I would use the mapping toolbox or this function and you can pretty much plot like normal. Everything is just converted into lat and long points. For example your code might look something like this:
% Use map toolbox to plot region of interest and coastlines
figure
worldmap('Canada')
c = load('coast.mat');
plotm(c.lat,c.long,'k')
% Combine lat, long, and magnitude and rearrange by magnitude
latLongMag = [latCoord; longCoord; magValueNum]';
latLongMag = sortrows(latLongMag,3);
% Create a color map for the data points (blue low red high)
colors = jet(length(latLongMag));
% Plot the results
scatterm(latLongMag(:,1),latLongMag(:,2),20,colors)
