( ( ( ( Underwater earthquakes ) and ) ( volcanic eruptions ) ) ( ( ( ( ( are ( ( the ( usual causes ) ) ( of tsunamis ) ) ) , ) also ) ( once ( called ( tidal waves ) ) ) ) . ) )
(ROOT (S (NP (NP (JJ Underwater) (NNS earthquakes)) (CC and) (NP (JJ volcanic) (NNS eruptions))) (VP (VP (VBP are) (NP (NP (DT the) (JJ usual) (NNS causes)) (PP (IN of) (NP (NNS tsunamis))))) (, ,) (ADVP (RB also)) (VP (ADVP (RB once)) (VBD called) (NP (JJ tidal) (NNS waves)))) (. .)))
Underwater earthquakes and volcanic eruptions are the usual causes of tsunamis, also once called tidal waves.
(ROOT (S (NP (DT A) (-LRB- -LRB-) (NN n) (-RRB- -RRB-) (NN underwater) (NN earthquake)) (VP (VBZ creates) (NP (DT a) (NN tsunami))) (. .)))