(S (S (BASENP (DT The) (NNP UN)) (VP (VBZ states) (SBAR (S (BASENP (JJ mass) (NNS rapes)) (VP (VBP have) (VP (VP (VBN occurred) (PP (IN in) (BASENP (DT the) (NN area)))) (PP (IN for) (NP (QP (ADVP (IN at) (JJS least)) (NN 15)) (NNS years))))))))) (PERIOD .))