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Posts Tagged ‘seismology’

journey-to-the-centre-of-the-earth-by-david-whitehouseJourney to the Centre of the Earth: A Scientific Exploration into the Heart of Our Planet, David Whitehouse (Weidenfeld & Nicolson 2015)

Can you touch anywhere on your body with your right hand? Replying quickly, you might say you can. But what about your right elbow? You can’t touch that with your right hand. Science is like that, because distant things are often easier to study and understand than close things. We have a good understanding of how stars work, for example, but not of how the earth’s magnetic field is generated.

And while we’ve been able to predict solar eclipses for millennia, we still can’t predict earthquakes or volcanic eruptions. Understanding the deep earth is difficult, so there are a lot of mysteries and conjectures in this well-written and compelling book about the interior of our home planet. Scientists have landed probes on Mars, millions of kilometres away, but the “deepest hole ever drilled on earth – the Kola Superdeep borehole in northern Russia” reached only 12,262 metres. That’s a mere pinprick by comparison with the radius of the earth. To get beyond that, scientists have had to study the shockwaves generated by earthquakes. The medium is the message: as the waves pass through or hit different regions and materials, they behave in different ways.

For example, when the Croatian scientist Andrija Mohorovičić (1857-1936) “studied the records from several seismometers” after an earthquake near Zagreb, “he realised that some of the shockwaves […] were being reflected back to the surface from a boundary region between the crust and mantle.” (pg. 82 of the 2016 paperback) The region is now called the Mohorovičić discontinuity. But that discovery was made before the First World War and deep geology hasn’t advanced very much in the intervening century. This book borrows the title of a Jules Verne novel published in 1864. If Verne came back to life, he would be pleased to see that his work is still popular, but he would be disappointed to see that the human race was no nearer reaching the centre of the earth.

Or would he? The American geologist Don Anderson says: “Almost everything known or inferred about the inner core from seismology or indirect inference is controversial.” (pg. 211) Deep geology is a difficult science, but that’s part of what makes it so interesting. Something else that makes it interesting is that the inner earth can visit catastrophes on the outer earth and the film of the life that clings there:

The big question is: can we see mass extinction events on the way up? Some scientists believe that we can by looking for the plumes [i.e., giant plumes of rising magma]. Such a thing is seen in the south-west Pacific near the Fiji Tonga subduction zone. It’s 700 km deep, has a structure consistent with a massive temperature anomaly and may be rising. It could render the earth uninhabitable for humans and it will reach the surface in an estimated 200 million years. (ch. 17, “Plumes”, pg. 146)

Asteroid impact and gamma-ray bursts are not the only catastrophes that threaten the continued existence of the human race. They may not even be the most likely. The film of life on the surface of the earth is fragile and one day it won’t be there any more.

But there’s also life deep inside the earth, living in conditions of extreme pressure, heat and darkness. We still know little about this “deep biosphere” and it may hold some big surprises. The rest of the deep earth almost certainly does. And the deep earth is just the beginning: as Whitehouse describes in chapter 26, there are “Other Worlds, Other Journeys” to come, including the even more extreme conditions at the heart of Jupiter, where the temperature is a “staggering 37,000 degrees C” and the pressure is “over ten times that found at the centre of the Earth.” (pg. 239)

Or so scientists estimate. Will scanners be invented to prove their theories? Will probes ever get there and find out for real? We can hope so. In the meantime, this book is an excellent introduction to the ideas, the pioneers and the modern researchers into mysteries that are right beneath our doorsteps. Whether it’s discussing diamonds, demons or “Double-D-Prime”, Journey to the Centre of the Earth is popular science that’s interesting, entertaining and informative all at once.

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The Million Death Quake by Roger MussonThe Million Death Quake: The Science of Predicting Earth’s Deadliest Natural Disaster, Roger Musson (Palgrave Macmillan 2012)

“As solid as the earth,” we say. That’s why even mild earthquakes are often frightening and always memorable. Suddenly you can’t rely on the earth any more: it’s not rock-steady, it’s dancing. And it might be about to dance you to death.

