9 July 2008

Lituya Bay – 50 years on

Posted by Dave Petley

Fifty years ago today, on the 9th July 1958, one of the most remarkable landslide events in recorded history occurred in Alaska. This was the Lituya Bay landslide, a large rockslide that collapsed catastrophically into a fjord in Alaska. Whilst the landslide itself was comparably unexceptional, though very large, the tsunami that it triggered most certainly was not.

Lituya Bay is located in the very southwest of Alaska (Figure 1). It consists of a narrow fjord some 12 km long and 2-3 km wide (Fig. 2). On the day in question at 10:16 pm, a magnitude 7.7 earthquake occurred on the Fairweather Fault about 21 km from Lituya Bay.

Fig. 1: USGS map showing the location of Lituya Bay

Fig. 2: Google Earth image showing the geometry of Lituya Bay

The earthquake triggered a collapse of a rockmass on a steep, recently debuttressed slope at the toe of the glacier at the top of the fjord (Fig. 3). The rockfall was large (probably about 40 million cubic metres) and catastrophic, falling as a single coherent mass into the fjord from a height of about 900 metres (fig. 4).

Fig. 3: Google Earth perspective view of Lituya Bay, highlighting the location of the rockfall. Click on the image for a better view (opens in a new window).

Fig. 4: USGS image showing the scar of the 1958 rockfall event

The fall triggered a wave in the fjord that raced from the landslide to the mouth of the fjord. The descriptions of this wave are remarkable, but are quite well shown in Fig. 5. The wave had a maximum run-up height (this is the vertical distance that it ran up the valley wall) of 530 metres. Whilst this sounds extreme, there is clear evidence that this was the case from sediments left by the wave and from the removal of trees by the water. This is the highest coastal wave ever recorded, although this very high run-up zone might be considered to me more of a splash than a coherent wave. The wave then travelled the length of the fjord, probably at a speed of about 150 km/hour, removing trees from above the waterline as it went. This removal of trees can be very clearly seen in Fig. 4 and is shown in more detail in Fig. 5. Whilst the height of the wave declined rapidly as it travelled down the fjord, it was still over 30 m high when it reached the mouth of the bay.

Fig. 4: USGS image showing the scar of the 1958 rockfall event

Fig. 5: USGS image showing the removal of trees by the tsunami wave

Fortunately of course Lituya Bay is located in a remote and essentially unsettled part of Alaska. There were however three small boats in the bay at the time of the landslide, the Edrie, the Badger and the Sunmore. The Edrie was at anchor when the wave, at that point about 30 metres high, struck her. Fortunately she rode over the wave and did not sink. The Badger was carried over the spit at the entrance to the fjord (see Fig. 4) and deposited in the open ocean, whereupon she sank. Fortunately the crew survived. The Sunmore tried to outrun the wave but was caught by it and engulfed. The boat and her crew were lost.