Roger...do you think there are a lot of the quakes where the depth is set (6.0 km, 10.0 km), or are these all relocated?. I'd suggest using one of the several catalogues of relocated quakes available online through the SCEC data center. These probably have location quality factors and you can toss those that are "poor" and maybe "fair" (I guess the quality factors are numerical?). You could toss those that are even km depth like 6.000 and 10.0000. But, looks like there is an interesting depth change that you and Glen picked up.

Chris