The Earth's sedimentary basins, once thought to be ideal locations for cities due to their flat terrain, have revealed a hidden danger. These basins, formed by tectonic activity, can act as natural resonance chambers during earthquakes, amplifying seismic waves and causing devastating damage. This phenomenon is particularly relevant for cities like Wellington, New Zealand, which is built on a sedimentary basin. The 2016 Kaikōura earthquake and the 1942 Wairarapa quake highlight the vulnerability of these areas, with the former causing severe damage to infrastructure and the latter destroying 10,000 chimneys. The key to understanding this lies in the shape and depth of the basin, which can trap and amplify seismic waves, creating 'seismic echoes'.
The research team, including myself, has developed a new model for the central Wellington basin, revealing it to be almost twice as deep and with a significantly different shape than previously thought. This discovery helps explain the stronger-than-expected shaking experienced during the Kaikōura earthquake. The basin's western edge, previously assumed to be the Wellington Fault, is actually defined by two low-activity faults, the Terrace and Lambton faults, which has significant implications for predicting future shaking. The model predicts amplifications of horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge, and this pattern aligns with the locations of damaged buildings during the Kaikōura earthquake.
This finding raises important questions about the risks faced by cities built on sedimentary basins, not just from local earthquakes but also from distant ones. The amplification of seismic waves due to the basin's shape and depth can create specific zones of extreme destruction, as seen in Mexico City in 1985, where high-rise buildings were destroyed despite the earthquake's epicenter being 350 kilometers away. This highlights the need for more granular zoning and a higher awareness of the risk to cities built on these basins.
In my opinion, this research is a crucial step towards understanding and mitigating the risks associated with sedimentary basins. It demonstrates the importance of accurate geophysical methods in urban areas and the need for more detailed computer simulations to predict amplified shaking. By doing so, we can better prepare for the potential impacts of earthquakes on cities built on these basins, ensuring the safety and resilience of our urban environments.
