Which of the following are common site effects?

• A. Soft soil amplification, basin resonance, topographic amplification, and weathering.
• B. River flooding and drought.
• C. Tsunami energy focusing.
• D. Global wind shear.

Answer: (A)

Local ground conditions shape how strongly earthquakes shake a site. This question focuses on those site effects—the ways the near-surface environment alters seismic waves as they reach the ground.

Soft soil amplification happens because very loose or soft layers slow down seismic waves and can resonate with the typical frequencies of an earthquake, making the surface shaking larger than it would be on hard rock. Basin resonance occurs when waves get trapped and reverberate in sediment-filled basins, which can boost shaking at certain frequencies. Topographic amplification arises where the landscape’s shape—like hills, ridges, or cliffs—changes how energy is focused or redirected, sometimes increasing shaking at specific spots. Weathering, which weakens rocks and soils and reduces stiffness near the surface, also tends to modify the ground response, often increasing the amplitude or changing the frequency content of shaking.

The other options describe hazards not tied to how the local ground responds to earthquake waves: river flooding and drought are hydrological events; tsunami energy focusing relates to ocean waves and coastal bathymetry rather than the ground’s response to shaking; global wind shear is an atmospheric phenomenon.