[ed]

To dissipate the energy of a M9 (which happens about once per decade) you'd need about 32,000 quakes of M6 (still big enough to collapse buildings).

Energy scales as 10^(1.5 × ΔM)

ΔM = 9.0 − 6.0 = 3.0

10^(1.5 × 3) = 10^4.5 ≈ 31,600