Question: what is a type of mechanical wave motion in an elastic medium
[{'answer': 'Rayleigh waves', 'provenance': [{'wikipedia_id': '162312', 'title': 'Mechanical wave', 'start_paragraph_id': 11, 'start_character': 231, 'end_paragraph_id': 11, 'end_character': 245, 'bleu_score': 1.0, 'section': 'Section::::Surface waves.\n'}]}, {'answer': 'ground roll', 'provenance': [{'wikipedia_id': '162312', 'title': 'Mechanical wave', 'start_paragraph_id': 12, 'start_character': 31, 'end_paragraph_id': 12, 'end_character': 42, 'bleu_score': 1.0, 'section': 'Section::::Surface waves.\n'}]}, {'provenance': [{'wikipedia_id': '162312', 'title': 'Mechanical wave', 'start_paragraph_id': 12, 'start_character': 0, 'end_paragraph_id': 12, 'end_character': 279, 'bleu_score': 0.854614595673399, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9353808492824036664}, 'section': 'Section::::Surface waves.\n'}]}]