Charles Darwin experienced a magnitude 8.5 earthquake in 1835, while he was in Chile. From The Voyage of the Beagle:
February 20th. - This day has been memorable in the annals of Valdivia, for the most severe earthquake experienced by the oldest inhabitant. I happened to be on shore and was lying down in the wood to rest myself. It came on suddenly, and lasted two minutes, but the time appeared much longer. The rocking of the ground was very sensible. The undulations appeared to my companion and myself to come from due east, whilst others thought they proceeded from south-west: this shows how difficult it sometimes is to perceive the directions of the vibrations. There was no difficulty in standing upright, but the motion made me almost giddy: it was something like the movement of a vessel in a little cross-ripple, or still more like that felt by a person skating over thin ice, which bends under the weight of his body.
Two weeks later the Beagle reached the epicentre of the earthquake:
March 4th. - We entered the harbour of Concepcion. While the ship was beating up to the anchorage, I landed on the island of Quiriquina. The mayor-domo of the estate quickly rode down to tell me the terrible news of the great earthquake of the 20th: - "That not a house in Concepcion or Talcahuano (the port) was standing; that seventy villages were destroyed; and that a great wave had almost washed away the ruins of Talcahuano." Of this latter statement I soon saw abundant proofs - the whole coast being strewed over with timber and furniture as if a thousand ships had been wrecked. Besides chairs, tables, book-shelves, etc. in great numbers, there were several roofs of cottages, which had been transported almost whole. The storehouses at Talcahuano had been burst open, and great bags of cotton, yerba, and other valuable merchantise were scattered on the shore. During my walk round the island, I observed that numerous fragments of rock, which, from the marine productions adhering to them, must recently have been lying in deep water, had been cast up high on the beach; one of these was six feet long, three broad, and two thick.
Darwin spends six more pages of his journal on his and the Beagle's crew's observations of the effects of the earthquake, and on the stories of survivors. He then starts theorising, first about the recession of the sea that preceded the wave, and then about the consequences of the uplift of the land resulting from repeated earthquakes:
The most remarkable effect of this earthquake was the permanent elevation of the land: it would probably be far more correct to speak of it as the cause. There can be no doubt that the land round the Bay of Concepcion was raised by two or three feet, but it deserves notice that, owing to the wave having obliterated the old lines of tidal action on the sloping sandy shores, I could discover no evidence for this fact, except in the united testimony of the inhabitants, that one little rocky shoal, now exposed, was formerly covered with water. At the island of S. Maria (about 30 miles distant) the elevation was greater; on one part, Captain Fitz Roy found beds of putrid mussel-shells still adhering to the rocks, ten feet above high water mark: the inhabitants had formerly dived at low-water spring tides for these shells. The elevation of this province is particularly interesting, from it having been the theatre of several other violent earthquakes, and from the vast number of sea-shells scattered over the land, up to a height of certainly 600, and I believe, of 1000 feet. At Valparaiso, as I have remarked, similar shells are found at a height of 1300 feet: it is hardly possible to doubt that this great elevation has been effected by successive small uprisings, such as that which accompanied or caused the earthquake of this year, and likewise by an insensibly slow rise, which is certainly in progress on some parts of this coast.