research team used a 3D model of the basin in a computer simulation to predict the shaking at frequencies of 0.7 Hertz. They found that the amplifications of horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge of the basin.
While there is some correlation between this predicted pattern and the actual locations of damaged buildings during the Kaikōura earthquake, other factors like reclaimed land and building design must also be considered.
Key Takeaways and Future Implications
Firstly, simple geophysical methods can now be used in urban areas to map out the depth and shape of basins, leading to more accurate predictions of amplified shaking and more precise zoning for vulnerable areas.
Secondly, there is a heightened awareness of the risk to cities built on sedimentary basins, not just from local earthquakes but also from distant ones. This knowledge is crucial for urban planning and disaster preparedness.
In my opinion, this research provides a fascinating insight into the hidden dangers beneath our cities and underscores the importance of continuous scientific exploration and understanding of our natural environment.
