= Solution
For a heterogeneous <fault slip> distribution, <seismic moment> is
$$
M_0=G\int_A D\,dA=GA\overline D,
$$
where $G=30\times10^9\ \mathrm{Pa}$ 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. Hence
$$
\int_A D\,dA\simeq40\{25(1.7+2.7+3.1+1.8+1.9+1.7+1.0)+15(0.3)\}\ \mathrm{km^2\,m}\simeq1.4\times10^{10}\ \mathrm{m^3},
$$
and \b[$M_0\simeq4.2\times10^{20}\ \mathrm{N\,m}$]. A reasonable graphical range is approximately $3$–$6\times10^{20}\ \mathrm{N\,m}$, dominated by colour averaging and the down-dip extent of small <fault slip>. The corresponding <moment magnitude> is about $M_w=7.7$, using $M_w=\frac23(\log_{10}(M_0/\mathrm{N\,m})-9.1)$. This is an estimate from the supplied distribution, rather than a substitution of a catalogued <earthquake> magnitude.
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