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An earthquake centroid is an effective average location of seismic moment release. It differs from the hypocenter, where rupture starts, and can depend on the source representation and observations. An associated centroid time similarly averages release rather than marking its first instant.

A logarithmic measure of earthquake size based on seismic moment. A commonly used SI relation is $M_w=\frac{2}{3}[{\log }_{10}(M_0/(\mathrm{N}\mathrm{m}))-9.1]$. Because seismic moment integrates fault slip over the fault plane and includes the shear modulus, a peak slip value cannot replace the area integral.

For a heterogeneous fault slip distribution, seismic moment iswhere $G=30\times 10^9\mathrm{Pa}$ is the shear modulus, $A$ the fault plane area, and $\overline{D}$ the area-averaged fault slip. The peak colour cannot be used as $\overline{D}$: most of the illustrated fault plane has much smaller fault slip.

One reproducible graphical estimate divides the approximately $190\mathrm{km}$ long and $40\mathrm{km}$ down-dip panel into seven $25\mathrm{km}$ strips and a final $15\mathrm{km}$ strip. Reading area averages from the colour scale, from southwest to northeast, gives roughly $1.7,2.7,3.1,1.8,1.9,1.7,1.0,0.3\mathrm{m}$. This includes the pale, nearly unslipped lower part of each strip. Henceand $M_0\simeq 4.2\times 10^{20}\mathrm{N}\mathrm{m}$. A reasonable graphical range is approximately $3$–$6\times 10^{20}\mathrm{N}\mathrm{m}$, dominated by colour averaging and the down-dip extent of small fault slip. The corresponding moment magnitude is about $M_w=7.7$, using $M_w=\frac{2}{3}({\log }_{10}(M_0/\mathrm{N}\mathrm{m})-9.1)$. This is an estimate from the supplied distribution, rather than a substitution of a catalogued earthquake magnitude.

For uniform slip on a shear fault, scalar seismic moment is $M_0=\mu Ad$, with shear modulus $\mu$, fault area $A$ and slip magnitude $d$. More generally it integrates rigidity-weighted slip. It measures source strength rather than radiated energy alone.