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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