How to calculate cumulative moment magnitude?
The moment magnitude ${\displaystyle M_{\mathrm {w} }}$  is a dimensionless number defined by Hiroo Kanamori as: 

$$M_{\mathrm {w} }={\frac {2}{3}}\log _{10}(M_{0})-10.7$$

To sum several moment magnitudes we need to first convert them to  ${\displaystyle M_{\mathrm {0} }}$ (seismic moment) which you can simply sum and then convert it back


for each of the magnitudes you have - insert the magnitude into ${\displaystyle M_{\mathrm {w} }}$, solve for ${\displaystyle M_{\mathrm {0} }}$ and write down the answer.
Sum all the answers
Insert the sum back into ${\displaystyle M_{\mathrm {0} }}$ and solve for ${\displaystyle M_{\mathrm {w} }}$