The Haiti tsunami

The State of the Planet blog, which is the blog of the Earth Institute at Columbia University, reports on an ongoing investigation of the tsunami that killed a small number of people to the west of Port-au-Prince (image below from the National Geographic).

The research has involved drilling cores into the sediment on the sea floor near to the tsunami. Their initial analysis suggests that the tsunami was caused by a lateral spread near to the coastline, which released a flow down the offshore slope into deeper water. It is this submarine landslide that was probably responsible for the localised tsunami.

They promise more analysis in the coming days, so watch this space.

An ancient tsunami in New York

The BBC are running a slightly strange story today about the possibility that the News York area was hit by a tsunami about 2,300 years ago. Given that by far the most likely source of a tsunami is a submarine landslide, this is of some interest. The odd part of this is the timing of the story - I cannot quite work out why it has popped up again now. A very similar story made the news in November last year - see here - and again in December (see here), so I can't quite see what has changed to bring this back to the forefront again. Perhaps a paper regarding this is about to be published - but there is no reference to this in the news report on the BBC.



The BBC news report focuses upon work being undertaken by Steve Goodbred and colleagues at Vanderbilt University in Tennessee. They have identified what appears to be a tsunami deposit from cores across New York and New Jersey. Tsunami deposits are interesting as the wave can transport coarse-grained particles (typically gravels) and even marine fossils from the near continental shelf to deposit them some way inland. Thus, they tend to be quite distinctive. Unfortunately, a major problem is that storm surges associated with very large storms can also bring in coarse shallow marine materials, which means that determining for sure that a deposit was generated by a tsunami is difficult.

In the case of the New York area, a quite extensive deposit has been found. Dr Goodbred and his colleagues suggest in the BBC article that this is probably a tsunami created by a submarine landslide, much like the 1929 Grand Banks (Newfoundland) event.



My sense is that considerable further work is needed here. First, we should all be distinctly pleased that there is no suggestion being made that this tsunami deposit was generated by the ludicrous hypothesis postulated a few years ago by Ward and Day that occasional flank collapses on the volcanoes of the Canary Islands can generate tsunamis that devastate the whole North Atlantic basin. However, the November 2008 news stories centred around a suggestion that a research team from Harvard and Colombia University had found evidence of an impact event origin for the tsunami deposit - i.e. that the wave was generated by a meteorite impact into the ocean. They claim that the deposit contains the types of particles that such an event would generate. I note that there does appear to be some difference in the size of the wave that the teams believe occurred - Goodbred suggests c.5 m, whilst the Harvard team suggest 20 m.

Personally I would like to see some peer reviewed publications on this before we make too much of it. There are a whole series of strands here that really need to go through peer review:
1. The structure of the deposit and its distribution;
2. Its age;
3. Analyses of its origin, looking in particular at its composition (particle mineralogies and sizes, fossil materials; etc), variations in its thickness, the presence of nano-diamonds that are supposedly indicative of an impact event, etc.

Of course the media interest in this is that the potential destruction of New York by a tsunami is rather exciting. We do need to understand Atlantic tsunamis rather better, but there is little doubt that they are really pretty rare and have rather localised impacts in terms of the tsunamis that they generate.

Two interesting recent landslides

Two interesting landslides to report in the last few days.

1. Landslide in Iztapalapa, Mexico City
On Thursday morning a 50 cubic metre landslide in the Itzapalapa, a poor suburb of Mexico City slid onto a house at the toe of the slope, killing two people. Chinagate has published a rather dramatic picture of the site:

The interesting thing about this image is the very large (apparently 5 m high) retaining wall that had been built between the slope and the houses. Clearly the wall has failed under the weight of the landslide (not surprising given the size of those boulders actually), but I wonder who built the wall (surely not the people living in these houses - it is far too big)?

2. Landslide-induced wave on Lake Roosevelt, Washington State, USA
The second is a report from the Spokane arm of Lake Roosevelt near to Spokane in Washington State. The website Spokesman Review is carrying a report that says:

"Property owners in the Spokane arm of Lake Roosevelt were swamped by a huge wave caused by a landslide last Friday. The 17-acre landslide on Jan. 16 near Mill Canyon – northeast of Davenport and downstream from Little Falls Dam – slid into the water, unleashing a wave that reached 30 feet above the full-pool mark across the lake into Breezy Bay, according to Adam Kelsey, chief ranger for the Lake Roosevelt National Recreation Area. Private docks and vessels were destroyed or damaged for at least 1.5 miles downstream from the site."

They also have an image of the site:


It is reasonable to assume that the displaced mass on the far side of the lake is the landslide as the Google Earth image does not show a slide at this site:

It is interesting to note that the section of slope that has failed shows clear signs of toe erosion and some interesting linear features on the Google Earth imagery:

Interestingly, in the years after Lake Roosevelt was filled there were a series of landslides. The Emergency Management Division of Washington Military Department has a document here that describes a series of events as the lake was filled and emptied:

