( Tsunamis ( ( are ( ( caused ( by ( an ( underwater disturbance ) ) ) ) ( usually ( an ( undersea earthquake ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Tsunamis)) (VP (VBP are) (VP (VBN caused) (PP (IN by) (NP (DT an) (JJ underwater) (NN disturbance))) (PP (RB usually) (NP (DT an) (NN undersea) (NN earthquake))))) (. .)))
Tsunamis are caused by an underwater disturbance usually an undersea earthquake.
(ROOT (S (NP (DT A) (-LRB- -LRB-) (NN n) (-RRB- -RRB-) (NN underwater) (NN earthquake)) (VP (VBZ creates) (NP (DT a) (NN tsunami))) (. .)))