Earthquakes can be particularly devastating for cities built on sedimentary basins, and now we have a better understanding of why. These basins, which are essentially depressions in the Earth's crust caused by tectonic activity, tend to be flat and are often chosen for urban development due to their accessibility and resources. However, during earthquakes, they can transform into natural resonance chambers, amplifying seismic waves and causing widespread destruction.
The phenomenon is akin to sound waves echoing around an empty hall, trapping and bouncing seismic waves within the basin. Depending on the basin's shape and depth, these 'seismic echoes' can become amplified, leading to severe damage to infrastructure and buildings. This is exactly what happened in New Zealand's capital city, Wellington, during the 2016 Kaikōura earthquake.
The quake, located 80 kilometers from the city, caused significant shaking in the central business district, exceeding design predictions. Many multi-story buildings were damaged or destroyed, and archival records show that a similar event in 1942 led to the destruction of 10,000 chimneys. Our recent research provides an updated model for the central Wellington basin, revealing it is almost twice as deep (about 500 meters) and shaped differently from previous assumptions.
This new understanding of the basin's shape and depth helps explain the stronger-than-expected shaking. 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 low-wave-speed sediments of the basin amplified the waves, creating standing waves and specific zones of extreme destruction.
Seismic waves become trapped and amplified due to two main reasons. First, as waves move 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. Second, resonance occurs when the wavelengths of the incoming seismic waves match the vertical and horizontal dimensions of the basin, and an edge effect can amplify waves near the basin's edges.
Surprisingly, the basin under Wellington's effective western edge is not the Wellington Fault but follows the Terrace and Lambton faults, cutting across the basin at a high angle. This new understanding has significant implications for predicting shaking and amplification in Wellington. Our computer simulation using a 3D model of the basin found that amplifications of horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge.
While there's 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 inadequate building design. However, our study highlights the importance of using simple geophysical methods to map out basin depths and shapes in urban areas, enabling more granular zoning and higher awareness of the risk to cities built on sedimentary basins, even from distant earthquakes.
