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 a lesser-known culprit: sedimentary basins. These depressions in the Earth's crust, favored for their flatness and suitability for urban development, can become natural resonance chambers during earthquakes, amplifying seismic waves and causing devastating damage.
The study, conducted by a team of geophysicists, focuses on Wellington, New Zealand, a city built on a sedimentary basin. The researchers found that the basin is almost twice as deep as previously thought, and its shape is significantly different from the old model. This new information helps explain why the shaking during the 2016 Kaikōura earthquake exceeded design predictions, even though the quake was located 80 kilometers from the city.
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 of moderate amplitude became trapped in the low-wave-speed sediments of the basin on which it is built, amplifying and causing extreme destruction.
Seismic waves become trapped and amplified for two main reasons. First, as the waves move from a fast wave-speed medium (solid basement rocks) to the low wave-speed of sedimentary rocks, the amplitude of the waves increases to compensate for the drop in wave speed. This is similar to a tsunami wave that travels across the deep ocean with a small amplitude but high speed, and then, as it comes closer to shore in shallow water, slows down but dramatically increases in amplitude.
The second reason for the amplification of seismic waves in a basin is resonance – when the wavelengths of the incoming seismic waves are similar to either the vertical and horizontal dimensions of the basin. If the basin has steep sides, an edge effect is also generated where strong amplification can occur close to the edge of the basin due to a buildup of different wave types.
The study's findings have significant implications for urban planning and earthquake preparedness. Simple geophysical methods can now be used in urban areas to map out the depth and shape of basins that cities are built on. From these models, we can then generate computer simulations to predict the location of amplified shaking, leading to more granular zoning for what parts of cities may be more vulnerable.
In conclusion, while sedimentary basins may be flat and ideal for building cities, they can also become deadly traps for seismic waves. As we continue to develop and populate these areas, it is crucial to understand the risks and take steps to mitigate them. Only then can we hope to build cities that are truly resilient to the forces of nature.
