How do ground motion amplification and basin effects influence city-scale shaking patterns in places like Mexico City or Los Angeles?

• A. Basins trap and amplify seismic waves, producing longer duration and higher amplitudes; soft soils amplify, focusing energy along corridors; local geology shapes damage.
• B. Basins dampen all shaking.
• C. Shaking patterns are independent of basin effects.
• D. Basins only affect water but not ground.

Answer: (A)

Ground motion amplification and basin effects determine how shaking varies across a city by changing how seismic energy moves through the ground. Soft soils and lake-bed or sediment-filled basins slow shear waves and let their amplitudes grow, so areas with these materials often experience stronger shaking than nearby rock sites. Basins also act like resonant containers that trap energy, causing reverberations that can last longer and reach higher amplitudes, especially at the edges and within the deepest parts of the basin. This combination means certain neighborhoods—where the geology is soft or where basin geometry concentrates energy—will shake more intensely, while others may feel less impact. In places like Mexico City, thick soft sediments can dramatically amplify motion and extend its duration, leading to widespread and prolonged shaking. In Los Angeles, basin structures can channel and amplify energy along particular corridors, concentrating damage in specific areas. Overall, local geology and basin configuration shape where and how strongly ground shaking is felt, influencing city-scale patterns of damage.