"1944 to 1953 – Massive landslides generated a number of inland tsunamis in Lake Roosevelt in Eastern Washington:
• April 8, 1944 - A four to five million cubic yard landslide from Reed Terrace generated a 30-foot wave, 5,000 feet away on the opposite shore of the lake about 98 miles above Grand Coulee Dam.
• July 27, 1949 - A two to three million cubic yard landslide near the mouth of Hawk Creek created a 65-foot wave that crossed the lake about 35 miles above Grand Coulee Dam; people 20 miles away observed the wave.
• February 23, 1951 – A 100,000 to 200,000 cubic yard landslide just north of Kettle Falls created a wave that picked up logs at the Harter Lumber Company Mill and flung them through the mill 10 feet above lake level.
• April 10 – 13, 1952 – A 15 million cubic yard landslide three miles below the Kettle Falls Bridge created a 65-foot wave that struck the opposite shore of the lake. People observed some waves six miles up the lake.
• October 13, 1952 – A landslide 98 miles upstream of Grand Coulee Dam created a wave that broke tugboats and barges loose from their moorings at the Lafferty Transportation Company six miles away. It also swept logs and other debris over a large area above lake level.
• February 1953 – A series of landslides about 100 miles upstream from Grand Coulee Dam generated a number of waves that crossed the lake and hit the opposite shore 16 feet above lake level. On average, observed waves crossed
the 5,000-foot wide lake in about 90 seconds.
• April – August 1953 – Landslides originating in Reed Terrace caused waves in the lake at least 11 different times. The largest wave to hit the opposite shore was 65 feet high and observed six miles away. Velocity of one of the series of waves was about 45 miles per hour."

Volcanic flank collapse and tsunamis

A few years ago the media got rather excited about a paper that suggested that there was the potential for a giant flank collapse on the Cumbre Vieja volcano in the Canary Islands. A model of the resultant tsunami suggested that it could be sufficiently large to cause huge losses throughout the coastal areas of the North Atlantic. Most scientists now believe that this tsunami was something of an exageration of a worse case scenario, (see for example this critique) but the potential for a flank collapse to generate a local tsunami event is very real, albeit comparatively rare. As a result, there is considerable interest in trying to understand these events better.

At the GSA Annual Meeting this week, Matthew Hornbach from the University of Texas presented a rather interesting study that has just been undertaken on the island of Tongatapu in Tonga. Tongatapu is located in the South Pacific with a number of active volcanoes nearby (Fig 1).

Fig. 1: Google Earth image of the island of Tongatapu showing the location of three unnamed submarine volcanoes to the west.

Hornbach found that on Tongatapu there are seven coral boulders located up to 400 m from the current coastline (Fig. 2). The largest of these boulders has a mass of about 1200 m. The boulders have clearly been lifted from the coral that fringes the island - Science News reports that for the largest boulder the point in the reef from where it has been removed is clear.

Fig. 2: Science News image of one of the boulders on the island of Tongatapu.

Now, this island is flat, meaning that the boulders cannot have rolled there. Hence, Hornbach has concluded that these boulders were emplaced by a huge tsunami, which they have provisionally dated to have occurred in the Holocene (i.e. in the last 10,000 years). This is potentially the largest tsunami deposit so far recorded - the wave must have had a run up of >40 m vertical distance. Hornback believes that the source of this tsunami was a submarine volcano located to the west of the island (Fig. 1). They report that bathymetric surveys of this feature show that it rises to to the water surface - some reports suggest within 65 m. This large volcano apparently has a section missing from the east side, which Hornbach interprets as being the result of a flank collapse - i.e. a giant landslide in which the side of the volcano collapses. Flank collapses undoubtedly can cause very large, localised tsunamis.

As an aside, it is interesting to note that the major population centres are on the sheltered north and east side of the islands (Fig. 3) . I wonder if this is a coincidence?

Fig. 3: Google Earth image of Tongatapu .

Submarine landslide in southern Africa?

The award for the strangest landslide story of the week goes to this one, from southern Africa. Late last week reports started to emerge of strange tidal patterns around the southern Cape of Africa (Fig. 1).

Fig. 1: Google Earth image of the southern Cape of Africa, showing the location of St Helena Bay and Plattenburg Bay.

In particular, on 21st August at about 8:45 am (local time) a series of tidal oscillations were recorded in St Helena Bay (Fig. 2). These tidal oscillations were reportedly large enough to damage some factories, pull a car into the sea and to cause problems for boats navigating into the harbours. A couple of days later, a report emerged from the National Sea Rescue Institute (NRSI) suggesting that the "mini-tsunami" was caused by a seismically-induced submarine landslide. Fig. 2 shows that some anomalous tidal activity certainly did occur.

Fig. 2: Tide gauge data for Port Nolloth showing the anomalous behaviour on the morning of 21st August 2008 (graph from the South African Weather Service).

Now, of course submarine landslides can trigger localised tsunamis (as occurred in Papua New Guinea in 1998), and the most likely cause of such an event is an earthquake. However, aspects of this cause do not seem to ring true. First, southern Africa is not a strongly seismic area. Second, no earthquake event has been recorded at the time that the tidal oscillations were observed. And third, the oscillations appear to have occurred over a long time period, and at markedly different times along the coast. One would expect that a landslide induced tsunami would be quite short-lived and to arrive at the coast over quite a short time period. Thus, the evidence does not really seem to support this mechanism.

In the last couple of days a more considered analysis from NSRI has been reported, recognising that the cause is more likely to be meteorological than the effects of a landslide. The South African Weather Service have proposed that atmospheric gravity waves might have been the cause, although they are sitting on the fence a little.

Whilst these tidal oscillations are clearly strange, a landslide cause does not really seem to be very likely.

Lituya Bay - 50 years on

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.