Earthquakes have long been a formidable force of nature, capable of wreaking havoc on cities and infrastructure. But what many people don't realize is that the very ground beneath our feet can play a significant role in amplifying the effects of these natural disasters. In this article, I'll delve into the fascinating and somewhat eerie phenomenon of how sedimentary basins, those depressions in Earth's crust caused by tectonic activity, can become natural resonance chambers during earthquakes. This is a topic that I find particularly intriguing, as it raises a deeper question about the interplay between nature and human construction, and the potential for unexpected consequences. So, let's explore this intriguing subject and uncover why it matters for cities built on these basins.
The Science Behind Seismic Echoes
Earthquakes generate seismic waves that travel through the Earth's crust. When these waves encounter a sedimentary basin, they can become trapped and amplified. This is due to two main reasons. Firstly, as the waves move from a fast wave-speed medium (solid basement rocks) to the low wave-speed of sedimentary rocks, the amplitude of the waves increases to compensate for the drop in wave speed. This is similar to a tsunami wave that travels across the deep ocean with a small amplitude but high speed, and then, as it comes closer to shore in shallow water, slows down but dramatically increases in amplitude. Secondly, the basin's shape and dimensions can create resonance, where the wavelengths of the incoming seismic waves are similar to either the vertical and horizontal dimensions of the basin. If the basin has steep sides, an edge effect is also generated where strong amplification can occur close to the edge of the basin due to a buildup of different wave types.
The Example of Wellington, New Zealand
New Zealand's capital city, Wellington, is built on a sedimentary basin. In the 2016 magnitude 7.8 Kaikōura earthquake, Wellington's central business district experienced shaking that exceeded design predictions. Even though the quake was located 80 kilometers from the city, it caused severe damage to infrastructure, with many multi-storey buildings damaged or destroyed. Archival records also show that during the 1942 magnitude 7.3 Wairarapa quake, which was located about 80 kilometers north of Wellington, some 10,000 chimneys were destroyed in the city. Our new research provides an updated model for the central Wellington basin, revealing it is almost twice as deep (about 500 meters) than previously thought and that its shape is significantly different from the previous model. These differences go some way toward explaining why the shaking was stronger than expected.
The Deadly Example of Mexico City
Historically, the most devastating example of seismic echoes was the 1985 Mexico City earthquake, which killed 8,000 people and destroyed high-rise buildings. The quake's epicenter was 350 kilometers west of the city, but when waves of moderate amplitude arrived in the city, they became trapped in the low-wave-speed sediments of the basin on which it is built and became amplified. As the waves bounced from side to side, they created a standing wave, similar to water waves in a bath. As a result, the city experienced specific narrow zones of extreme destruction, underscoring the risk from even very distant earthquakes for cities built on sedimentary basins.
The Shape of the Basin Under Wellington
Perhaps the most surprising of our findings is the shape of the basin under Wellington. Its effective western edge is not the Wellington Fault, as previously assumed. Instead, the edge cuts across the basin at a high angle to the Wellington fault and follows the line of two previously identified, low-activity faults - the Terrace and Lambton faults. These differences between the new and old basin models have significant impacts on the predicted shaking Wellington might expect. In particular, there will be an effect linked to the newly described edge, and the predicted amplification will be higher for a deeper basin.
The Implications and Future Developments
Our study highlights two key points. Firstly, simple geophysical methods can now be used in urban areas to map out the depth and shape of basins that cities are built on. From these models, we can then generate computer simulations to predict the location of amplified shaking. This will lead to more granular zoning for what parts of cities may be more vulnerable. Secondly, there is a higher awareness of the risk to cities built on sedimentary basins from not only local but also distant earthquakes. This knowledge can inform better building codes and urban planning, potentially saving lives and reducing the impact of future earthquakes.
Personal Perspective
From my perspective, this research is a fascinating insight into the complex relationship between nature and human construction. It raises important questions about the resilience of our cities and the potential for unexpected consequences. As we continue to build and develop in areas prone to earthquakes, it's crucial that we consider the role of the ground beneath our feet and the potential for seismic echoes to amplify the effects of these natural disasters. In my opinion, this research is a step towards a more resilient and sustainable future, where we can better understand and mitigate the risks posed by earthquakes to our cities and infrastructure.
