Measured energy in Japan quake

Lay and Kanamori reply: Our article states that the total radiated energy release estimated for the Tohoku event, as directly measured by integration of seismic-wave ground-velocity recordings and the source time function, is 4.2 × 10¹⁷ J. That number compares with David von Seggern’s energy value for a 100-megaton explosion of “roughly 4 × 10¹⁷ J.” Thus we seem to agree that our estimate of the seismic wave energy release from the earthquake corresponds to total energy from a 100-MT explosion.

The wording in the first paragraph of our article, however, should have been “total radiated energy” rather than “total strain energy.” Some strain energy goes into heating the fault zone and other dissipative processes, so total strain energy will always exceed seismically radiated energy by an amount that cannot be measured by seismology. Von Seggern computes a number for “seismic energy” using a formula (apparently the Gutenberg–Richter relation) for radiated energy as a function of seismic magnitude; that is quite different from estimating radiated seismic energy directly as we did. His estimate of seismic energy is about a factor of five larger than our directly measured radiated energy estimate. Scaling relations between seismic magnitude and energy have very large spread, so we prefer direct measures of radiated energy from seismic waves.