Dust flux, Vostok ice core

Dust flux, Vostok ice core
Two dimensional phase space reconstruction of dust flux from the Vostok core over the period 186-4 ka using the time derivative method. Dust flux on the x-axis, rate of change is on the y-axis. From Gipp (2001).

Monday, October 17, 2011

Boys, boys, boys . . . (updated)

. . . don't worry overly much about the tsunami that has been forecast to join the Occupy Wall Street movement.

The idea that one or more of the Canary Islands will suffer some sort of catastrophic failure due to a volcanic eruption causing an enormous landslide that will generate a tsunami sufficient to devastate eastern North America, Europe, Northwest Africa and parts of South and Central America has been around for some time.

Although the USGS is well aware of the possibility, and has carried out some preperatory work in terms of planning, there are a number of factors to consider in the risk assessment of such an event.

First of all, the video below is complete bullshit.

I don't doubt the sincerity of those who promote this idea, but there are two basic problems with the premise.

Although it is true that landslides create large tsunami--in fact the largest tsunami on record (Lituya Bay) and the largest tsunami ever inferred from the geological record were both generated from landslides. The problem is that none of these events have ever generated the so-called teletsunami--one that crosses the ocean to wreak havoc on the other side.

A major part of the reason has to do with the geometry of the source. For a tsunami to travel long distances, it has to have been formed from a long, linear source. If the source of the tsunami is a point, as is the case for submarine volcanism or landslides, then much of the energy is dispersed along the rapidly expanding front of the wave as it moves away from the source. Further dissipation occurs as the wave crosses seamounts, all of which act to reduce the impact of such a wave as it crosses the ocean.

Consequently, the tsunami generated from submarine earthquakes (usually in subduction zones) start off smaller than landslide-generated earthquakes, but there is little dispersion of energy as the leading edge of the wave does not much lengthen as the wave crosses the ocean. There are numerous examples of large subduction earthquakes generating tsunami which crossed the ocean with enough force to cause severe property damage and casualties, including in the Indian Ocean in 2004, and Hawaii in both 1946 (source-the Aleutian Islands) and again in 1960 (source-Chile).

Geologists have been able to infer that such teletsunami have occurred in the past, on the basis of certain types of deposits, and, in some cases, the historical record. The geologic record goes farther than the historical record, and necessarily forms the basis of much risk assessment in areas prone to tsunami.

Volcanic islands have generated very large tsunami in the geologic past. The Hawaiian Islands are notorious for them. Despite their much larger initial height than earthquake-generated tsunami, there are yet to be any discoveries of tsunami deposits around the edge of the Pacific basin that can be tied to the Hawaiian landslides.

The only geological evidence for a teletsunami generated from a point source that I am aware of is the (still controversial) interpretation of some units in the Brazos River Sandstone in Texas as a result of the Chicxuclub asteroid impact at the end of the Cretaceous. The distance between the impact and the tsunami deposits is considerably less than the distance between the Canary Islands and North America--furthermore the energy provided by the bolide impact is orders of magnitude larger than would be expected from a Canary Islands landslide.

The second serious problem with the Canary Island megatsunami idea is that the collapse of a very large mass of rock is not likely to occur as a single impulse. The events are usually complex, characterized by multiple episodes of failure over a period of minutes to hours to days. The models usually assume a single impulse, which generates the most devastating results (and possibly results in greater funding opportunities). A more realistic model would generate a much smaller and more complex series of waves, the energy of which will disperse as the wave crosses the Atlantic as discussed above.

This is not to say that there is no chance whatsoever for damage on the eastern seaboard--it is just grotesquely exaggerated.

However--as local phenomena, these landslide-generated tsunami are enormous, and can cause tremendous damage locally. Thus, I would not argue with the decision to evacuate portions of the Canary Islands, as the swarm of earthquakes does suggest an imminent risk, and it would be impossible to evacuate in response to an event--it would hit too fast and too hard. But the risk level for North America is low.

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Update (October 21)

This is an abstract from Geochemistry, Geophysics Geosystems Journal (AGU) of an in-press paper (meaning that as of this writing it is not yet ready for purchase). The key point is that large, possibly tsunamigenic landslides in the Canary Islands tend to occur in stages, often separated by days. This would greatly reduce the size of the resulting tsunami.


Sedimentological and geochemical evidence for multistage failure of volcanic island landslides: a case study from Icod landslide on north Tenerife, Canary Islands

James Edward Hunt
Russell Wynn
Douglas Masson
Peter Talling
Damon A.H. Teagle

Volcanic island landslides can pose a significant geohazard through landslide-generated tsunamis. However, a lack of direct observations means that factors influencing tsunamigenic potential of landslides remain poorly constrained. The study of distal turbidites generated from past landslides can provide useful insights into key aspects of the landslide dynamics and emplacement process, such as total event volume and whether landslides occurred as single or multiple events. The northern flank of Tenerife has undergone multiple landslide events, the most recent being the Icod landslide dated at ~165 ka. The Icod landslide generated a turbidite with a deposit volume of ~210 km3, covering 355,000 km2 of seafloor off northwest Africa. The Icod turbidite architecture displays a stacked sequence of seven normally graded sand and mud intervals (named subunits SBU1-7). Evidence from subunit bulk geochemistry, volume, basal grain size, volcanic glass composition and sand mineralogy, combined with petrophysical and geophysical data, suggests that the subunit facies represents multistage retrogressive failure of the Icod landslide. The basal subunits (SBU1-3) indicate that the first three stages of the landslide had a submarine component, whereas the upper subunits (SBU4-7) originated above sea level. The presence of thin, non-bioturbated, mud intervals between subunit sands suggests a likely time interval of at least several days between each stage of failure. These results have important implications for tsunamigenesis from such landslides, as multistage retrogressive failures, separated by several days and with both a submarine and subaerial component, will have markedly lower tsunamigenic potential than a single-block failure.

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