Which phenomenon explains why eastern U.S. earthquakes are felt over longer distances than California earthquakes of similar size?

• A. The crust is old and cold in the Eastern US and transmits the waves more effectively
• B. California's crust is younger and warmer
• C. There are more faults closer to the surface in the East
• D. Seismic waves travel slower in the East due to moisture

Answer: (A)

Seismic energy travels farther when attenuation is low, meaning the waves don’t lose much energy as they propagate. The eastern United States sits on ancient, cold, relatively rigid crust that conducts seismic waves with less energy loss and with less scattering. That lets P and S waves travel farther and still be felt at greater distances from the source. In California, the crust is younger and warmer, more fractured, and structurally complex, which increases attenuation and scattering of waves, so shaking dies out more quickly with distance. The other ideas don’t fit the pattern: near-surface faults in the East would boost local shaking, moisture effects don’t make waves travel farther, and slower waves from moisture would actually reduce, not extend, felt distances.