What magnitude is widely cited for the 1964 Great Alaska Earthquake?

• A. 9.5
• B. 9.2
• C. 8.8
• D. 9.0

Answer: (B)

Understanding how earthquake size is quantified helps explain why 9.2 is the figure you’ll see cited for the Great Alaska Earthquake. That event is evaluated using the moment magnitude scale, which measures the total energy released by a rupture on a fault. For the 1964 Alaska quake, scientists analyze the rupture’s area, the amount of slip, and the seismic waves recorded globally to estimate this energy release. The result is a magnitude of about 9.2 on the moment magnitude scale, placing it among the largest earthquakes in recorded history and the largest on North American soil. This number reflects the overall power of the event, not just how violently any single location shook. The quake occurred along the Alaska–Aleutian subduction zone, where a massive plate boundary slip occurred over a long stretch of fault, which is why the energy release—and thus the magnitude—reached such a high value. Tsunamis generated by the rupture crossed the Pacific, underscoring how megathrust earthquakes can have far-reaching effects. While older or different-scale estimates have appeared in various sources, 9.2 on the moment magnitude scale is the widely accepted, modern figure.