Earthquakes and Sedimentary Basins: A Recipe for Disaster?
The ground beneath our feet is a complex and dynamic system, and when it rumbles, it can have devastating consequences for cities built upon it. Sedimentary basins, those flat depressions in the Earth's crust caused by tectonic activity, are particularly vulnerable to the destructive forces of earthquakes. These basins, favored locations for urban development, can become natural resonance chambers, amplifying seismic waves and causing localized, intense shaking.
The case of Wellington, New Zealand, is a stark reminder of this phenomenon. The city, built on a sedimentary basin, experienced severe damage during the 2016 Kaikōura earthquake, despite being located 80 kilometers away. The shaking exceeded design predictions, and the central business district suffered significant infrastructure damage. Archival records also reveal that a similar event occurred during the 1942 Wairarapa quake, where 10,000 chimneys were destroyed.
But what makes this scenario even more intriguing is the role of the basin's shape and depth. Our research, published in the journal Geophysics, reveals that the central Wellington basin is almost twice as deep as previously thought, and its shape differs significantly from the old model. This updated understanding of the basin's geometry provides a partial explanation for the stronger-than-expected shaking.
The deadliest example of seismic echoes, or trapped waves, can be found in the 1985 Mexico City earthquake. The quake's epicenter was 350 kilometers west of the city, but the waves became trapped in the low-wave-speed sediments of the basin, amplifying and creating standing waves. This resulted in specific narrow zones of extreme destruction, highlighting the risk from distant earthquakes for cities built on sedimentary basins.
Seismic waves become trapped and amplified due to two primary reasons. Firstly, as waves transition from a fast wave-speed medium (solid basement rocks) to the low wave-speed of sedimentary rocks, their amplitude increases to compensate for the drop in wave speed, similar to a tsunami wave. Secondly, resonance plays a crucial role when the wavelengths of incoming seismic waves align with the vertical and horizontal dimensions of the basin.
One of the most surprising findings of our study is the shape of the basin beneath Wellington. Contrary to previous assumptions, the effective western edge of the basin is not the Wellington Fault. Instead, it cuts across the basin at a high angle, following the line of two low-activity faults: the Terrace and Lambton faults. These new insights significantly impact the predicted shaking Wellington might experience, with higher amplification expected in a deeper basin.
Using a 3D model of the basin, we conducted computer simulations to predict the shaking at frequencies of 0.7 Hertz. Our findings indicate that the amplifications of horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge of the basin. Interestingly, when we compared this predicted pattern to the actual damage locations during the Kaikōura earthquake, we observed a correlation with the western edge of the basin.
However, it's essential to approach this comparison with caution, as other factors, such as reclaimed land distribution and building design, could also influence the damage pattern. Nevertheless, our study emphasizes two critical points. Firstly, geophysical methods can now be employed in urban areas to map out the depth and shape of basins, enabling computer simulations to predict amplified shaking.
This advancement allows for more precise zoning, identifying vulnerable areas within cities. Secondly, the risk to cities built on sedimentary basins is not limited to local earthquakes but also extends to distant quakes. As we continue to unravel the mysteries of the Earth's crust, understanding and mitigating these risks will be crucial in building safer and more resilient urban environments.
