Measured energy in Japan quake

The article by Thorne Lay and Hiroo Kanamori titled “Insights from the great 2011 Japan earthquake” (PHYSICS TODAY, December 2011, page 33 ) is an interesting one. As a seismologist who worked in the field of underground nuclear explosions, I was caught by the following statement in the first paragraph: “Total strain energy equivalent to a 100-megaton explosion was released during the sliding.” Some familiarity with this subject led me to think this is not right. If the authors would carefully review their calculations using the energy equivalent in TNT, the relationship between seismic moment and magnitude, and the relationship between strain energy and seismic moment, they would find that the seismic energy equivalent of the 2011 Japan earthquake is roughly 2 × 10¹⁸ J, while that of a 100-megaton nuclear bomb is roughly 4 × 10¹⁷ J. Thus the 2011 Japan subduction event released approximately five times as much energy as a 100-megaton device, which is approximately twice the largest nuclear detonation ever—a 50-megaton atmospheric explosion by the former Soviet Union in October 1961.

The 1964 Chilean earthquake had still more energy by a factor of about 3, or 15 times that of a 100-megaton nuclear device. I believe the authors used the relation for seismic energy release rather than total strain energy release. The seismic energy underestimates the total strain energy release by a variable that depends on friction on the fault plane. Accounting for total strain energy release would increase the earthquake energy number by orders of magnitude.

Despite the catastrophic damage potential of nuclear bombs, the forces of nature occasionally unleash much larger energy releases. Although the nuclear bombs are under our control, earthquakes, volcanic eruptions, and extreme weather events are not. However, by judicious preparation and avoidance measures, humans can significantly diminish the damage of natural events.