The Earth's sedimentary basins, once thought to be safe havens for urban development, are now revealed as potential death traps. These depressions in the Earth's crust, favored for their flatness and suitability for city construction, can transform into natural resonance chambers during earthquakes. This phenomenon, akin to sound waves echoing in an empty hall, results in seismic waves becoming trapped and amplified, leading to 'seismic echoes' that can be highly destructive. The 2016 Kaikōura earthquake in New Zealand's Wellington, located 80 kilometers from the city, serves as a stark reminder of this. The city's central business district experienced shaking that exceeded design predictions, with many multi-storey buildings damaged or destroyed. This is not an isolated incident; historical records show that during the 1942 Wairarapa quake, some 10,000 chimneys were destroyed in Wellington, and the 1985 Mexico City earthquake, located 350 kilometers from the city, resulted in 8,000 deaths and the destruction of high-rise buildings. The reason for this lies in the shape and depth of the basin, which can act as a natural amplifier for seismic waves. The new model of the central Wellington basin, found to be almost twice as deep and with a significantly different shape, provides some explanation for the stronger shaking. The deadliest example of seismic echoes was the 1985 Mexico City earthquake, where waves became trapped in the low-wave-speed sediments of the basin and amplified, creating standing waves that caused specific narrow zones of extreme destruction. The amplification of seismic waves in a basin occurs due to two main reasons: the interchange between wave amplitude and speed, and resonance when the wavelengths of the incoming seismic waves are similar to the vertical and horizontal dimensions of the basin. The shape of the basin under Wellington is particularly surprising, with its effective western edge not being the Wellington Fault, but rather following the line of two previously identified, low-activity faults - the Terrace and Lambton faults. This new understanding of the basin's shape and depth has significant implications for predicting the shaking Wellington might expect, with the predicted amplification being higher for a deeper basin. Simple geophysical methods can now be used in urban areas to map out the depth and shape of basins, leading to more granular zoning for vulnerable areas. However, the risk to cities built on sedimentary basins is not limited to local earthquakes; distant quakes can also cause severe damage due to the amplification of seismic waves. This raises a deeper question: how can we better prepare for and mitigate the risks posed by these natural resonance chambers?
