Analogues and analogies. Standard dictionaries define these as a comparison, correspondence, or similarity between one thing and another, that can apply to concepts, ideas or physical entities. They are tools, used to illustrate concepts, particularly abstract ideas, to help explain phenomena or theories. Science makes frequent use of analogies. It does so because many phenomena that it attempts to investigate and explain, extend beyond normal human experience, beyond what is visible to the unaided eye, beyond what we can touch. Well-chosen analogies can help us understand the universe without, and the universe within. Continue reading
Groundwater is always on the move. Under some conditions, in fractures or other large conduits, it can move quickly; almost at a walking pace. Under other conditions it moves inexorably slowly, like fractions of a millimeter a year. Regardless, it is always compelled to move. Movement requires energy. Where does this energy come from? What drives the flow of groundwater? Answers to these questions provide the foundations to the science of hydrogeology. Continue reading
Sunday in Pisa proved to be a welcome change from the usual tourist-cramped, shoulder-barging throngs of popular attractions in Tuscany. No problem finding a seat in a decent café, en route to the Piazza del Miricoli. Cross the street, turn a corner and there – the massive, white-marbled Pisa Duomo, Romanesque grandeur with a veneer of 21st Century scaffolding. But the sense of balance normally attributed to cathedrals, is disrupted by the stand-alone bell tower that leans precariously, like a drunk looking for a lamppost. The Leaning Tower of Pisa has been looking for a lamppost for almost one thousand years. And for a thousand years, people have been drawn to the tower not because it is particularly beautiful, but because it looks like it is about to fall over. Continue reading
I like a good detective thriller. Danish, Norwegian, Swedish and Britain’s BBC networks have produced some quality shows over the past few years. Forensics is usually equated with ‘who dunnit?’ but science also makes use of forensic-like tools to help unravel mysteries and solve problems. This post looks at certain chemical compounds found in hydrocarbon deposits. The compounds are specific, complex organic molecules called biomarkers. Biomarkers provide scientific fingerprints of oil deposits, that help scientists and oil explorationists decipher the where, when and how such deposits formed, and environmental scientists monitoring the migration and degradation of spilled oil. Continue reading
You could be excused for labeling this title conspiratorial, the brutal reality encapsulated in Alan Parsons prog rock group’s signature song or a Helen Mirren thriller. And if that is your inclination, don’t bother reading any further because this post deals with far more mundane uses of remotely sensed data. The data sets are generated by two groups of satellites that measure very different attributes of the earth, gravity and light spectra. Teasing the data has given us multiple stories of how systems like surface and groundwater are responding to human activities and natural processes. Some of these stories make grim reading.
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.
Of 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.
Earthquake 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 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).
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