Shaky ground

A little less than six years apart -- on 1 November 1755 and 31 March 1761 -- two major earthquakes struck the country of Portugal, each time generating a tsunami that devastated the capital city of Lisbon.

They were both huge, although given that this was before the invention of the seismometer, we can only guess at how big; estimates are that the 1761 quake was around 8.5 on the Richter Scale, while the 1755 one may have been as high as 9.0.  Each time, the tremors were felt far from the epicenter.  The shaking from the 1755 quake was recorded as far away as Finland.

The effects in Portugal and nearby nations were devastating.  In 1755 the combined death toll in Portugal, Spain, and Morocco -- mostly from the tsunami -- is estimated at fifty thousand.  Over eighty percent of the buildings in Lisbon were damaged or completely destroyed -- and five and a half years later, many of the ones that had survived in 1755 collapsed.

Ruins of the Convento do Carmo, which was destroyed in the Great Lisbon Earthquake of 1755 [Image licensed under the Creative Commons Chris Adams, Convento do Carmo ruins in Lisbon, CC BY-SA 3.0]

What's curious is that Portugal isn't ordinarily thought to be high on the list of seismically-active nations.  It's not on the Ring of Fire, where the majority of the world's earthquakes and volcanoes occur.  The fact is, though, there is a poorly-studied (and poorly-understood) fault zone offshore -- the Azores-Gibraltar Transform Fault -- that is thought to have been responsible for both of the huge eighteenth century quakes, as well as a smaller (but still considerable) earthquake in 1816.

The AGTF, and how it's evolving, was the subject of a paper in the journal Geology last week.  The big picture here has to do with the Wilson Cycle -- named after plate tectonics pioneer John Tuzo Wilson -- which has to do with how the Earth's crust is formed, moved, and eventually destroyed.

At its simplest level, the Wilson Cycle has two main pieces -- divergent zones (or rifts) where oceanic crust is created, pushing plates apart, and convergent zones (or trenches) where oceanic crust is subducted back into the mantle and destroyed.  Right now, one of the main divergent zones is the Mid-Atlantic Rift, which is why the Atlantic Ocean is gradually widening; the Pacific, on the other hand, is largely surrounded by convergent zones, so it's getting smaller.

Of course, the real situation is considerably more complex.  In some places the plates are moving parallel to the faults; these are transform (or strike-slip) faults, like the AGTF and the more famous San Andreas Fault.  And what the new paper found was that the movement along the AGTF doesn't just involve side-by-side movement, but there's a component of compression.

So the Azores-Gibraltar Transform Fault, in essence, is trying to turn into a new subduction zone.

"[These are] some of the oldest pieces of crust on Earth, super strong and rigid -- if it were any younger, the subducting plate would just break off and subduction would come to a halt," said João Duarte, of the University of Lisbon, who lead the research, in an interview with Science Daily.  "Still, it is just barely strong enough to make it, and thus moves very slowly."

The upshot is that subduction appears to be invading the eastern Atlantic, a process that (in tens or hundreds of millions of years) will result in the Atlantic Ocean closing up once more.  The authors write:

[T]he Atlantic already has two subduction zones, the Lesser Antilles and the Scotia arcs.  These subduction zones have been forced from the nearby Pacific subduction zones.  The Gibraltar arc is another place where a subduction zone is invading the Atlantic.  This corresponds to a direct migration of a subduction zone that developed in the closing Mediterranean Basin.  Nevertheless, few authors consider the Gibraltar subduction to be still active because it has significantly slowed down in the past millions of years.  Here, we use new gravity-driven geodynamic models that reproduce the evolution of the Western Mediterranean, show how the Gibraltar arc formed, and test if it is still active.  The results suggest that the arc will propagate farther into the Atlantic after a period of quiescence.  The models also show how a subduction zone starting in a closing ocean (Ligurian Ocean) can migrate into a new opening ocean (Atlantic) through a narrow oceanic corridor.

So the massive Portugal quakes of the eighteenth and nineteenth centuries seem to be part of a larger process, where compression along a (mostly) transform fault is going to result in the formation of a trench.  It's amazing to me how much we've learned in only sixty-odd years -- Wilson and his colleagues only published their seminal papers that established the science of plate tectonics between 1963 and 1968 -- and how much we are still continuing to learn.

And along the way elucidating the processes that generated some of the biggest earthquakes ever recorded.

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The rocks drawn down

The map of the world's continents is so familiar that for most of us, it seems permanent.  Even when you find out that the Earth's land masses have moved dramatically, that (geologically speaking) you don't have to go back very far to get to a time when the map would be unrecognizable, it's a little hard to fathom that the map is still changing now.  The tectonic plates are sliding as you read this, and every earthquake adjusts their positions a little bit.

