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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A whole lot of shakin'

I didn't realize how complicated it is to calculate the magnitude of an earthquake.

Most of us are probably familiar with the Richter Scale, the one most commonly used in the media.  It was developed in 1935 by seismologist Charles Francis Richter to give a standard scale to measure the power of earthquakes.  The scale is logarithmic; each increase in one on the scale represents a ten-fold increase in intensity.  The scale is based upon the displacement amplitude on a seismograph at a distance of one hundred kilometers from the epicenter, starting with a magnitude 0 earthquake causing the needle to move with an amplitude of one micron.  The scale extends up to an unspecified "greater than 9" -- because at that point, pretty much everything in the vicinity, including the seismograph, gets completely pulverized.

When you start looking more closely, though, the problems with the scale start to become obvious.  First of all, if the measurement is being made one hundred kilometers from the epicenter, the terrain in between is a significant factor.  Tremors passing through material with a high amount of shear (such as sand or mud) will lose intensity fast, as compared to ones going through a material that is rigid (such as solid rock).  Second, the origin of the earthquake usually isn't at the epicenter, which is the point on the surface nearest the source; the origin is the hypocenter, directly underneath -- but which can be at any depth from right near the surface down to hundreds of kilometers down.  (The deepest earthquake ever recorded was a minor tremor off the island of Vanuatu in 2004, which had a hypofocus 736 kilometers deep.)  Then there's the fact that earthquakes can be of different durations -- a less powerful earthquake that lasts longer can do as much damage as a more powerful, but shorter, tremor.

Another problem is that earthquakes can result in differences in the oscillation of the waves relative to the direction they're moving.  This is largely due to the fact that there are three basic sorts of faults.  There are thrust faults or convergent faults, where two tectonic plates are moving toward each other; what happens then can be one plate being pushed underneath the other (subduction), which is what causes the quakes (and the volcanoes) in Indonesia and Japan, or the two plates kind of smashing together into a jumble, which is the process that created the Himalayas.  There are extension faults or divergent faults, where the two plates are moving apart; this usually creates smaller but more frequent quakes, and lots of volcanism as magma bubbles up from the underlying mantle.  This is happening in Iceland, and is also the cause of the Great Rift Valley in Africa, which will eventually peel off the Horn of Africa (Somalia and parts of Ethiopia, Kenya, and Tanzania) and open up a new ocean.  Last, there are strike-slip faults or transform faults, where the plates are moving in opposite directions on each side of the fault, such as the famous San Andreas Fault in California.

Map of the (known) tectonic plates [Image is in the Public Domain courtesy of NASA/JPL]

The problems with the Richter Scale have led to the development of several other scales of intensity, such as the Surface-wave Magnitude Scale (which is pretty much just what it sounds like, and doesn't take into account source depth), the Moment Magnitude Scale (which is based on the amount of energy released as measured by the amount and distance of rock moved), the Duration Magnitude Scale (which figures in how long the tremor lasts), and so on.  But these all use different numerical benchmarks, and given that the Richter Scale is more widely known, a lot of people have continued to use that one despite its downsides.

The reason all this comes up is a new study from the University of Southampton that has identified evidence of what appears to be the biggest earthquake known; an almost unimaginable 9.5 on the Richter Scale quake that happened in Chile 3,800 years ago.  Trying to find the epicenter brings up yet another problem with measuring quake intensity, because the evidence is that this particular quake originated from the rupture of a part of the thrust fault between the Nazca Plate and the South American Plate off the coast of the Atacama Desert -- a rupture that was one thousand kilometers long.

The result was a tsunami that deposited marine sediments and fossils of oceanic animals several kilometers inland, and then traveled across the Pacific Ocean and slammed into New Zealand, tossing boulders the size of cars over distances of hundreds of meters.  That region of the Atacama Desert had been inhabited prior to the quake -- astonishing considering how dry and inhospitable the place is -- but it was (understandably) abandoned by the survivors for a long while afterward.

"The local population there were left with nothing," said geologist James Goff, who co-authored the study.  "Our archaeological work found that a huge social upheaval followed as communities moved inland beyond the reach of tsunamis.  It was over a thousand years before people returned to live at the coast again, which is an amazing length of time given that they relied on the sea for food.  It is likely that traditions handed down from generation to generation bolstered this resilient behavior, although we will never know for sure.  This is the oldest example we have found in the Southern Hemisphere where an earthquake and tsunami had such a catastrophic impact on people’s lives.  There is much to learn from this."

