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, known as seismic echoes, has been a subject of study for geologists and engineers, with a recent focus on its impact on cities like Wellington, New Zealand. The city, built on a sedimentary basin, experienced severe shaking during the 2016 Kaikōura earthquake, which exceeded design predictions. This event, along with historical records, highlights the vulnerability of cities built on these basins, even when earthquakes are located far away.
The key to understanding this phenomenon lies in the shape and depth of the basin. Our research, published in the Journal of Geophysics, reveals that the central Wellington basin is almost twice as deep as previously thought, with a significantly different shape. This discovery provides a partial explanation for the stronger-than-expected shaking. The basin's effective western edge, which was previously assumed to be the Wellington Fault, is actually defined by two low-activity faults, the Terrace and Lambton faults. This new understanding of the basin's geometry has significant implications for predicting the amplified shaking that cities like Wellington might experience.
The deadliest example of seismic echoes in history is 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 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, emphasizing the risk from distant earthquakes for cities built on sedimentary basins.
Seismic waves become trapped and amplified for two main reasons. First, as waves move from a fast wave-speed medium (solid basement rocks) to the low wave-speed of sedimentary rocks, the amplitude increases to compensate for the drop in wave speed, similar to a tsunami wave that increases in amplitude as it approaches shore. Second, resonance occurs when the wavelengths of the incoming seismic waves are similar to the vertical and horizontal dimensions of the basin, amplifying the waves further. If the basin has steep sides, an edge effect can also generate strong amplification close to the basin's edge due to the buildup of different wave types.
One of the most surprising findings of our research is the shape of the basin under Wellington. The effective western edge is not the Wellington Fault, as previously assumed, but rather a high-angle cut across the basin following the line of two low-activity faults. This new understanding of the basin's geometry has significant implications for predicting the amplified shaking that cities like Wellington might experience. Our 3D model of the basin, used in a computer simulation, predicted amplifications of horizontal ground motion up to 2.5-3 times the background level adjacent to the western edge of the basin.
While there is some correlation between the predicted pattern of amplified shaking and the actual damaged buildings during the Kaikōura earthquake, we must be cautious. This pattern could be linked to other factors, such as the distribution of reclaimed land and clustering of inadequately designed buildings. However, our study highlights two key points. First, simple geophysical methods can now be used in urban areas to map out the depth and shape of basins that cities are built on, leading to more granular zoning for vulnerable areas. Second, there is a higher awareness of the risk to cities built on sedimentary basins, not only from local but also distant earthquakes.
In conclusion, the Earth's sedimentary basins, once thought to be safe locations for cities, have revealed a hidden danger. As we continue to study and understand this phenomenon, we must be aware of the risks and take steps to mitigate them. The future of urban planning and design must consider the potential for seismic echoes, ensuring the safety and resilience of our cities in the face of natural disasters.