But Robert Musson, author of this excellent guide to the history and future of seismology, points out that even in a big earthquake you’ll usually be safe in the open away from buildings. The problem is that few people spend much time like that. Cities are getting bigger and more crowded, which is why he suggests that one day an earthquake could kill a million people or more. Tehran is one candidate. So is this:

The case of Istanbul is unnerving for another reason. The North Anatolian Fault, the great strike-slip fault that starts in eastern Turkey and dies out in the middle of the Aegean, has an interesting property. Earthquakes along it tend to occur in sequences, starting in the east and moving progressively west. Each quake, as it occurs, throws more stress on the next section of fault to the west, which then fails a few years to a decade or so later. it’s like a series of dominoes toppling. […] The current sequence began with a 7.8 magnitude event near Erzincan, at the eastern end of the fault line, in 1939. This was followed by quakes progressively further west in 1942, 1943, 1944, 1957 and 1967. Then, after a lull, the next most westerly stretch of fault broke in 1999 with the Izmit earthquake. The next stretch of fault to the west goes straight through the Sea of Marmara, just south of Istanbul. This is the next domino to fall, and it could happen at any time. (ch. 12, “Stay Safe”, pp. 233-4)

Or there could be another lull. That is one of the interesting things about earthquakes: their unpredictability. The subtitle of this book is misleading, because there is no reliable science of prediction for earthquakes. Seismologists can say in great detail why and how they occur, but they can’t say where or when or what size. We are far better at predicting the behaviour of the sky above our heads than we are at predicting the behaviour of the earth beneath our feet. Meteorologists are refining and extending their forecasts further all the time. Astronomers have been accurately predicting eclipses and planetary orbits for thousands of years.

Seismologists would like to make their discipline predictive rather than reactive, but it’s proving very difficult. Masson discusses one team of Greek seismologists who claimed to be able to predict quakes using “seismic electrical signals, or SES for short” released by “rocks once they are stressed beyond a certain degree” (ch. 8, “Next Year’s Earthquakes”, pg. 172). But the team, led by Professor Panayotis Varotsos, made their predictions by sending telegrams to each other rather than informing an official body. When the earthquake occurred, they would produce the telegram and its date-stamp: “The question that was whispered in the corridors at conference sessions was this: How many telegrams were quietly burned when the prediction failed?”

Then a “moderate earthquake” hit Athens in 1999 and although the team claimed to have predicted it, they hadn’t said so in public. Apparently stung by the criticism that followed, Professor Varotsos issued a public prediction of a larger earthquake on its way in central Greece. But it never happened and the team were no longer taken seriously.

It’s not difficult to understand why earthquake prediction is so difficult: rocks aren’t transparent and gathering data from the depths of the earth is much harder than gathering data from the sky. Seismologists would be delighted if they could realize the suggestion made by Arthur C. Clarke in his short story “The Fires Within” (1949):

Sonar, as you will know, is the acoustic equivalent of radar, and although less familiar is older by some millions of years, since bats use it very effectively to detect insects and obstacles at night. Professor Hancock intended to send high-powered supersonic pulses into the ground and to build up from the returning echoes an image of what lay beneath. The picture would be displayed on a cathode ray tube and the whole system would be exactly analogous to the type of radar used in aircraft to show the ground through cloud.

Nearly seventy years on, we’re still waiting for a geoscope like that. Seismology is still a hobbled science and earthquakes are still mysterious and frightening things. As Sherlock Holmes says in “The Adventure of the Copper Beeches” (1892): “Data! data! Data! … I can’t make bricks without clay.” But seismologists have done a lot with the limited data they’ve got, as you’ll learn here. Writing clearly and colloquially, Masson traces the history of mankind’s attempts to understand earthquakes, describes their effects on history, discusses related phenomena like volcanoes and tsunamis, and explains why seismologists don’t use the “Richter scale”. The Million Death Quake has a hyperbolic title and a misleading subtitle, but it’s one of the best popular science books I’ve come across.

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