(S (S (SBAR (IN If) (S (BASENP (DT an) (NN earthquake)) (VP (VBZ happens) (PP (IN at) (BASENP (PDT such) (DT a) (JJ low) (NN depth)))))) (COMMA ,) (S (BASENP (DT the) (NNS waves)) (VP (VP (VBP move) (PP (IN along) (NP (BASENP (JJ low) (NNS layers)) (COMMA ,) (NP (ADVP (RB practically)) (BASENP (DT the) (NN mantle)))))) (CONJP (COMMA ,) (CC but)) (VP (VP (VBP weaken) (ADVP (RB significantly))) (PP (IN before) (S (VP (VBG reaching) (BASENP (DT the) (NN earth) (NN surface))))))))) (PERIOD .))