Tag Archives: tsunami

Islands with attitude; the devastation wrought by collapse of oceanic volcanoes

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Krakatoa, 1883, and the seas shivered. The eruption, one of the largest in recorded history, delivered tsunamis that swept away entire villages around Indonesia and its neighbours; little more than the flotsam and jetsam of nature’s fickleness.  Five years later, in the same general neighbourhood, nature was at it again.

Ritter Island, barely a speck on most maps, is a volcanic edifice rooted to the floor of Bismarck Sea between Papua New Guinea and New Britain. In 1888, most of the island slid beneath the waves, creating avalanches of rocky debris.  Eye-witness accounts tell of multiple tsunamis over a 3-hour period, and waves at least 8m high with run-ups to 15m above sea level.  Ritter Island is an active volcano, but at that time it was not erupting in any major way.  The island landslide is probably the largest in recent history – more than 4 cubic kilometres of volcanic rock were dislodged and redeposited along the seafloor. Slope failures like this are called volcanic sector collapses. Continue reading

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Tsunamis behave as shallow-water waves

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May 26, 1983 tsunami near coast of Honshu, Japan. Boats in foreground for scale

Tsunami statistics make grim reading, which is why I am not going to quote any.  There are some great documentaries and websites that will regale you with all the stats you need. There’s even a couple of movies, where, if you sift through the hype, you may see a smidgen of science, or hear a bit of terminology added to the dialogue to give the impression of knowledgeable heroes.

The word Tsunami derives from two Japanese words; Tsu meaning harbour, and nami wave; an appropriate etymology given that these forces of nature really come into their own along shallow coasts and harbours. About 80% of tsunamis are generated by powerful earthquakes (particularly those beneath the sea floor); the remaining 20% result from large landslides, volcanic eruptions, and less frequently (fortunately) meteorite impacts. They are sometimes referred to, incorrectly, as tidal waves. Tides result from astronomical forces.  We can think of the succession of high and low tides as the passing of a wave that has a period of about 12 hours (the time from one high tide to the next). Tidal waves move along coasts such that a high tide at one location (i.e. the crest of the wave) will occur at a different time to that at a more distant location.  Tides also move water masses; waves do not.

Sea and lake surface waves are generated by wind. The wind provides the energy which is transferred to surface waters.  As a general rule, the stronger the wind, the greater are wave amplitude, wavelength, and speed. Surface waves are basically pulses of energy. As such , a water mass does not move in concert with the wave. Instead, water particles beneath waves have a circular or elliptical motion (referred to as orbitals); the larger orbitals occurring immediately below the crest,  decreasing in size to a depth that equates to about half the wavelength.  This means that in deep water, waves do not interact with the sea floor. This kind of surface wave is given the name deep-water wave, the speed of which depends only on the ratio of wavelength to wave period. Deep-water waves occur where water depth is greater than half the wavelength.

As waves approach the coast, the wave orbitals begin to touch the sea floor (also referred to as wave-base) and wave speed decreases.  At these depths (depth is less than half the wavelength), loose sediment can be moved by the wave orbitals. Some energy is transferred to the sea floor, but to conserve energy, the height, or wave amplitude must also increase. As you can see in the diagram, the orbitals also become flattened. At this stage, the waves have become shallow-water waves.

Although it may seem counterintuitive, tsunamis behave as shallow-water waves. They have long wavelengths, commonly measured in 10s to 100s of kilometres. The speed of shallow-water waves, including tsunamis, is independent of their wavelength, but is dependent on water depth in the following way:

Speed = (g . depth) (g = gravitational constant, 9.8m/s2; depth in metres)

In the case of tsunamis, the wavelength is many times greater than water depth, even in oceans more than 4000m deep. For example, a tsunami traveling across ocean that is 4000m deep will have a speed of 198m/second, or 713 km/hour. This animation of the 2010, M8.8 Chile earthquake and tsunami gives an impression of the speed of wave propagation across oceans, and the shape of the wave fronts. Tsunami waves commonly pass unnoticed beneath ships at sea or offshore rigs.  As they approach shallower water, their speed decreases to between 40-80km/hour (because speed is dependent on water depth), but the amount of energy in the wave changes very little; to compensate, the wave amplitude must increase. Earthquakes that generate tsunamis create several waves that spread out from the epicentre. All these waves can be destructive, and in some cases the first wave is the least harmful. It is also possible for a wave trough to reach the coast before the first wave crest; this results in a rapid drawdown of the water-level, exposing parts of the foreshore that would not normally be seen at even the lowest tides. Unfortunately, in all too short a time, the absence of water is replaced by a more menacing prospect.

