The spell says:
You create a minor earthquake that can trip creatures. Make a single trip attack and use the result against every creature in the area. Those that fail are tripped and fall prone. You do not provoke an attack of opportunity for this trip attack (though you do provoke one for casting the spell), nor can you knock yourself prone with a bad roll."
Being a spell caster and new to pathfinder, do I just roll a D20 and add my CMB (Base attack bonus + Strength modifier + special size modifier) and then compare to their CMD's?