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.


GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, doi:10.1029/2011GC003740

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.

Nuclear power and geological hazards

There is considerable excitement over the damaged nuclear reactors in Japan after the earthquake and tsunami.



The explosion at the Fukushima nuclear power plant looks bad at the surface. What's behind it?

As soon as the earthquake occurred, the power plants shut down automatically. The control rods enter the core to stop the nuclear reaction. There is a great deal of residual heat, which is handled by the plant's normal cooling system. But the earthquake damage was severe enough to knock out power to the plants, so the cooling system had to be run off backup diesel generators.

The backups have failed because diesel generators don't run well underwater.

So the Japanese have to run around on their destroyed infrastructure and bring power to these plants.

Notably, the power plants were designed to withstand a magnitude 8.2 earthquake, so they have done well to be still there after the 8.9.

However, the sequence of events since Friday does not appear to have been anticipated by the engineers.

Tsunami dynamics--why they can cross an ocean

Friday's earthquake off the coast of Japan reminds us of the power of water.



But first, a little animation of a model of the tsunami.



Here is a model of amplitude of wave height as posted by NOAA.

This model is constructed first from knowledge of the motion of the earthquake, which can be seen here.

The beachballs in the figure above give you a sense of the motion of the earthquake. To interpret them, imagine a beachball divided into four longitudinal quadrants. One pair of opposing quadrants is black, the other pair is white. The black refers to the portion of the globe where the first motion of the ground after the earthquake is compressive (i.e., the ground is pushed away from the focal point). The white areas represent the areas on the globe where the first motion is extensional (i.e., the ground is pulled toward the focal point).

In order to divide the beachball into four quadrants, you have two intersecting planes. In the diagram above, we can see that of the two planes, one must be approximately vertical, and the other is nearly horizontal. One of these planes is the fault plane--the plane along which the earth has broken. In the diagram above, we are not given enough information to tell which one it is, but additional information on the USGS website allows us to tell it was the nearly horizontal plane.

The other plane is perpendicular to the direction of motion along the fault plane. Knowing that the nearly horizontal plane is the fault plane, we can tell that the direction of motion is perpendicular to the nearly vertical plane, which is oriented approximately NNE-SSW. The direction of motion is therefore either north of west, or south of east. Since the compressive motion is on the west side, we know the motion was towards the west.

Notice the thin arc of white around the Harv and CPPT solutions. The ground in the black has moved upward/outward from the focal point, and the white areas have moved toward the focal point. Hence Japan, to the west of the fault has moved towards it, but much of the seafloor between Japan and the focus has moved upwards, spawning the tsunami.



The tsunami starts off from the earthquake epicentre and flows outwards. In order to model what happens next, you need a model of the topography of the ocean basin.

The velocity of the tsunami is a function of water depth--the deeper the water the faster it goes. When the wave hits shallower water it slows down, and piles up as the faster moving water behind it catches up to it All of those little and islands and seamounts will diffract some of the energy (in doing so they act like new, but smaller sources of radial waves). Either the islands or the topography may act to focus the energy of the wave along some portions of the wave front, explaining how central California was hit by higher waves than areas either north or south of it.

The key to the energy transmission on a global scale is the geometry of the source. If you throw a pebble into a pond, the ripples you get have the form of a series of expanding circles.

Consider the portion of the ripple outlined in red (between the two yellow lines). Let us say that the quantity of energy represented in that portion of the ripple is a. As the ripple expands, the amount of energy in the ripple remains the same (it actually declines due to friction and internal factors). However as the length of the red arc increases as the ripple expands, the amount of energy per unit length of the wave declines with distance travelled (in addition to friction and other losses).

An earthquake occurs along a plane. The motion does not occur along the plane everywhere at once, but propogates at a finite speed.

Diagram showing both magnitude of motion along the fault (colours) and the length

of time before motion was initiated at points along the fault (contours). Source here.

Thus instead of a point source for the wave, you have a line source (we are only considering the ocean surface at this point). The effect of a line source makes a tremendous difference in energy propagation.

Note here that as the waves propagate, the lengths of the red arcs stay constant. Thus there is no loss of energy to expansion of the wavefront over parts of the tsunami (not the case at the ends in the diagram above). There are still losses due to friction and internal dissipation.

Here is a screen capture of the NOAA model (from the model above).

Right at the moment of this capture, the wavefront heading towards South America is nearly linear, meaning there is little loss of energy as the wave flows. Where the front is curved, energy is declining dramatically.