( ( When ( ( the volcano ) ( is swelling ) ) ) ( , ( ( the tiltmeter ) ( ( can ( measure ( ( exactly ( how much ) ) ( ( ( the slope ) ( of ( the volcano ) ) ) ( is ( ( tilting away ) ( from ( ( the center ) ( of uplift ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NN volcano)) (VP (VBZ is) (VP (VBG swelling))))) (, ,) (NP (DT the) (NN tiltmeter)) (VP (MD can) (VP (VB measure) (SBAR (WHADJP (RB exactly) (WRB how) (JJ much)) (S (NP (NP (DT the) (NN slope)) (PP (IN of) (NP (DT the) (NN volcano)))) (VP (VBZ is) (VP (VBG tilting) (PRT (RP away)) (PP (IN from) (NP (NP (DT the) (NN center)) (PP (IN of) (NP (NN uplift))))))))))) (. .)))
When the volcano is swelling, the tiltmeter can measure exactly how much the slope of the volcano is tilting away from the center of uplift.
(ROOT (S (NP (NNS Tiltmeters)) (VP (VBP are) (VP (VBN used) (S (VP (TO to) (VP (VB measure) (NP (NP (DT the) (NN angle)) (PP (IN of) (NP (NP (DT the) (NN slope)) (PP (IN of) (NP (DT a) (NN volcano))))))))))) (. .)))