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 focus on the immediate destruction caused by these natural disasters, there's a hidden danger lurking beneath the surface: sedimentary basins. These depressions in the Earth's crust, formed by tectonic activity, can turn into deadly traps for cities built upon them.
In a recent study, researchers have uncovered a fascinating yet alarming phenomenon. Sedimentary basins, with their flat and often shallow nature, can act as natural resonance chambers during earthquakes. This means that seismic waves, instead of dissipating, can get trapped and amplified within these basins, creating a deadly echo effect.
The consequences of this discovery are far-reaching. Take, for instance, the city of Wellington, New Zealand. Built on a sedimentary basin, it has experienced devastating earthquakes in the past. During the 2016 Kaikōura earthquake, the central business district of Wellington endured shaking that exceeded design predictions, despite the quake being located 80 kilometers away. The damage was severe, with many multi-story buildings damaged or destroyed.
Archival records tell a similar tale. The 1942 Wairarapa earthquake, also centered near Wellington, destroyed 10,000 chimneys in the city. These events highlight the vulnerability of cities built on sedimentary basins, even when the earthquake's epicenter is relatively distant.
The 1985 Mexico City earthquake is a grim reminder of the deadly potential of seismic echoes. With a death toll of 8,000 and high-rise buildings reduced to rubble, this disaster occurred 350 kilometers from the city. The culprit? The low-wave-speed sediments of the basin on which Mexico City stands, which trapped and amplified the seismic waves, creating a standing wave effect.
So, what's the secret behind this deadly phenomenon? Seismic waves become trapped and amplified for two main reasons. Firstly, as waves transition 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 gaining strength as it approaches the shore.
Secondly, resonance plays a crucial role. When the wavelengths of incoming seismic waves align with the vertical and horizontal dimensions of the basin, amplification occurs. Steep-sided basins can also generate edge effects, where strong amplification happens close to the basin's edges due to the buildup of different wave types.
One of the most surprising findings of this research is the shape of the basin beneath Wellington. Contrary to previous assumptions, the effective western edge of the basin is not the Wellington Fault. Instead, it cuts across the basin at a high angle, following the lines of two low-activity faults: the Terrace and Lambton faults.
These new insights have significant implications for predicting earthquake-induced shaking in Wellington. The newly described edge and the deeper basin shape will likely lead to higher amplification of ground motion, particularly at frequencies of 0.7 Hertz, the dominant shaking frequency recorded during past quakes.
In a computer simulation, the researchers found that horizontal ground motion could be 2.5-3 times the background level adjacent to the western edge of the basin. Interestingly, this pattern of amplified shaking correlates with the actual damage locations during the Kaikōura earthquake, although other factors, such as reclaimed land and building design, may also play a role.
This study offers a glimmer of hope. By using simple geophysical methods to map out the depth and shape of sedimentary basins in urban areas, we can now generate computer simulations to predict the locations of amplified shaking. This enables more granular zoning, identifying vulnerable areas within cities.
Moreover, it raises awareness of the risk posed by not only local but also distant earthquakes to cities built on sedimentary basins. As we continue to build and develop our urban landscapes, understanding and mitigating these geological risks become increasingly crucial. The fate of cities like Wellington and Mexico City hangs in the balance, and it's up to us to learn from their tragic stories and take action to ensure a safer future.
