The devastating impact of earthquakes on cities built atop sedimentary basins has long been a concern, but recent research sheds new light on this phenomenon. In my opinion, the findings are both fascinating and alarming, as they reveal the hidden dangers lurking beneath these seemingly stable urban landscapes.
The Resonance Effect
Sedimentary basins, those natural depressions in the Earth's crust caused by tectonic activity, have a unique ability to trap and amplify seismic waves. Just as sound echoes in an empty hall, these basins can create "seismic echoes" during earthquakes, leading to intense shaking and potential destruction. This effect is particularly pronounced in cities like Wellington, New Zealand, which is built on such a basin.
Historical Evidence
The 2016 Kaikōura earthquake, with a magnitude of 7.8, caused severe damage to Wellington's central business district, even though the quake was located 80 kilometers away. Archival records from the 1942 Wairarapa quake further highlight the vulnerability of the city, with 10,000 chimneys destroyed despite the quake's epicenter being 80 kilometers north of Wellington.
A Deeper Understanding
Our new research provides an updated model of the central Wellington basin, revealing it to be almost twice as deep as previously thought, with a significantly different shape. This updated model helps explain the stronger-than-expected shaking experienced during earthquakes. The deadliest example of this phenomenon is the 1985 Mexico City earthquake, which killed 8,000 people and destroyed high-rise buildings despite its epicenter being 350 kilometers away.
The Science Behind the Amplification
Seismic waves become trapped and amplified due to two main factors. Firstly, as the waves move from solid basement rocks to sedimentary rocks, their amplitude increases to compensate for the drop in wave speed, similar to a tsunami wave transforming as it approaches land. Secondly, resonance occurs when the wavelengths of incoming seismic waves match the dimensions of the basin, creating a standing wave effect.
Surprising Discoveries
One of the most surprising findings is the shape of the basin beneath Wellington. Its western edge is not the Wellington Fault, as previously assumed, but instead cuts across the basin at a high angle, following the lines of two low-activity faults. This new understanding of the basin's shape and depth has significant implications for predicting the intensity of future earthquakes and the potential damage they could cause.
Practical Applications
The research highlights the importance of using simple geophysical methods to map out the depth and shape of basins beneath cities. From these models, computer simulations can predict the locations of amplified shaking, leading to more precise zoning for vulnerable areas. Additionally, it raises awareness of the risk to cities built on sedimentary basins not only from local but also distant earthquakes.
Conclusion
This research provides a deeper understanding of the complex relationship between earthquakes and the cities built on sedimentary basins. It underscores the need for continued research and preparedness to mitigate the devastating impacts of these natural disasters. As we continue to explore and understand these phenomena, we can work towards building more resilient urban environments.