The obvious next question is, "Could this happen again?"  The answer is not just that it could, but it will.  Probably not in the same spot, but somewhere along the many tectonic boundaries in the world.  Nor do we know when.  Earthquake prediction is very far from an exact science.  We have instruments like strain gauges to estimate the tension rock is experiencing, but that doesn't tell you what's going on deeper in the ground, nor when the rock will fracture and release that energy as an earthquake.  Predicting volcanic eruptions is much easier; vulcanologists have gotten pretty good at detecting magma movement underground, and recognizing when a volcano is likely to blow.  (This is why the ongoing hoopla about the Yellowstone Supervolcano is all hype; sure, it'll probably erupt again, but some time in the next hundred thousand years or so, and it's showing no signs of an imminent eruption.)

The Earth is a dynamic planet, and the plates on the surface are in constant motion, jostling, coming together, moving apart, a bit like ice sheets on a river when they begin to break up in the spring.  You can't help but be fascinated by the amount of power it's capable of -- a catastrophic release of energy so large that the scales we've developed to measure such things are all but incapable of expressing.

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Forecasting on a fault line

Living in an earthquake zone is risky business.

I lived for ten years in Seattle, which is immediately adjacent to the Cascadia Subduction Zone, widely considered to be one of the most potentially dangerous faults in the world.  The little Juan de Fuca plate -- all that's left of a much larger piece of oceanic crust that once lay underneath Panthalassa, the ocean that surrounded the supercontinent Pangaea back around the time of the Permian-Triassic Extinction of 251 million years ago -- is slowly disappearing as it gets pulled underneath the North American Plate by convection currents in the mantle.  Subduction zone earthquakes occur along trenches that form the boundaries between plates that are moving toward each other, generating a "thrust fault" as one plate dives beneath the other.  Not only do these produce some of the most massive earthquakes known, they also generate volcanoes like Mount Saint Helens and Mount Rainier.

So lovely as the Seattle area is, it's kind of a disaster waiting to happen.  If you have a high tolerance for being freaked out by the power of the natural world, or you don't live in the Pacific Northwest (or both), you should read journalist Kathryn Schulz's wonderful analysis "The Really Big One" that appeared in The New Yorker in 2015.  Her predictions for what will happen to the area when Cascadia ruptures are truly terrifying -- and would be enough to keep me from ever moving back there, much as I loved western Washington for its culture, climate, and natural beauty.

[Image is in the Public Domain]

If you read the article hoping that Schulz (or the geologists she interviewed) can tell you when the "Really Big One" is going to occur, you're not going to find what you're looking for.  We have a pretty good idea of where earthquakes occur and the types of faults that cause them, but predicting when they'll happen is far more problematic.  And sometimes, even the "where" isn't predictable.  In November of 2019 a 5.0 magnitude quake hit the Rhône Valley in France, along the La Rouvière Fault -- a fault zone that we thought was last active twenty million years ago.

Just last week, though, three papers came out looking at the warning signs that a fault is about to rupture, and methods we may be able to use to predict when they'll happen and how big they'll be.  Getting better at this is imperative for the millions of people who live in quake-prone areas, and could potentially save countless lives.

The first, in the journal Nature, was by a team led by Jonathan Bedford of Helmholtz Centre Potsdam.  In "Months-Long Thousand-Kilometre-Scale Wobbling Before Great Subduction Earthquakes," we learn that there are warning signs -- a slow backward drag on the plate margin that ends with a massive slip in the opposite direction, a little like pulling backward on a bowstring and then letting go suddenly.  The authors write:

[We used] a recently developed trajectory modelling approach that is designed to isolate secular tectonic motions from the daily GNSS time series to show that the 2010 Maule, Chile (moment magnitude 8.8) and 2011 Tohoku-oki, Japan (moment magnitude 9.0) earthquakes were preceded by reversals of 4–8 millimetres in surface displacement that lasted several months and spanned thousands of kilometres.  Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan.

