Question: when was the worst earthquake in san francisco
[{'answer': 'April 18 , 1906', 'provenance': [{'wikipedia_id': '20110714', 'title': '1906 San Francisco earthquake', 'start_paragraph_id': 57, 'start_character': 77, 'end_paragraph_id': 57, 'end_character': 92, 'bleu_score': 1.0, 'section': 'Section::::Centennial commemorations.\n'}]}]