Mike's comments are accurate. Part of the reason a normal faulting earthquake of M8 would be rare in the oceans is that normal-faulting earthquakes tend to be on faults that dip about 45 deg (they are supposed to be 60 deg but I think for large quakes are more like 45). Thrust quakes are flatter, so can cover more area within crust, so can be larger magnitude. A flatter fault would affect a larger areas of sea floor. So, a flatter fault might have a bigger tsunami for same size quake. Chris