There was a noticeable tremor in SD in the early '80's. One son was studying in the college library at the college the state converted to a prison some years later. He couldn't figure out what was happening, but definitely felt, saw things moving that shouldn't have been. I was awake, reading, and as I recall it may have been around 9:30 or 10M and I thought it might be a sonic boom, but never heard the "boom". Heard on the news next morning what it was. Really strange feeling and I don't want to experience a bigger one!