The second paper, in Science, looked at what's happening deep underground beneath one of the most famous fault zones, the strike-slip San Andreas Fault.  In "Excitation of San Andreas Tremors by Thermal Instabilities Below the Seismogenic Zone," geologists Lifeng Wang of the China Earthquake Administration and Sylvain Barbot of the University of Southern California found that temperature patterns can predict the likelihood of a fault suddenly giving way.  For a while, the pieces of the plate margin can slowly, steadily grind past each other, but that motion generates frictional heating.  This can lead to rapid fault failure as the warming rock becomes more plastic.  "Just like rubbing our hands together in cold weather to heat them up, faults heat up when they slide. The fault movements can be caused by large changes in temperature," said study co-author Sylvain Barbot, in an interview with Science Daily.  "This can create a positive feedback that makes them slide even faster, eventually generating an earthquake."

Last, in Nature Communications, geologists Claudia Hulbert and Romain Jolivet (of the École Normale Superieure) and Bertrand Rouet-LeDuc and Paul Johnson (of the Geophysics Group at Los Alamos National Laboratory) turned the power of machine learning on past patterns of seismic instability, and found that large "megathrust" earthquakes were preceded by as much as a year-long slow slip.  Where this slip is occurring, and how fast, might give us advance warning of a major fault rupture:

Slow slip events result from the spontaneous weakening of the subduction megathrust and bear strong resemblance to earthquakes, only slower.  This resemblance allows us to study fundamental aspects of nucleation that remain elusive for classic, fast earthquakes.  We rely on machine learning algorithms to infer slow slip timing from statistics of seismic waveforms.  We find that patterns in seismic power follow the 14-month slow slip cycle in Cascadia, arguing in favor of the predictability of slow slip rupture.  Here, we show that seismic power exponentially increases as the slowly slipping portion of the subduction zone approaches failure, a behavior that shares a striking similarity with the increase in acoustic power observed prior to laboratory slow slip events.  Our results suggest that the nucleation phase of Cascadia slow slip events may last from several weeks up to several months.

Even though such a pattern of slow slips might tell us that a major earthquake is imminent, it's unlikely we'll ever be able to say "... and it's going to happen next Friday at ten A.M."  And given our penchant for ignoring science unless it can give us pinpoint accuracy, we're probably not going to see much change in our behavior.  After all, that tendency is at the heart of the United States's failure to address the COVID-19 pandemic -- the scientists were saying back in December and January, "this has the capacity to be deadly and fast-spreading," and government officials said, "How fast and how deadly?"  The scientists had to say, "We're not sure yet," and that was insufficient for leaders to take swift and decisive action.  (And that's not even taking into consideration that Donald Trump knew about the danger, admitted up front the potential devastation COVID-19 could cause, and deliberately decided to lie about it because he was afraid it would hurt his chances of being re-elected.)

So we're not so good at reacting to clear and present dangers if the remedy is inconvenient or costly.  As James Burke said, in his frighteningly prescient 1991 documentary After the Warming, "The scientists said that devastating climate change was going to happen at some point, but for most people that wasn't good enough.  We wouldn't pay for what amounts to climate insurance, even though we happily insure our lives and our property against far less likely occurrences."

Be that as it may, I'm glad we're seeing this progress being made.  Earthquakes are notorious amongst natural disasters at giving no warning whatsoever, so anything we could do to figure out how to predict them more accurately could potentially save lives.

But even so, I don't think I'd want to live in the Pacific Northwest again.

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Humans have always looked up to the skies.  Art from millennia ago record the positions of the stars and planets -- and one-off astronomical events like comets, eclipses, and supernovas.

And our livelihoods were once tied to those observations.  Calendars based on star positions gave the ancient Egyptians the knowledge of when to expect the Nile River to flood, allowing them to prepare to utilize every drop of that precious water in a climate where rain was rare indeed.  When to plant, when to harvest, when to start storing food -- all were directed from above.

As Carl Sagan so evocatively put it, "It is no wonder that our ancestors worshiped the stars.  For we are their children."

In her new book The Human Cosmos: Civilization and the Stars, scientist and author Jo Marchant looks at this connection through history, from the time of the Lascaux Cave Paintings to the building of Stonehenge to the medieval attempts to impose a "perfect" mathematics on the movement of heavenly objects to today's cutting edge astronomy and astrophysics.  In a journey through history and prehistory, she tells the very human story of our attempts to comprehend what is happening in the skies over our heads -- and how our mechanized lives today have disconnected us from this deep and fundamental understanding.

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