Practically speaking -- and to any readers who are geologists, I will apologize for what will seem to you a drastic oversimplification -- there are three ways plates can move relative to one another:

1. Divergent plate boundaries -- where the two plates are moving apart.  This is what's happening in the middle of the Atlantic Ocean, where the Mid-Atlantic Rift Zone is upwelling magma that drags the North American and South American Plates toward the west and the European and African Plates toward the east.  New divergent boundaries can tear a continent in half, which is currently happening in eastern Africa, where the East African Rift Zone is eventually going to rip the continent in two, sending most of Ethiopia, Kenya, and Tanzania, and all of Somalia, eastward, and creating a new ocean in between.
2. Convergent plate boundaries -- where two plates are moving toward each other.  If both are thick, cool continental plates, this causes a pile-up -- i.e., non-volcanic mountains, such as the Himalayas.  If one or both is a thin oceanic plate, one will dive underneath the other and melt, creating a line of volcanoes more or less parallel to the plate boundary.  Examples include Japan, Indonesia, and the Cascades.
3. Strike-slip boundaries -- where two plates are moving alongside one another in opposite directions.  An example is the famous San Andreas Fault and the other bits and pieces of the southern California fault system.

The result is that the continents are being moved around, torn apart, and slammed back together all the time, just at a pace so slow we usually aren't aware of it.  But the reality is that the western bits of California are in the process of sliding northward along the North American coast and will eventually smash into Alaska; Australia will run into New Guinea, and north Africa into southern Europe; and Antarctica will slide northward, away from the South Pole and into what is now the southern Pacific.

[Image is in the Public Domain]

But new plate boundaries can form, as the subterranean forces in the mantle create new breaks or seams.  That's actually what brings this whole topic up; a friend and loyal reader of Skeptophilia sent me a link about research that's a few years old but that I'd somehow missed, that there's an "embryonic" subduction zone -- a convergent plate boundary -- forming off the coast of Portugal.  These things don't happen overnight, and the surmise is that this developing subduction zone is responsible for the devastating earthquake that hit Lisbon on November 1, 1755, and damn near flattened every building in the city.  It killed an estimated twenty thousand people, and geologists estimate that it had a magnitude of 8.4, more powerful than the more famous San Francisco Earthquake of 1906.

This complicates the picture in the Atlantic, however. The conventional wisdom is that the divergent boundary in the middle of the Atlantic -- shown in red in the above picture -- is moving North and South America away from Europe and Africa, but if there's a convergent boundary off the coast of Portugal, that'll eat up bits of the oceanic plate off the coast and pull Europe closer to North America.  (It also creates the possibility of Andes-type volcanoes in Portugal and Spain.)  Geologists are still investigating how, and how fast, this new convergent zone is moving, and what its capacity is for generating earthquakes and tsunamis -- we really only have the one significant data point, the 1755 earthquake, to make a stab at what the potential for seismic activity is.  And how this will affect the positions of the continents in the long term is at the moment anyone's guess.

It's endlessly fascinating to me how the face of the Earth can change -- for example, there have been at least three times that more or less all the land masses were fused together into one supercontinent (and the rest of the world was covered by one superocean).  Mountains and oceans have been a symbol of something eternal, unchanging, but in reality everything is in flux.  It recalls to mind the lines from Percy Shelley's evocative poem "Mont Blanc," which seems a fitting way to end:

Yet not a city, but a flood of ruin
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d.  The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost.  The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known.  Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.

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This week's Skeptophilia book recommendation of the week is about as cutting-edge as you can get, and is as scary as it is fascinating.  A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, by Jennifer Doudna and Samuel Sternberg, is a crash course in the new genetic technology called CRISPR-Cas9 -- the gene-editing protocol that Doudna herself discovered.  This technique allows increasingly precise cut-and-paste of DNA, offering promise in not just treating, but curing, deadly genetic diseases like cystic fibrosis and Huntington's disease.

But as with most new discoveries, it is not without its ethical impact.  The cautious are already warning us about "playing God," manipulating our genes not to eliminate disease, but to enhance intelligence or strength, to change personal appearance -- or personality.

A Crack in Creation is an unflinching look at the new science of gene editing, and tries to tease out the how much of what we're hearing is unwarranted fear-talk, and how much represents a genuine ethical minefield.  Doudna and Sternberg give the reader a clear understanding of what CRISPR-Cas9 is likely to be able to do, and what it won't, and maps out a direction for the discussion to take based on actual science -- neither panic and alarmism, nor a Panglossian optimism that everything will sort itself out.  It's a wonderful introduction to a topic that is sure to be much in the news over the next few years.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]