Earthquakes and Sedimentary Basins: A Recipe for Disaster?
The ground beneath our feet can be a treacherous place, especially when it comes to earthquakes. While we often think of earthquakes as destructive forces that originate deep within the Earth, a new study highlights an often-overlooked factor: the role of sedimentary basins in amplifying seismic activity.
Sedimentary basins, essentially depressions in the Earth's crust caused by tectonic activity, are flat and favored locations for building cities. However, during earthquakes, these basins can transform into natural resonance chambers, trapping and amplifying seismic waves, leading to "seismic echoes" that can be highly destructive.
This phenomenon has been observed in New Zealand's capital city, Wellington, which is built on a sedimentary basin. The 2016 Kaikōura earthquake, located 80 kilometers away, caused severe damage to infrastructure, exceeding design predictions. Archival records also reveal that the 1942 Wairarapa quake, also 80 kilometers north of Wellington, destroyed 10,000 chimneys in the city.
The study, which provides an updated model for the central Wellington basin, reveals that it is almost twice as deep (about 500 meters) and has a significantly different shape than previously thought. These differences help explain the stronger-than-expected shaking.
The deadliest example of seismic echoes occurred in 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 that caused specific zones of extreme destruction.
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.
The second reason is resonance, where the wavelengths of incoming seismic waves are similar to the vertical and horizontal dimensions of the basin. Steep-sided basins can also generate edge effects, leading to strong amplification close to the basin's edges.
One of the study's most surprising findings is the shape of the basin under Wellington. Its effective western edge is not the Wellington Fault but rather a high-angle cut across the basin, following the Terrace and Lambton faults. This new understanding has significant implications for predicting shaking in Wellington.
The study used a 3D model of the basin in a computer simulation, finding that amplifications of horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge of the basin. This pattern correlated with the actual damaged buildings during the Kaikōura earthquake, but further analysis is needed to confirm the relationship.
This research highlights the importance of using simple geophysical methods to map out the depth and shape of sedimentary basins in urban areas. By generating computer simulations, we can predict the location of amplified shaking, leading to more granular zoning and a higher awareness of the risk to cities built on these basins, not just from local but also distant earthquakes.
In conclusion, while earthquakes are a natural phenomenon, the role of sedimentary basins in amplifying seismic activity is a critical factor that needs to be considered in urban planning and disaster preparedness. As we continue to build and develop our cities, understanding and accounting for these geological features will be essential in ensuring the safety and resilience of our urban environments.
