Category Archives: Geophysics

Subcutaneous oceans on distant moons; Enceladus and Europa

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Our blue Earth, rising above the lunar horizon, is an abiding image of our watery state that must evoke an emotional response in any sensible person. Cloud-swirled, Van Gogh-like. Such a blue – there’s nothing like it, at least in our own solar system.  A visitor to Mars three billion years ago might have also seen a red planet daubed blue, but all those expanses of water have since vanished, replaced by seas of sand.

Earth’s oceans are unique in our corner of the universe. Except for a thin carapace of ice at the poles, they are in a liquid state, and are in direct contact with the atmosphere to the extent that feed-back processes control weather patterns and climates.  Sufficient gravitational pull plus the damping effect of our atmosphere, prevents H2O from being stripped from our planet by solar radiation (again, unlike Mars). Our oceans exist because of this finely tuned balancing act. Continue reading

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An Italian job; seismic risk-assessment at risk

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Life is a risky business. Not a day goes by when some aspect of our lives comes under the gaze of risk assessment, an analysis of potential adversity, the probability that some event will impact our well-being. No black and white determinism here; we have become probabilistic entities. The seemingly simple act of driving your car, is translated into an actuarial assessment that determines the cost of insurance, a government’s health budget, a funeral director’s business plan, or a vehicle manufacturer’s liability. All are predicated on the probability of some event taking place – one chance in x occurrences. No luck, or absence of luck? Luck is when you win the lottery without buying a ticket.

Predicting natural phenomena, like volcanic eruptions, tornadoes, or earthquakes, is the stuff of science. The problem with these kinds of events is that they can have global impacts. How does Continue reading

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Which satellite is that? What does it measure?

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Space may well be the final frontier (there are one or two on earth that still require some work), but the space around our own planet is decidedly crowded. Folk at NASA’s Goddard Space Center (Maryland) estimate about 2300 satellites now orbit Earth; vehicles in various states of repair, use or disuse, of which a little more than 1400 are operational Continue reading

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The earth moved; GPS, earthquakes, and slow-slip

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It is often useful to know where you are, in a spatial sense. In the old days (LOL), field geologists, the kind that make maps of rocks and earth structures would, armed with topography maps and compass, determine their location from some vantage point using line-of-sight and triangulation.  I don’t hanker for a return to these days. I’m grateful for the kind of location data instantly available on my smart phone – the little blue dot that seems to follow my course across some digital representation of the universe. But I acknowledge a kind of smugness, in the event the digital world nosedives, knowing that I can still find my way; no General Panic Stations (GPS) if the satellite-based Global Positioning System (the other GPS) fails.

GPS devices can also be attached to bits of the earth’s crust.  This is useful because the crust, whether continent, sea floor, or volcanic island, is always on the move. Continue reading

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So, adding CO2 does increase surface heating; how science has filled another gap in our knowledge

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Read any scientific paper or blog on climate and you’re bound to come across the phrase radiative forcing.  Radiative forcing is central to all climate science. Radiation from the sun heats our atmosphere and earth surface.  Some of this radiation is reflected back to space. If there is a balance between incoming and outgoing radiation then average global atmospheric temperatures neither increase or decrease. However, if the balance is perturbed, climate will warm or cool. Radiative forcing causes climate imbalances.  Thus, volcanic aerosols tend to cool things off, decreasing albedo will tend to warm them. Continue reading

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Volcanism does not cause glaciations; let’s turn this statement on its head

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It is almost a truism that volcanic eruptions influence climate. Cold winters and even failed crops, particularly in the northern hemisphere, followed on the heels the Tambora, Krakatoa, and Pinatubo eruptions.  But these climate aberrations were relatively short-lived, counted in years; the stratospheric aerosols and fine volcanic ash that reflect solar radiation back into space, eventually succumb to gravity and fall to earth.  Eruptions of this kind do not result in long-lived, or permanent changes; they are temporary blips on an evolving earth and an evolving climate. Continue reading

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Tsunamis behave as shallow-water waves

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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. Water particles beneath waves have a circular or elliptical motion (referred to as orbitals); the larger circles occurring immediately below the crest, and 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.

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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