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, new research highlights a lesser-known culprit: sedimentary basins. These depressions in the Earth's crust, favored locations for city building, can transform into natural resonance chambers during earthquakes, amplifying seismic waves and causing devastating damage.
In a recent study, scientists have uncovered the reasons why earthquakes can be particularly destructive for cities built on sedimentary basins. By analyzing the case of Wellington, New Zealand, and comparing it to historical examples like Mexico City, they have shed light on the phenomenon of seismic echoes and their potential impact on urban areas.
The Wellington Enigma
Wellington, New Zealand's capital, is built on a sedimentary basin, and its experience during the 2016 Kaikōura earthquake provides a striking example of this phenomenon. Despite being located 80 kilometers from the epicenter, the city's central business district suffered severe damage, with many multi-story buildings destroyed or damaged. Archival records also reveal that a similar event occurred during the 1942 Wairarapa earthquake, where 10,000 chimneys were destroyed.
The study's authors propose a new model for the central Wellington basin, revealing it to be almost twice as deep (approximately 500 meters) and with a significantly different shape than previously thought. These findings help explain the stronger-than-expected shaking experienced in the city.
Seismic Echo Chambers
Seismic waves become trapped and amplified in sedimentary basins due to two primary 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 that slows down but grows in amplitude as it approaches 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 study's most surprising findings is the shape of the basin beneath Wellington. Contrary to previous assumptions, the 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 the shaking Wellington might experience.
A Historical Perspective
The 1985 Mexico City earthquake serves as a grim reminder of the potential devastation caused by seismic echoes. Located 350 kilometers west of the city, the quake's epicenter was far away, yet the city experienced extreme destruction. The low-wave-speed sediments of the basin trapped and amplified the seismic waves, creating standing waves similar to those in a bathtub.
This resulted in specific narrow zones of extreme destruction, highlighting the risk posed by even distant earthquakes to cities built on sedimentary basins.
Mapping the Risks
The research emphasizes the importance of using simple geophysical methods to map the depth and shape of sedimentary basins in urban areas. By creating computer simulations, scientists can predict the locations of amplified shaking, leading to more granular zoning and identifying vulnerable areas within cities.
This newfound understanding of seismic echoes and their impact on urban areas is a crucial step towards enhancing earthquake preparedness and resilience in cities built on sedimentary basins. As we continue to unravel the complexities of our planet's geology, we must also consider the intricate relationship between the Earth's structure and the safety of our urban environments.