Landslides can also produce monster waves; Lituya Bay in Alaska, 1958 is a good example with first-hand witnesses to the 15-22m wave. A prime example of volcanic eruption-derived waves is the cataclysmic 1883 Krakatoa eruption; a 30m tsunami wreaked havoc in Indonesia and across Sunda Strait.

Tsunami warning systems generally involve an international effort to, in the first instance, detect and pinpoint the epicentre of large earthquakes, and secondly, to detect tsunamis and predict their arrival times at different locations. There is a particular focus on submarine and near-coast, shallow crust seismic events of magnitude 7 and greater; high magnitude earthquakes deeper than about 100km generally do not produce destructive tsunamis.  Tsunami detection buoys have been installed in 59 deep ocean locations, most around the Pacific rim.  The map shows the buoys to be located along tectonically active plate margins, such as the west coasts of North and South America, the Aleutian Arc, and other volcanic arcs – subduction zones from Japan through to New Zealand.

Tsunami detection buoys

The deep-water buoys are anchored to the sea floor; for each sea-bottom buoy there is a linked surface buoy that relays data via satellite.  The deep buoys measure subtle changes in water pressure that can be used to calculate changes in sea-surface height.  The latest models have two-way communications so that a particular buoy can be programmed to search for pressure changes if an earthquake is known to have occurred.  Of course, all this is fine if a region has several hours to prepare for possible inundation.  Those close to epicentres may only have a few minutes to react.

The technology for tsunami prediction and warning is always improving. This is particularly the case for new generations of satellite that are tasked with collecting all manner of climate-related data, data relating to short- and long-term sea-level changes, and subtle changes in gravity and magnetic fields associated with earth’s ever-changing profile.

National Tsunami Warning Centre

Some Tsunami video clips

Boxing Day tsunami 2004 (Cornell Univ. animation)

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When nature casually flicks a finger at us

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Trim line of forest devastation from the Lituya Bat tsunami

I always feel a sense of unease when hearing of natural disasters.  I live in a country where earthquakes, volcanic eruptions, landslides, floods and the threat of tsunamis all occur within a relatively small land area.  In Canada, I lived on a small island (a daily ferry commute), nestled in a typical West coast fiord near Vancouver and more than once I pondered the possibility, even inevitability of those towering rock walls suddenly obeying the laws of gravity.

Fiords exist because of ponderous gouging and plucking of the solid earth by rivers of ice; testaments to ancient glacial climates and the forces of nature.  Fiord landscapes are stunning, beautiful in and of themselves and, I suspect because they invoke a sense of awe.  But this is a façade.  Rock walls, almost vertical in places and towering 100s if not 1000s of metres are inherently unstable; any glaciers at the head of the fiord or hanging off the valley walls exacerbate the problems.  Landslides, rockfalls and glacier calving inevitably end up in the sea and the resulting tsunamis can be terrifying.  The video here shows these guys had a lucky escape after venturing a bit too close to a calving, Greenland glacier.

 

All that energy focused through a narrow gap

Terrestrial or submarine landslides that occur on open coasts can produce locally massive tsunami waves (10s of metres high) but these tend to dissipate radially.  There are well known examples on oceanic volcanoes like Canary Islands and Hawaii.  From the North Sea, the Storegga submarine landslide about 8000 years ago must have devastated parts of Britain and northern Europe coasts with wave run-up locally 20m and more. The Youtube Video here shows a recreation of this event.

Tsunamis generated in fiords react differently because the wave energy is focused, generally along the length of the fiord.  Some of the largest waves on record, either observed directly or the immediate aftermath, have been generated in fiords.  There are many examples – historical and prehistoric.  The three I have chosen are well documented and pretty spectacular.  They provide a sober reminder of what can happen when nature casually flicks a finger at us.

 

Talfjord, Norway, 1934

On April 7, about 2 million cubic metres of rock fell about 700m into the fiord.  Displacement of water instantly created a wave that initially was 62m high near the landslide impact.  By the time the wave had reached the village of Talfjord some 5 km away it was still 16m. It was about 3am and most in the town were asleep.   Geological mapping has shown that there have been at least 10 landslides along a 7km stretch of the fiord, and along the adjacent Storfjorden 108 rockslides have been identified, all occurring since deglaciation about 14,500 years ago (Source: Harbitz et al. 2014, Coastal Engineering, v 88).  It was common knowledge at the time that the rock mass above the fiord was unstable, where tension cracks were forming.  Unfortunately, there are no real geotechnical solutions to a problem like this, other than monitoring.

