Define 'aftershock' and explain why aftershocks occur after the main event.

• A. A larger earthquake that happens before the main event.
• B. A smaller earthquake following a larger mainshock, due to crustal readjustment and stress redistribution after rupture.
• C. A random tremor with no relation to the main event.
• D. An earthquake caused by a volcanic eruption.

Answer: (B)

Aftershocks are smaller earthquakes that occur after a larger mainshock as the crust settles into a new balance. When the main rupture happens, it redistributes stress in the surrounding rocks, so nearby faults or patches of the same fault may fail in quick succession as they adjust to the new conditions. This sequence is typically strongest soon after the main event and tends to decrease in both size and frequency over time. Aftershocks stay tied to the same region and fault system as the initial quake, unlike random tremors with no relation or events caused by volcanic activity.