Monthly Archives: November 2016

The Shunga Event; did a Precambrian mass extinction give rise to an ancient supergiant oil field?

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Earth continues to evolve.  So far it has taken, notwithstanding Bishop Ussher’s different view of things, about 4.6 billion years for the atmosphere, hydrosphere, lithosphere (the solid earth), and biosphere to get to where they are today.  Over that time there have been (rewording a well-known expression) long geological periods of inexorably slow change punctuated by catastrophes.  Mass extinctions caused by blink-of-an-eye bolides and episodes of rampant volcanism (e.g. the Deccan Traps) completely changed the course of biological evolution.  Contrast events like these with the life and death of oceans counted over 100-200 million years.  Meteorites, volcanoes, and glaciations have all played their part in moulding our planet. Continue reading

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Liquefaction; more than a sloppy puddle at the beach

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Most of us at some time have gleefully created our own liquefied puddle by stomping on wet beach sand.  I once showed my kids, intent on explaining the scientific intricacies of liquefaction, but being teenagers at the time they walked off in embarrassment.  But liquefaction is much more than a trick at the beach; it is a process that can have devastating consequences for built structures and natural slope stability.

Earth movement during an earthquake is cyclic; each back-and-forth or up-and-down motion is one cycle.  There can be many cycles during those few terrifying seconds.  One way to express the intensity of the movement (and earthquake magnitude) is to measure the acceleration of ground displacement (called Peak Ground Acceleration, PGA).  A useful analogy is a car that, when the gas pedal is depressed, accelerates to some maximum speed; the acceleration is the rate at which this speed increases.  Take your foot off the gas pedal and the car decelerates; your vehicle has completed one cycle. A PGA value is usually expressed as a fraction of ‘g’ (the acceleration due to gravity); the greater the fraction, the greater the intensity. During severe earthquakes most damage occurs because of physical, mechanical shaking.  However, there is another process that, in some circumstances, produces severe and widespread damage to buildings and infrastructure – liquefaction.

liquefaction-clean-upOf the two major earthquakes to hit Christchurch, New Zealand (M7.1 September 2010 and M6.2 February 2011) the second, lower magnitude (but greater intensity) event resulted in the most damage and loss of life (because the epicentre was less than 10km from the city and high PGAs).  Christchurch lies on a relatively flat, low elevation flood plain that is underlain by unconsolidated (soft) layers of silt, clay, sand and gravel.  The watertable is less than 2m beneath much of the city area.  During the 2011 earthquake, liquefaction resulted in water and sediment expulsion (to the surface) over about a third of the city, damaged about 15,000 buildings (6000 beyond repair) and buried water-sewerage pipes, and deposited more than 400,000 tonnes of sand and silt on streets and backyards.

Most of the damage from earthquake shaking takes place above the land surface; most of the damage from liquefaction occurs below it.

 

What is liquefaction?

Under normal conditions sand deposits have strength, imparted by all those sand grains being in contact with other grains around them; this is the reason you can walk along a beach without fear of sinking up to your neck.  This condition also applies to sands below a watertable but in this case all the pore spaces between grains are filled with water.  The groundwater fluid pressure in this situation is normal.

liquefactionEarthquake shaking applies a cyclic external force to the sand grains and the ground water.  If shaking is strong enough the sand grains begin to separate until they reach a point where most are ‘floating’ in the surrounding water.  At this point, the fluid now consists not only of water but also the floating grains and a consequence of this is that fluid pressure increases.  The sand is now liquefied. In this state it no longer has sufficient strength to support surface loads and their foundations sink.  Buried pipes will also move, some breaching the land surface.

The excess fluid (water + grains) in liquefied sediment is at a higher than normal pressure and will flow towards the surface as boils or sand volcanoes.  The 2011 Christchurch earthquake provided some excellent examples of this process (although pretty disheartening for those directly affected).  As water plus sediment is expelled the sand grains beneath the surface begin to settle and eventually the process stops.  Another spectacular example of foundation failure took place during the 1964 Niigata earthquake on Honshu, Japan.  Here, entire 4- and 5-storey apartment blocks literally fell over because of liquefaction beneath the buildings.

liquefaction-niigata

Liquefaction beneath the waves

Sediment beneath the sea or lake bed can respond in a similar way to seismic shaking.  However, seismic events are not the only cause of submarine liquefaction; the impact of storm waves on the seabed can also produce liquefied sediment.  Like earthquake events, this process is also cyclic where fluid pressures increase and decrease as each wave passes.  Liquefaction of seabed sediment can result in serious damage and movement of structures such as pipeline, cables, oil rigs, reclaimed land and jetties; it can also trigger submarine landslides and bottom-hugging flows of mud, sand and gravel.

 

A simple experiment

The inset demonstrates a simple experiment where a structure, comfortably resting on a sandy surface, will sink during an experimental seismic event.  All you need is a suitable container, sand, water and Lego (or whatever takes your fancy for a surface structure).

liquefaction-expt

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When nature casually flicks a finger at us

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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

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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 trimlines.

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.

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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 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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