OK, the seismicity is much shallower south than north. I pulled out the fault map to see where this is. I don't know the area but may be higher heat flow south than north, and more contraction north than south (transpression vs. transtension?). So, sure, there are probably geologic causes. A fault that is locked to a deeper depth can give off more seismic energy, everything else being the same. Probably shaking is greater in transpression than transtension (or, at least thrust shakes harded than normal). Loma Prieta hypocenter was in area with deeper seismicity before the quake than other parts of the San Andreas.

I'm not an earthquake seismologist, but I think that what you are looking for is worthwhile...although it may take 1000 years to get good at this in any one area. You could look at worldwide quakes in similar fault settings to cut down the wait time (although then it would start to be a job rather than a hobby). You could look into Google Earth and see if earthquakes can be plotted in 3D, or animated with time, on this or some other free 3D software...maybe Roger would be in to helping. This just came up yesterday ...a colleague sent HW an image from Google Earth of eastern Marmara Sea and suroundings...but it looked like the offshore Bathymetry was pasted in to a Google earth oblique view to match, using Illustrator, rather than displaying the bathymetry with Google Earth. I actually don't know what is possible, but think Google Earth can be used for such things.

In research, free is good...until it takes so long to use that time also becomes valuable.
Chris