You cause the earth within the area to shake like an earthquake, which acts as a trip attack against all creatures in the area. This trip-like attack is not subject to the size limitations of a normal trip attack. Your CMB for this trip attack is equal to your caster level plus your Intelligence, Wisdom, or Charisma modifier, whichever is highest. This trip attack does not provoke an attack of opportunity.  Those who fail are tripped and fall prone, those who succeed are unaffected. If your attack fails by 10 or more, you are not knocked prone instead. You cannot use Improved Trip or similar feats in conjunction with this spell. The spell only affects creatures in contact with the affected area (for example, a standing or climbing creature is affected, while a flying creature is not).