Why is site effect important in seismic hazard, and what are common site effects?

• A. Local geology affects only groundwater, not shaking.
• B. It has no impact on shaking.
• C. Local geology can amplify or dampen shaking; common effects include soft soil amplification, basin resonance, topographic amplification, and weathering.
• D. It only influences climate change.

Answer: (C)

Local ground conditions change how earthquake energy arrives at the surface, so site effects can make shaking stronger or weaker than what bedrock would suggest. Softer near-surface materials have lower shear-wave velocities and can trap and amplify energy, especially at certain frequencies, which is why areas built on thick soft soils often experience more intense shaking. Basin resonance is a related idea: basins with layered sediments can set up standing waves that boost motion at specific frequencies, stretching the shaking out in space and time inside the basin. Topographic amplification occurs where the landscape itself—like hills or ridges—focuses and strengthens waves, leading to higher shaking at the crest or along steep slopes. Weathering of surface rock or soils lowers stiffness and velocity, creating conditions that amplify ground motion, particularly for longer-period shaking relevant to taller structures. Together, these effects mean that the same earthquake can produce very different shaking across a region, so understanding local geology is essential for accurate seismic hazard assessment and safer design. The other statements overlook how local materials and geometry influence shaking, so they don’t describe site effects accurately.