A similarly unstable, but much larger rock mass in Storfjorden (about 50 million m3) has been monitored for several years.  Signs of instability include tension gashes that seem to become larger every year.  Early warning systems are in place although the warning time for the closest villages is likely to be no more than a few minutes.  Fingers are crossed.

Paatuut, Greenland, November 21, 2000

A large landslide at Paatuut located in a fiord on Greenland’s west coast, moved about 90 million m3 of rocky debris about a third of which splashed into the fiord.  The landslide was initiated 1000-1400m above sea level.  No one witnessed the landslide but the resulting tsunami was recorded in a small village about 40km away.  On the side of the fiord opposite Paatuut an abandoned mining town (Qullissat) about 25km away was almost destroyed.  Wave run-up at Qullissat was 28m (observed in the lines of debris) and it is estimated that the wave near the landslide itself had a run-up of about 50m.  Seven kilometres northwest of Paatuut several icebergs were left stranded up to 700m from the coast on an alluvial fan.  Fishing crates and other flotsam were stranded to 800m inland and 40m above sea level.

 

Lituya Bay, Alaska, July 9, 1958

Aerial view (1960) of the trim-line Lituya Bay

One of the largest waves ever observed (and survived) took place in Lituya Bay, Alaska during a 1958 magnitude 7.8 earthquake along the Fairweather Fault (Fairweather Fault is part of an extensive system of active faults that mark the boundary between the Pacific and North American plates – it is a fundamental structure in the Alaskan Panhandle.).

The tsunami was triggered by a 30 million m3 rock mass falling more than 900m into Gilbert Inlet at the head of Lituya Bay.  It seems likely that collapse of the glacier front at the head of the Bay also played a role in generating massive waves. The wave nearest the landslide had an incredible 524m run-up (1720 feet), but subsided to about 10m near the entrance to the open sea.  Wave run-up was easily mapped along Lituya Bay by the line of devastated forest; also known as trim lines.

Howard Urlich and his 7 year-old son, from their fishing boat anchored in a sheltered bay, provided first-hand accounts of what must have been a terrifying experience.  They awoke to a wall of water, that from their estimates was 50-75 feet high (15-22m) coming at them.  The boat was anchored in about 10m of water.  Unable to release the anchor Urlich let the anchor chain run free – it snapped at about 40 fathoms (73m – note however that the boat would have been carried forward at this point in their adventure and the 73m does not indicate true wave height; but it does give some indication).  The boat was carried along the front of the wave over the adjacent shore and several metres above the trees. It was then washed back into the bay, still upright.  A couple of other boats in the bay didn’t fare so well.  The Youtube video shows a dramatic reconstruction of events.

Trimline 55 m above sea level; below, evidence for the force of the wave

Read the first USGS account by Don Miller, 1960 here,  and some old images here.

The list of terrifyingly spectacular fiord tsunamis goes on.  For example, a more recent event at Taan Fiord Alaska, October 17, 2015 – as if Lituya Bay wasn’t enough; this one is still under investigation, but it looks like the wave was capable of hedge-trimming activities to about 150m and tossing boulders even higher.

 

Postscript

The day I wrote this post ended with a magnitude 7.5 jolt in northeast South Island, New Zealand (north of Christchurch).  We live about 800 km north of the Kaikoura epicentre and our house shook and swayed, trees swayed, and water in the pool sloshed over the sides.  No damage to us, but considerable building and infrastructure damage farther south. Some aftershocks have been greater than magnitude 6. A rude awakening…

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Submarine landslides; danger lurks in the ocean deep

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Source: Newfoundland and Labrador Heritage Website. You can watch this short video on Youtube

Five pm, November 18 1929 in the sleepy fishing village on Burin Peninsula, Newfoundland (at that time Newfoundland and Labrador were a Dominion of Britain.  They did not become part of the Canadian Federation until 1949).  Most people felt the tremors from the Grand Banks 7.2M earthquake, centered about 260km south of Burin but apparently went about their business as usual.  About 7.30 the same evening, there was a sudden drop in sea level, exposing the local shore and stranding boats.  The follow-up was totally unexpected – three massive waves inundated coastal dwellings, killing 28 people and leaving hundreds homeless.  The waves were 3-7m high in most places, but along some narrow inlets the tsunami energy had focussed into 27m-high monsters.  The tsunami was caused not by the earthquake itself, but by a massive submarine landslide. (Check out some images here). Continue reading

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