Earthquakes and Sedimentary Basins: A Recipe for Disaster?
The devastating impact of earthquakes on cities built on sedimentary basins is a well-known phenomenon, but the underlying reasons for this vulnerability have been a subject of ongoing research and debate. A recent study has shed new light on this complex relationship, offering a more nuanced understanding of why these basins can become deadly traps for seismic activity.
The Basin's Role in Seismic Amplification
Sedimentary basins, characterized by their flat and depression-like features, are often favored locations for urban development due to their accessibility and resource availability. However, during earthquakes, these basins can transform into natural resonance chambers, amplifying seismic waves and causing localized, intense destruction.
The key to this amplification lies in the interaction between the seismic waves and the basin's geometry. As waves travel from solid basement rocks to the low-wave-speed sedimentary rocks within the basin, their amplitude increases, compensating for the drop in wave speed. This phenomenon is akin to a tsunami wave, which slows down in shallow water but grows in amplitude, a principle known as the 'tsunami effect'.
Additionally, the basin's shape and dimensions play a crucial role in resonance. When the wavelengths of incoming seismic waves align with the vertical and horizontal dimensions of the basin, resonance occurs, further amplifying the waves. This resonance effect is particularly pronounced in basins with steep sides, where edge effects can lead to localized amplification.
The Case of Wellington, New Zealand
The study's findings are particularly relevant to New Zealand's capital city, Wellington, which is built on a sedimentary basin. During the 2016 Kaikōura earthquake, the city's central business district experienced shaking that exceeded design predictions, despite the quake being located 80 kilometers away. Archival records also reveal the destructive potential of distant earthquakes, such as the 1942 Wairarapa quake, which destroyed 10,000 chimneys in Wellington.
The new research updates the model of the central Wellington basin, revealing it to be almost twice as deep (approximately 500 meters) and significantly different in shape from previous assumptions. These differences help explain the stronger-than-expected shaking in the city. The basin's effective western edge, for instance, is not the Wellington Fault but rather a high-angle intersection with the Terrace and Lambton faults, leading to higher predicted amplification.
Historical Precedent: Mexico City
The most devastating example of seismic echoes and trapped waves was the 1985 Mexico City earthquake, which resulted in 8,000 fatalities and the destruction of high-rise buildings. The city's location on a sedimentary basin, with low-wave-speed sediments, caused the waves to become trapped and amplified, creating standing waves similar to those in a bathtub. This led to specific zones of extreme destruction, highlighting the risk posed by distant earthquakes to cities built on such basins.
Implications and Future Directions
The study's findings have significant implications for urban planning and risk assessment. By employing simple geophysical methods to map the depth and shape of sedimentary basins, cities can now generate computer simulations to predict the locations of amplified shaking. This enables more granular zoning, identifying vulnerable areas and informing more effective building codes and infrastructure design.
Moreover, the research underscores the need for heightened awareness of the risks associated with cities built on sedimentary basins, not just from local earthquakes but also from distant ones. This knowledge is crucial for developing comprehensive earthquake preparedness and response strategies, ensuring the safety and resilience of urban populations in the face of seismic events.
In conclusion, the interplay between earthquakes and sedimentary basins is a complex and often deadly dance. By understanding the underlying mechanisms and applying this knowledge to urban planning, we can work towards mitigating the risks and building more resilient cities in the future.
