stacking rock

Hey guys, i recently add 20 lbs of live rocks to my 30 gallons tank, and i dont know if the way i stack on the picture can make it secure. I live in California, and have earthquake sometime, just don't want some rock fall off and break the glass. I also want to stack it a little higher because...

I was curious, do most of you free stack your rock, or are you using some epoxy or something to put some bits together? If so, what are you using? I had read somewhere about people using a drill and specially made dowels to do certain stacking in order to keep things movable, although this...